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Title: Man's Place in the Universe - A Study of the Results of Scientific Research in Relation - to the Unity or Plurality of Worlds, 3rd Edition
Author: Wallace, Alfred R.
Language: English
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MAN'S PLACE IN THE UNIVERSE



MAN'S PLACE IN THE UNIVERSE

A Study of the Results of Scientific Research in Relation to the Unity or
Plurality of Worlds


BY ALFRED R. WALLACE LL.D., D.C.L., F.R.S., ETC.


    'O, glittering host! O, golden line!
      I would I had an angel's ken,
    Your deepest secrets to divine,
      And read your mysteries to men.'


_THIRD EDITION_

LONDON CHAPMAN AND HALL LIMITED 1904

    'I said unto my inmost heart,
      Shall I don corslet, helm, and shield,
    And shall I with a Giant strive,
      And charge a Dragon on the field?'

    J.H. DELL.



PREFACE


This work has been written in consequence of the great interest excited by
my article, under the same title, which appeared simultaneously in _The
Fortnightly Review_ and the _New York Independent_. Two friends who read
the manuscript were of opinion that a volume, in which the evidence could
be given much more fully, would be desirable, and the result of the
publication of the article confirmed their view.

I was led to a study of the subject when writing four new chapters on
Astronomy for a new edition of _The Wonderful Century_. I then found that
almost all writers on general astronomy, from Sir John Herschel to
Professor Simon Newcomb and Sir Norman Lockyer, stated, as an indisputable
fact, that our sun is situated _in_ the plane of the great ring of the
Milky Way, and also very nearly in the centre of that ring. The most recent
researches also showed that there was little or no proof of there being any
stars or nebulæ very far beyond the Milky Way, which thus seemed to be the
limit, in that direction, of the stellar universe.

Turning to the earth and the other planets of the Solar System, I found
that the most recent researches led to the conclusion that no other planet
was likely to be the seat of organic life, unless perhaps of a very low
type. For many years I had paid special attention to the problem of the
measurement of geological time, and also that of the mild climates and
generally uniform conditions that had prevailed throughout all geological
epochs; and on considering the number of concurrent causes and the delicate
balance of conditions required to maintain such uniformity, I became still
more convinced that the evidence was exceedingly strong against the
probability or possibility of any other planet being inhabited.

Having long been acquainted with most of the works dealing with the
question of the supposed _Plurality of Worlds_, I was quite aware of the
very superficial treatment the subject had received, even in the hands of
the most able writers, and this made me the more willing to set forth the
whole of the available evidence--astronomical, physical, and biological--in
such a way as to show both what was proved and what suggested by it.

The present work is the result, and I venture to think that those who will
read it carefully will admit that it is a book that was worth writing. It
is founded almost entirely on the marvellous body of facts and conclusions
of the New Astronomy together with those reached by modern physicists,
chemists, and biologists. Its novelty consists in combining the various
results of these different branches of science into a connected whole, so
as to show their bearing upon a single problem--a problem which is of very
great interest to ourselves.

This problem is, whether or no the logical inferences to be drawn from the
various results of modern science lend support to the view that our earth
is the only inhabited planet, not only in the Solar System but in the
whole stellar universe. Of course it is a point as to which absolute
demonstration, one way or the other, is impossible. But in the absence of
any direct proofs, it is clearly rational to inquire into probabilities;
and these probabilities must be determined not by our prepossessions for
any particular view, but by an absolutely impartial and unprejudiced
examination of the tendency of the evidence.

As the book is written for the general, educated body of readers, many of
whom may not be acquainted with any aspect of the subject or with the
wonderful advance of recent knowledge in that department often termed the
New Astronomy, a popular account has been given of all those branches of it
which bear upon the special subject here discussed. This part of the work
occupies the first six chapters. Those who are fairly acquainted with
modern astronomical literature, as given in popular works, may begin at my
seventh chapter, which marks the commencement of the considerable body of
evidence and of argument I have been able to adduce.

To those of my readers who may have been influenced by any of the adverse
criticisms on my views as set forth in the article already referred to, I
must again urge, that throughout the whole of this work, neither the facts
nor the more obvious conclusions from the facts are given on my own
authority, but always on that of the best astronomers, mathematicians, and
other men of science to whose works I have had access, and whose names,
with exact references, I generally give.

What I claim to have done is, to have brought together the various facts
and phenomena _they_ have accumulated; to have set forth the hypotheses by
which _they_ account for them, or the results to which the evidence clearly
points; to have judged between conflicting opinions and theories; and
lastly, to have combined the results of the various widely-separated
departments of science, and to have shown how they bear upon the great
problem which I have here endeavoured, in some slight degree, to elucidate.

As such a large body of facts and arguments from distinct sciences have
been here brought together, I have given a rather full summary of the whole
argument, and have stated my final conclusions in six short sentences. I
then briefly discuss the two aspects of the whole problem--those from the
materialistic and from the spiritualistic points of view; and I conclude
with a few general observations on the almost unthinkable problems raised
by ideas of Infinity--problems which some of my critics thought I had
attempted in some degree to deal with, but which, I here point out, are
altogether above and beyond the questions I have discussed, and equally
above and beyond the highest powers of the human intellect.

    BROADSTONE, DORSET,

    _September_ 1903.


    'The wilder'd mind is tost and lost,
      O sea, in thy eternal tide;
    The reeling brain essays in vain,
      O stars, to grasp the vastness wide!
    The terrible tremendous scheme
      That glimmers in each glancing light,
    O night, O stars, too rudely jars
      The finite with the infinite!'

    J.H. DELL.



                             CONTENTS



  CHAP.                                                            PAGE

     I. EARLY IDEAS,                                                  1

    II. MODERN IDEAS,                                                 7

   III. THE NEW ASTRONOMY,                                           24

    IV. THE DISTRIBUTION OF THE STARS,                               47

     V. DISTANCES OF STARS: THE SUN'S MOTION,                        73

    VI. UNITY AND EVOLUTION OF THE STAR-SYSTEM,                      99

   VII. ARE THE STARS INFINITE?                                     135

  VIII. OUR RELATION TO THE MILKY WAY,                              156

    IX. THE UNIFORMITY OF MATTER AND ITS LAWS,                      183

     X. THE ESSENTIAL CHARACTERS OF ORGANISMS,                      191

    XI. PHYSICAL CONDITIONS ESSENTIAL FOR LIFE,                     206

   XII. THE EARTH IN RELATION TO LIFE,                              218

  XIII. THE ATMOSPHERE IN RELATION TO LIFE,                         243

   XIV. THE OTHER PLANETS ARE NOT HABITABLE,                        262

    XV. THE STARS: HAVE THEY PLANETS? ARE THEY
          USEFUL TO US?                                             282

   XVI. STABILITY OF THE STAR-SYSTEM: IMPORTANCE OF
          CENTRAL POSITION: SUMMARY AND CONCLUSION,                 295

        INDEX,                                                      326

_EIGHT DIAGRAMS IN THE TEXT AND TWO STAR CHARTS AT END._



    'Who is man, and what his place?
      Anxious asks the heart, perplext
    In this recklessness of space,
      Worlds with worlds thus intermixt:
    What has he, this atom creature,
      In the infinitude of Nature?'

    F.T. PALGRAVE.



MAN'S PLACE IN THE UNIVERSE



CHAPTER I

EARLY IDEAS AS TO THE UNIVERSE AND ITS RELATION TO MAN


When men attained to sufficient intelligence for speculations as to their
own nature and that of the earth on which they lived, they must have been
profoundly impressed by the nightly pageant of the starry heavens. The
intense sparkling brilliancy of Sirius and Vega, the more massive and
steady luminosity of Jupiter and Venus, the strange grouping of the
brighter stars into constellations to which fantastic names indicating
their resemblance to various animals or terrestrial objects seemed
appropriate and were soon generally adopted, together with the apparently
innumerable stars of less and less brilliancy scattered broadcast over the
sky, many only being visible on the clearest nights and to the acutest
vision, constituted altogether a scene of marvellous and impressive
splendour of which it must have seemed almost impossible to attain any real
knowledge, but which afforded an endless field for the imagination of the
observer.

The relation of the stars to the sun and moon in their respective motions
was one of the earliest problems for the astronomer, and it was only
solved by careful and continuous observation, which showed that the
invisibility of the former during the day was wholly due to the blaze of
light, and this is said to have been proved at an early period by the
observed fact that from the bottom of very deep wells stars can be seen
while the sun is shining. During total eclipses of the sun also the
brighter stars become visible, and, taken in connection with the fixity of
position of the pole-star, and the course of those circumpolar stars which
never set in the latitudes of Greece, Egypt, and Chaldea, it soon became
possible to frame a simple hypothesis which supposed the earth to be
suspended in space, while at an unknown distance from it a crystal sphere
revolved upon an axis indicated by the pole-star, and carried with it the
whole host of heavenly bodies. This was the theory of Anaximander (540
B.C.), and it served as the starting-point for the more complex theory
which continued to be held in various forms and with endless modifications
down to the end of the sixteenth century.

It is believed that the early Greeks obtained some knowledge of astronomy
from the Chaldeans, who appear to have been the first systematic observers
of the heavenly bodies by means of instruments, and who are said to have
discovered the cycle of eighteen years and ten days after which the sun and
moon return to the same relative positions as seen from the earth. The
Egyptians perhaps derived their knowledge from the same source, but there
is no proof that they were great observers, and the accurate orientation,
proportions, and angles of the Great Pyramid and its inner passages may
perhaps indicate a Chaldean architect.

The very obvious dependence of the whole life of the earth upon the sun, as
a giver of heat and light, sufficiently explains the origin of the belief
that the latter was a mere appanage of the former; and as the moon also
illuminates the night, while the stars as a whole also give a very
perceptible amount of light, especially in the dry climate and clear
atmosphere of the East, and when compared with the pitchy darkness of
cloudy nights when the moon is below the horizon, it seemed clear that the
whole of these grand luminaries--sun, moon, stars, and planets--were but
parts of the terrestrial system, and existed solely for the benefit of its
inhabitants.

Empedocles (444 B.C.) is said to have been the first who separated the
planets from the fixed stars, by observing their very peculiar motions,
while Pythagoras and his followers determined correctly the order of their
succession from Mercury to Saturn. No attempt was made to explain these
motions till a century later, when Eudoxus of Cnidos, a contemporary of
Plato and of Aristotle, resided for some time in Egypt, where he became a
skilful astronomer. He was the first who systematically worked out and
explained the various motions of the heavenly bodies on the theory of
circular and uniform motion round the earth as a centre, by means of a
series of concentric spheres, each revolving at a different rate and on a
different axis, but so united that all shared in the motion round the polar
axis. The moon, for example, was supposed to be carried by three spheres,
the first revolved parallel to the equator and accounted for the diurnal
motion--the rising and setting--of the moon; another moved parallel to the
ecliptic and explained the monthly changes of the moon; while the third
revolved at the same rate but more obliquely, and explained the inclination
of the moon's orbit to that of the earth. In the same way each of the five
planets had four spheres, two moving like the first two of the moon,
another one also moving in the ecliptic was required to explain the
retrograde motion of the planets, while a fourth oblique to the ecliptic
was needed to explain the diverging motions due to the different obliquity
of the orbit of each planet to that of the earth. This was the celebrated
Ptolemaic system in the simplest form needed to account for the more
obvious motions of the heavenly bodies. But in the course of ages the Greek
and Arabian astronomical observers discovered small divergences due to the
various degrees of excentricity of the orbits of the moon and planets and
their consequent varying rates of motion; and to explain these other
spheres were added, together with smaller circles sometimes revolving
excentrically, so that at length about sixty of these spheres, epicycles
and excentrics were required to account for the various motions observed
with the rude instruments, and the rates of motion determined by the very
imperfect time-measurers of those early ages. And although a few great
philosophers had at different times rejected this cumbrous system and had
endeavoured to promulgate more correct ideas, their views had no influence
on public opinion even among astronomers and mathematicians, and the
Ptolemaic system held full sway down to the time of Copernicus, and was
not finally given up till Kepler's _Laws_ and Galileo's _Dialogues_
compelled the adoption of simpler and more intelligible theories.

We are now so accustomed to look upon the main facts of astronomy as mere
elementary knowledge that it is difficult for us to picture to ourselves
the state of almost complete ignorance which prevailed even among the most
civilised nations throughout antiquity and the Middle Ages. The rotundity
of the earth was held by a few at a very early period, and was fairly well
established in later classical times. The rough determination of the size
of our globe followed soon after; and when instrumental observations became
more perfect, the distance and size of the moon were measured with
sufficient accuracy to show that it was very much smaller than the earth.
But this was the farthest limit of the determination of astronomical sizes
and distances before the discovery of the telescope. Of the sun's real
distance and size nothing was known except that it was much farther from us
and much larger than the moon; but even in the century before the
commencement of the Christian era Posidonius determined the circumference
of the earth to be 240,000 stadia, equal to about 28,600 miles, a
wonderfully close approximation considering the very imperfect data at his
command. He is also said to have calculated the sun's distance, making it
only one-third less than the true amount, but this must have been a chance
coincidence, since he had no means of measuring angles more accurately than
to one degree, whereas in the determination of the sun's distance
instruments are required which measure to a second of arc.

Before the discovery of the telescope the sizes of the planets were quite
unknown, while the most that could be ascertained about the stars was, that
they were at a very great distance from us. This being the extent of the
knowledge of the ancients as to the actual dimensions and constitution of
the visible universe, of which, be it remembered, the earth was held to be
the centre, we cannot be surprised at the almost universal belief that this
universe existed solely for the earth and its inhabitants. In classical
times it was held to be at once the dwelling-place of the gods and their
gift to man, while in Christian ages this belief was but slightly, if at
all, changed; and in both it would have been considered impious to maintain
that the planets and stars did not exist for the service and delight of
mankind alone but in all probability had their own inhabitants, who might
in some cases be even superior in intellect to man himself. But apparently,
during the whole period of which we are now treating, no one was so daring
as even to suggest that there were other worlds with other inhabitants, and
it was no doubt because of the idea that we occupied _the_ world, the very
centre of the whole surrounding universe which existed solely for us, that
the discoveries of Copernicus, Tycho Brahé, Kepler, and Galileo excited so
much antagonism and were held to be impious and altogether incredible. They
seemed to upset the whole accepted order of nature, and to degrade man by
removing his dwelling-place, the earth, from the commanding central
position it had always before occupied.



CHAPTER II

MODERN IDEAS AS TO MAN'S RELATION TO THE UNIVERSE


The beliefs as to the subordinate position held by sun, moon, and stars in
relation to the earth, which were almost universal down to the time of
Copernicus, began to give way when the discoveries of Kepler and the
revelations of the telescope demonstrated that our earth was not specially
distinguished from the other planets by any superiority of size or
position. The idea at once arose that the other planets might be inhabited;
and when the rapidly increasing power of the telescope, and of astronomical
instruments generally, revealed the wonders of the solar system and the
ever-increasing numbers of the fixed stars, the belief in other inhabited
worlds became as general as the opposite belief had been in all preceding
ages, and it is still held in modified forms to the present day.

But it may be truly said that the later like the earlier belief is founded
more upon religious ideas than upon a scientific and careful examination of
the whole of the facts both astronomical, physical, and biological, and we
must agree with the late Dr. Whewell, that the belief that other planets
are inhabited has been generally entertained, not in consequence of
physical reasons but in spite of them. And he adds:--'It was held that
Venus, or that Saturn was inhabited, not because anyone could devise, with
any degree of probability, any organised structure which would be suitable
to animal existence on the surfaces of those planets; but because it was
conceived that the greatness or goodness of the Creator, or His wisdom, or
some other of His attributes, would be manifestly imperfect, if these
planets were not tenanted by living creatures.' Those persons who have only
heard that many eminent astronomers down to our own day have upheld the
belief in a 'Plurality of Worlds' will naturally suppose that there must be
some very cogent arguments in its favour, and that it must be supported by
a considerable body of more or less conclusive facts. They will therefore
probably be surprised to hear that any direct evidence which may be held to
support the view is almost wholly wanting, and that the greater part of the
arguments are weak and flimsy in the extreme.

Of late years, it is true, some few writers have ventured to point out how
many difficulties there are in the way of accepting the belief, but even
these have never examined the question from the various points of view
which are essential to a proper consideration of it; while, so far as it is
still upheld, it is thought sufficient to show, that in the case of some of
the planets, there seem to be such conditions as to render life possible.
In the millions of planetary systems supposed to exist it is held to be
incredible that there are not great numbers as well fitted to be inhabited
by animals of all grades, including some as high as man or even higher,
and that we must, therefore, believe that they are so inhabited. As in the
present work I propose to show, that the probabilities and the weight of
direct evidence tend to an exactly opposite conclusion, it will be well to
pass briefly in review the various writers on the subject, and to give some
indication of the arguments they have used and the facts they have set
forth. For the earlier upholders of the theory I am indebted to Dr.
Whewell, who, in his _Dialogue on the Plurality of Worlds_--a Supplement to
his well-known volume on the subject--refers to all writers of importance
known to him.

The earliest are the great astronomers Kepler and Huygens, and the learned
Bishop Wilkins, who all believed that the moon was or might probably be
inhabited; and of these Whewell considers Wilkins to have been by far the
most thoughtful and earnest in supporting his views. Then we have Sir Isaac
Newton himself who, at considerable length, argued that the sun was
probably inhabited. But the first regular work devoted to the subject
appears to have been written by M. Fontenelle, Secretary to the Academy of
Sciences in Paris, who in 1686 published his _Conversations on the
Plurality of Worlds_. The book consisted of five chapters, the first
explaining the Copernican Theory; the second maintaining that the moon is a
habitable world; the third gives particulars as to the moon, and argues
that the other planets are also inhabited; the fourth gives details as to
the worlds of the five planets; while the fifth declares that the fixed
stars are suns, and that each illuminates a world. This work was so well
written, and the subject proved so attractive, that it was translated into
all the chief European languages, while the astronomer Lalande edited one
of the French editions. Three English translations were published, and one
of these went through six editions down to the year 1737. The influence of
this work was very great and no doubt led to that general acceptance of the
theory by such men as Sir William Herschel, Sir John Herschel, Dr.
Chalmers, Dr. Dick, Dr. Isaac Taylor, and M. Arago, although it was wholly
founded on pure speculation, and there was nothing that could be called
evidence on one side or the other.

This was the state of public opinion when an anonymous work appeared (in
1853) under the somewhat misleading title of _The Plurality of Worlds: An
Essay_. This was written, as already stated, by Dr. Whewell, who, for the
first time, ventured to doubt the generally accepted theory, and showed
that all the evidence at our command led to the conclusion that some of the
planets were _certainly_ not habitable, that others were _probably_ not so,
while in none was there that close correspondence with terrestrial
conditions which seemed essential for their habitability by the higher
animals or by man. The book was ably written and showed considerable
knowledge of the science of the time, but it was very diffuse, and the
larger part of it was devoted to showing that his views were not in any way
opposed to religion. One of his best arguments was founded on the
proposition that '_the Earth's Orbit is the Temperate Zone of the Solar
System_,' that there only is it possible to have those moderate variations
of heat and cold, dryness and moisture, which are suitable for animal
life. He suggested that the outer planets of the system consisted mainly of
water, gases, and vapour, as indicated by their low specific gravity, and
were therefore quite unsuitable for terrestrial life; while those near the
sun were equally unsuited, because, owing to the great amount of solar
heat, water could not exist on their surfaces. He devotes a great deal of
space to the evidence that there is no animal life on the moon, and taking
this as proved, he uses it as a counter argument against the other side.
They always urge that, the earth being inhabited, we must suppose the other
planets to be so too; to which he replies:--We know that the moon is not
inhabited though it has all the advantage of proximity to the sun that the
earth has; why then should not other planets be equally uninhabited?

He then comes to Mars and admits that this planet is very like the earth so
far as we can judge, and that it may therefore be inhabited, or as the
author expresses it, 'may have been judged worthy of inhabitants by its
Maker.' But he urges the small size of Mars, its coldness owing to distance
from the sun, and that the annual melting of its polar ice-caps will keep
it cold all through the summer. If there are animals they are probably of a
low type like the saurians and iguanodons of our seas during the Wealden
epoch; but, he argues, as even on our earth the long process of preparation
for man was carried on for countless millions of years, we need not discuss
whether there are intelligent beings on Mars till we have some better
evidence that there are any living creatures at all.

Several of the early chapters are devoted to an attempt to minimise the
difficulties of those religious persons who feel oppressed by the immensity
and complexity of the material universe as revealed by modern astronomy;
and by the almost infinite insignificance of man and his dwelling-place,
the earth, in comparison with it, an insignificance vastly increased if not
only the planets of the solar system, but also those which circle around
the myriads of suns, are also theatres of life. And these persons are
further disquieted because the very same facts are used by sceptics of
various kinds in their attacks upon Christianity. Such writers point out
the irrationality and absurdity of supposing that the Creator of all this
unimaginable vastness of suns and systems, filling for all we know endless
space, should take any _special_ interest in so mean and pitiful a creature
as man, the imperfectly developed inhabitant of one of the smaller worlds
attached to a second or third-rate sun, a being whose whole history is one
of war and bloodshed, of tyranny, torture, and death; whose awful record is
pictured by himself in such books as Josephus' _History of the Jews_, the
_Decline and Fall of the Roman Empire_, and even more forcibly summarised
in that terrible picture of human fiendishness and misery, _The Martyrdom
of Man_; while their character is indicated by one of the kindest and
simplest of their poets in the restrained but expressive lines:--

    'Man's inhumanity to man
    Makes countless thousands mourn.'

It is for such a being as this, they say, that God should have specially
revealed His will some thousands of years ago, and finding that His
commands were not obeyed, His will not fulfilled, yet ordained for their
benefit the necessarily unique sacrifice of His Son, in order to save a
small portion of these 'miserable sinners' from the natural and
well-deserved consequence of their stupendous follies, their unimaginable
crimes? Such a belief they maintain is too absurd, too incredible, to be
held by any rational being, and it becomes even less credible and less
rational if we maintain that there are countless other inhabited worlds.

It is very difficult for the religious man to make any adequate reply to
such an attack as this, and as a result many have felt their position to be
untenable and have accordingly lost all faith in the special dogmas of
orthodox Christianity. They feel themselves really to be between the horns
of a dilemma. If there are myriads of other worlds, it seems incredible
that they should each be the object of a special revelation and a special
sacrifice. If, on the other hand, we are the only intelligent beings that
exist in the material universe, and are really the highest creative product
of a Being of infinite wisdom and power, they cannot but wonder at the vast
apparent disproportion between the Creator and the created, and are
sometimes driven to Atheism from the hopelessness of comprehending so mean
and petty a result as the sole outcome of infinite power.

Whewell tells us that the great preacher, Dr. Chalmers, in his Astronomical
Discourses, attempted a reply to these difficulties, but, in his opinion,
not a very successful one; and a large part of his own work is devoted to
the same purpose. His main point seems to be that we know too little of the
universe to arrive at any definite conclusions on the question at issue,
and that any ideas that we may have as to the purposes of the Creator in
forming the vast system we see around us are almost sure to be erroneous.
We must therefore be content to remain ignorant, and must rest satisfied in
the belief that the Creator had a purpose although we are not yet permitted
to know what it was. And to those who urge that in other worlds there may
be other laws of nature which may render them quite as habitable by
intelligent beings as our world is for us, he replies, that if we are to
suppose new laws of nature in order to render each planet habitable, there
is an end of all rational inquiry on the subject, and we may maintain and
believe that animals may live on the moon without air or water, and on the
sun exposed to heat which vaporises earths and metals.

His concluding argument, and perhaps one of his strongest, is that founded
upon the dignity of man, as conferring a pre-eminence upon the planet which
has produced him. 'If,' he says, 'man be not merely capable of Virtue and
Duty, of universal Love and Self-Devotion, but be also immortal; if his
being be of infinite duration, his soul created never to die; then, indeed,
we may well say that one soul outweighs the whole unintelligent creation.'
And then, addressing the religious world, he urges that, if, as they
believe, God _has_ redeemed man by the sacrifice of His Son, and _has_
given to him a revelation of His will, then indeed no other conception is
possible than that he is the sole and highest product of the universe.
'The elevation of millions of intellectual, moral, religious, spiritual
creatures, to a destiny so prepared, consummated, and developed, is no
unworthy occupation of all the capacities of space, time, and matter.' Then
with a chapter on 'The Unity of the World,' and one on 'The Future,'
neither of which contains anything which adds to the force of his argument,
the book ends.

The publication of this able if rather vague and diffuse work, contesting
popular opinions, was followed by a burst of indignant criticism on the
part of a man of considerable eminence in some branches of physics--Sir
David Brewster, but who was very inferior, both in general knowledge of
science and in literary skill, to the writer whose views he opposed. The
purport of the book in which he set forth his objections is indicated by
its title--_More Worlds than One, the Creed of the Philosopher and the Hope
of the Christian_. Though written with much force and conviction it appeals
mainly to religious prejudices, and assumes throughout that every planet
and star is a special creation, and that the peculiarities of each were
designed for some special purpose. 'If,' he says, 'the moon had been
destined to be merely a lamp to our earth, there was no occasion to
variegate its surface with lofty mountains and extinct volcanoes, and cover
it with large patches of matter that reflect different quantities of light
and give its surface the appearance of continents and seas. It would have
been a better lamp had it been a smooth piece of lime or of chalk.' It is,
therefore, he thinks, prepared for inhabitants; and then he argues that all
the other satellites are also inhabited. Again he says that 'when it was
found that Venus was about the same size as the Earth, with mountains and
valleys, days and nights, and years analogous to our own, the _absurdity_
of believing that she had no inhabitants, when no other rational purpose
could be assigned for her creation, became an argument of a certain amount
that she was, like the Earth, the seat of animal and vegetable life.' Then,
when it was found that Jupiter was so gigantic 'as to require four moons to
give him light, the argument from analogy that _he_ was inhabited became
stronger also, because it extended to _two_ planets.' And thus each
successive planet having certain points of analogy with the others becomes
an additional argument; so that when we take account of all the planets,
with atmosphere, and clouds, and arctic snows, and trade-winds, the
argument from analogy becomes, he urges, very powerful;--'and the absurdity
of the opposite opinion, that planets should have moons and no inhabitants,
atmospheres with no creatures to breathe in them, and currents of air
without life to be fanned, became a formidable argument which few minds, if
any, could resist.'

The work is full of such weak and fallacious rhetoric and even, if
possible, still weaker. Thus after describing double stars, he adds--'But
no person can believe that two suns could be placed in the heavens for no
other purpose than to revolve round their common centre of gravity'; and he
concludes his chapter on the stars thus:--'Wherever there is matter there
must be Life; Life Physical to enjoy its beauties--Life Moral to worship
its Maker, and Life Intellectual to proclaim His wisdom and His power.' And
again--'A house without tenants, a city without citizens, presents to our
minds the same idea as a planet without life, and a universe without
inhabitants. Why the house was built, why the city was founded, why the
planet was made, and why the universe was created, it would be difficult
even to conjecture.' Arguments of this kind, which in almost every case beg
the question at issue, are repeated _ad nauseam_. But he also appeals to
the Old Testament to support his views, by quoting the fine passage in the
Psalms--'When I consider Thy heavens the work of Thy fingers, the moon and
the stars which Thou hast ordained; what is man that Thou art mindful of
him?' on which he remarks--'We cannot doubt that inspiration revealed to
him [David] the magnitude, the distances, and the final cause, of the
glorious spheres which fixed his admiration.' And after quoting various
other passages from the prophets, all as he thinks supporting the same
view, he sets forth the extraordinary idea as a confirmatory argument, that
the planets or some of them are to be the future abode of man. For, he
says--'Man in his future state of existence is to consist, as at present,
of a spiritual nature residing in a corporeal frame. He must live,
therefore, upon a material planet, subject to all the laws of matter.' And
he concludes thus:--'If there is not room, then, on our globe for the
millions of millions of beings who have lived and died on its surface, we
can scarcely doubt that their future abode must be on some of the primary
or secondary planets of the solar system, whose inhabitants have ceased to
exist, or upon planets which have long been in a state of preparation, as
our earth was, for the advent of intellectual life.'

It is pleasant to turn from such weak and trivial arguments to the only
other modern works which deal at some length with this subject, the late
Richard A. Proctor's _Other Worlds than Ours_, and a volume published five
years later under the title--_Our Place Among Infinities_. Written as these
were by one of the most accomplished astronomers of his day, remarkable
alike for the acuteness of his reasoning and the clearness of his style, we
are always interested and instructed even when we cannot agree with his
conclusions. In the first work mentioned above, he assumes, like Sir David
Brewster, the antecedent probability that the planets are inhabited and on
much the same theological grounds. So strongly does he feel this that he
continually speaks as if the planets _must_ be inhabited unless we can show
very good reason that they _cannot_ be so, thus throwing the burden of
proving a negative on his opponents, while he does not attempt to prove his
positive contention that they are inhabited, except by purely hypothetical
considerations as to the Creator's purpose in bringing them into existence.

But starting from this point he endeavours to show how Whewell's various
difficulties may be overcome, and here he always appeals to astronomical or
physical facts, and reasons well upon them. But he is quite honest; and,
coming to the conclusion that Jupiter and Saturn, Uranus and Neptune,
cannot be habitable, he adduces the evidence and plainly states the result.
But then he thinks that the satellites of Jupiter and Saturn _may_ be
habitable, and if they may be, then he concludes that they _must_. One
great oversight in his whole argument is, that he is satisfied with showing
the possibility that life may exist now, but never deals with the question
of whether life could have been developed from its earliest rudiments up to
the production of the higher vertebrates and man; and this, as I shall show
later, is the _crux_ of the whole problem.

With regard to the other planets, after a careful examination of all that
is known about them, he arrives at the conclusion that if Mercury is
protected by a cloud-laden atmosphere of a peculiar kind it may possibly,
but not probably, support high forms of animal life. But in the case of
Venus and Mars he finds so much resemblance to and so many analogies with
our earth, that he concludes that they almost certainly are so.

In the case of the fixed stars, now that we know by spectroscopic
observations that they are true suns, many of which closely resemble our
sun and give out light and heat as he does, Mr. Proctor argues, that 'The
vast supplies of heat thus emitted by the stars not only suggest the
conclusion that there must be worlds around these orbs for which these
heat-supplies are intended, but point to the existence of the various forms
of force into which heat may be transmuted. We know that the sun's heat
poured upon our earth is stored up in vegetable and animal forms of life;
is present in all the phenomena of nature--in winds and clouds and rain, in
thunder and lightning, storm and hail; and that even the works of man are
performed by virtue of the solar heat-supplies. Thus the fact that the
stars send forth heat to the worlds which circle around them suggests at
once the thought that on those worlds there must exist animal and vegetable
forms of life.' We may note that in the first part of this passage the
presence of worlds or planets is 'suggested,' while later on 'the worlds
which circle round them' is spoken of as if it were a proved fact from
which the presence of vegetable and animal life may be inferred. A
suggestion depending on a preceding suggestion is not a very firm basis for
so vast and wide-reaching a conclusion.

In the second work referred to above there is one chapter entitled, 'A New
Theory of Life in other Worlds,' where the author gives his more matured
views of the question, which are briefly stated in the preface as being
'that the weight of evidence favours my theory of the (relative) paucity of
worlds.' His views are largely founded on the theory of probabilities, of
which subject he had made a special study. Taking first our earth, he shows
that the period during which life has existed upon it is very small in
comparison with that during which it must have been slowly forming and
cooling, and its atmosphere condensing so as to form land and water on its
surface. And if we consider the time the earth has been occupied by man,
that is a very minute part, perhaps not the thousandth part, of the period
during which it has existed as a planet. It follows that even if we
consider only those planets whose physical condition seems to us to be such
as to be able to sustain life, the chances are perhaps hundreds to one
against their being at that particular stage when life has begun to be
developed, or if it has begun has reached as high a development as on our
earth.

With regard to the stars, the argument is still stronger, because the
epochs required for their formation are altogether unknown, while as to the
conditions required for the formation of planetary systems around them we
are totally ignorant. To this I would add that we are equally ignorant as
to the probability or even possibility of many of these suns producing
planets which, by their position, size, atmosphere, or other physical
conditions can possibly become life-producing worlds. And, as we shall see
later, this point has been overlooked by all writers, including Mr. Proctor
himself. His conclusion is, then, that although the worlds which possess
life at all approaching that of our earth may be relatively few in number,
yet considering the universe as practically infinite in extent, they may be
really very numerous.

It has been necessary to give this sketch of the views of those who have
written specially on the question of the Plurality of Worlds, because the
works referred to have been very widely read and have influenced educated
opinion throughout the world. Moreover, Mr. Proctor, in his last work on
the subject, speaks of the theory as being 'identified with modern
astronomy'; and in fact popular works still discuss it. But all these
follow the same general line of argument as those already referred to, and
the curious thing is that while overlooking many of the most essential
conditions they often introduce others which are by no means essential--as,
for instance, that the atmosphere must have the same proportion of oxygen
as our own. They seem to think that if any of our quadrupeds or birds taken
to another planet could not live there, no animals of equally high
organisation could inhabit it; entirely overlooking the very obvious fact
that, supposing, as is almost certain, that oxygen is necessary for life,
then, whatever proportion of oxygen within certain limits was present, the
forms of life that arose would necessarily be organised in adaptation to
that proportion, which might be considerably less or greater than on the
earth.

The present volume will show how extremely inadequate has been the
treatment of this question, which involves a variety of important
considerations hitherto altogether overlooked. These are extremely numerous
and very varied in their character, and the fact that they all point to one
conclusion--a conclusion which so far as I am aware no previous writer has
reached--renders it at least worthy of the careful consideration of all
unbiassed thinkers. The whole subject is one as to which no direct evidence
is obtainable, but I venture to think that the convergence of so many
probabilities and indications towards a single definite theory, intimately
connected with the nature and destiny of man himself, raises this theory to
a very much higher level of probability than the vague possibilities and
theological suggestions which are the utmost that have been adduced by
previous writers.

In order to make every step of my argument clearly intelligible to all
educated readers, it will be necessary to refer continually to the
marvellous extension of our knowledge of the universe obtained during the
last half-century, and constituting what is termed the New Astronomy. The
next chapter will therefore be devoted to a popular exposition of the new
methods of research, so that the results reached, which will have to be
referred to in succeeding chapters, may be not only accepted, but clearly
understood.



CHAPTER III

THE NEW ASTRONOMY


During the latter half of the nineteenth century discoveries were made
which extended the powers of astronomical research into entirely new and
unexpected regions, comparable to those which were opened up by the
discovery of the telescope more than two centuries before. The older
astronomy for more than two thousand years was purely mechanical and
mathematical, being limited to observation and measurement of the apparent
motions of the heavenly bodies, and the attempts to deduce, from these
apparent motions, their real motions, and thus determine the actual
structure of the solar system. This was first done when Kepler established
his three celebrated laws: and later, when Newton showed that these laws
were necessary consequences of the one law of gravitation, and when
succeeding observers and mathematicians proved that each fresh irregularity
in the motions of the planets was explicable by a more thorough and minute
application of the same laws, this branch of astronomy reached its highest
point of efficiency and left very little more to be desired.

Then, as the telescope became successively improved, the centre of interest
was shifted to the surfaces of the planets and their satellites, which
were watched and scrutinised with the greatest assiduity in order if
possible to attain some amount of knowledge of their physical constitution
and past history. A similar minute scrutiny was given to the stars and
nebulæ, their distribution and grouping, and the whole heavens were mapped
out, and elaborate catalogues constructed by enthusiastic astronomers in
every part of the world. Others devoted themselves to the immensely
difficult problem of determining the distances of the stars, and by the
middle of the century a few such distances had been satisfactorily
measured.

Thus, up to the middle of the nineteenth century it appeared likely that
the future of astronomy would rest almost entirely on the improvement of
the telescope, and of the various instruments of measurement by means of
which more accurate determinations of distances might be obtained. Indeed,
the author of the Positive Philosophy, Auguste Comte, felt so sure of this
that he deprecated all further attention to the stars as pure waste of time
that could never lead to any useful or interesting result. In his
_Philosophical Treatise on Popular Astronomy_ published in 1844, he wrote
very strongly on this point. He there tells us that, as the stars are only
accessible to us by sight they must always remain very imperfectly known.
We can know little more than their mere existence. Even as regards so
simple a phenomenon as their temperature this must always be inappreciable
to a purely visual examination. Our knowledge of the stars is for the most
part purely negative, that is, we can determine only that they do _not_
belong to our system. Outside that system there exists, in astronomy, only
obscurity and confusion, for want of indispensable facts; and he concludes
thus:--'It is, then, in vain that for half a century it has been
endeavoured to distinguish two astronomies, the one solar the other
sidereal. In the eyes of those for whom science consists of real laws and
not of incoherent facts, the second exists only in name, and the first
alone constitutes a true astronomy; and I am not afraid to assert that it
will always be so.' And he adds that--'all efforts directed to this subject
for half a century have only produced an accumulation of incoherent
empirical facts which can only interest an irrational curiosity.'

Seldom has a confident assertion of finality in science received so
crushing a reply as was given to the above statements of Comte by the
discovery in 1860 (only three years after his death) of the method of
spectrum-analysis which, in its application to the stars, has
revolutionised astronomy, and has enabled us to obtain that very kind of
knowledge which he declared must be for ever beyond our reach. Through it
we have acquired accurate information as to the physics and chemistry of
the stars and nebulæ, so that we now know really more of the nature,
constitution, and temperature of the enormously distant suns which we
distinguish by the general term stars, than we do of most of the planets of
our own system. It has also enabled us to ascertain the existence of
numerous invisible stars, and to determine their orbits, their rate of
motion, and even, approximately, their mass. The despised stellar astronomy
of the early part of the century has now taken rank as the most profoundly
interesting department of that grand science, and the branch which offers
the greatest promise of future discoveries. As the results obtained by
means of this powerful instrument will often be referred to, a short
account of its nature and of the principles on which it depends must here
be given.

The solar spectrum is the band of coloured light seen in the rainbow and,
partially, in the dew-drop, but more completely when a ray of sunlight
passes through a prism--a piece of glass having a triangular section. The
result is, that instead of a spot of white light we have a narrow band of
brilliant colours which succeed each other in regular order, from violet at
one end through blue, green, and yellow to red at the other. We thus see
that light is not a simple and uniform radiation from the sun, but is made
up of a large number of separate rays, each of which produces in our eyes
the sensation of a distinct colour. Light is now explained as being due to
vibrations of ether, that mysterious substance which not only permeates all
matter, but which fills space at least as far as the remotest of the
visible stars and nebulæ. The exceedingly minute waves or vibrations of the
ether produce all the phenomena of heat, light, and colour, as well as
those chemical actions to which photography owes its wonderful powers. By
ingenious experiments the size and rate of vibration of these waves have
been measured, and it is found that they vary considerably, those forming
the red light, which is least refracted, having a wave-length of about
1/326000 of an inch, while the violet rays at the other end of the spectrum
are only about half that length or 1/630000 of an inch. The rate at which
the vibrations succeed each other is from 302 millions of millions per
second for the extreme red rays, to 737 millions of millions for those at
the violet end of the spectrum. These figures are given to show the
wonderful minuteness and rapidity of these heat and light waves on which
the whole life of the world, and all our knowledge of other worlds and
other suns, directly depends.

But the mere colours of the spectrum are not the most important part of it.
Very early in the nineteenth century a close examination showed that it was
everywhere crossed by black lines of various thicknesses, sometimes single,
sometimes grouped together. Many observers studied them and made accurate
drawings or maps showing their positions and thicknesses, and by combining
several prisms, so that the beam of sunlight had to pass through them
successively, a spectrum could be produced several feet long, and more than
3000 of these dark lines were counted in it. But what they were and how
they were caused remained a mystery, till, in the year 1860, the German
physicist Kirchhoff discovered the secret and gave to chemists and
astronomers a new and quite unexpected engine of research.

It had already been observed that the chemical elements and various
compounds, when heated to incandescence, produced spectra consisting of
coloured lines or bands which were constant for each element, so that the
elements could at once be recognised by their characteristic spectra; and
it had also been noticed that some of these bands, especially the yellow
band produced by sodium, corresponded in position with certain black lines
in the solar spectrum. Kirchhoff's discovery consisted in showing that,
when the light from an incandescent body passes through the same substance
in a state of vapour or gas, so much of the light is absorbed that the
coloured lines or bands become black. The mystery of more than half a
century was thus solved; and the thousands of black lines in the solar
spectrum were shown to be caused by the light from the incandescent matter
of the sun's surface passing through the heated gases or vapours
immediately above it, and thereby having the bright coloured lines of their
spectra changed, by absorption, to comparative blackness.

Chemists and physicists immediately set to work examining the spectra of
the elements, fixing the position of the several coloured lines or bands by
accurate measurement, and comparing them with the dark lines of the solar
spectrum. The results were in the highest degree satisfactory. In a large
proportion of the elements the coloured bands corresponded exactly with a
group of dark lines in the spectrum of the sun, in which, therefore, the
same terrestrial elements were proved to exist. Among the elements first
detected in this manner were hydrogen, sodium, iron, copper, magnesium,
zinc, calcium, and many others. Nearly forty of the elements have now been
found in the sun, and it seems highly probable that all our elements really
exist there, but as some are very rare and are present in very minute
quantities they cannot be detected. Some of the dark lines in the sun were
found not to correspond to any known element, and as this was thought to
indicate an element peculiar to the sun it was named Helium; but quite
recently it has been discovered in a rare mineral. Many of the elements
are represented by a great number of lines, others by very few. Thus iron
has more than 2000, while lead and potassium have only one each.

The value of the spectroscope both to the chemist in discovering new
elements and to the astronomer in determining the constitution of the
heavenly bodies, is so great, that it became of the highest importance to
have the position of all the dark lines in the solar spectrum, as well as
the bright lines of all the elements, determined with extreme accuracy, so
as to be able to make exact comparisons between different spectra. At first
this was done by means of very large-scale drawings showing the exact
position of every dark or bright line. But this was found to be both
inconvenient and not sufficiently exact; and it was therefore agreed to
adopt the natural scale of the wave-lengths of the different parts of the
spectrum, which by means of what are termed diffraction-gratings can now be
measured with great accuracy. Diffraction-gratings are formed of a polished
surface of hard metal ruled with excessively fine lines, sometimes as many
as 20,000 to an inch. When sunlight falls upon one of these gratings it is
reflected, and by interference of the rays from the spaces between the fine
grooves, it is spread out into a beautiful and well-defined spectrum,
which, when the lines are very close, is several yards in length. In these
diffraction spectra many dark lines are seen which can be shown in no other
way, and they also give a spectrum which is far more uniform than that
produced by glass prisms in which minute differences in the composition of
the glass cause some rays to be refracted more and others less than the
normal amount.

The spectra produced by diffraction-gratings are double; that is, they are
spread out on both sides of the central line of the ray which remains
white, and the several coloured or dark lines are so clearly defined that
they can be thrown on a screen at a considerable distance, giving a great
length to the spectrum. The data for obtaining the wave-lengths are the
distance apart of the lines, the distance of the screen, and the distance
apart of the first pair of dark lines on each side of the central bright
line. All these can be measured with extreme accuracy by means of
telescopes with micrometers and other contrivances, and the result is an
accuracy of determination of wave-lengths which can probably not be
equalled in any other kind of measurement.

As the wave-lengths are so excessively minute, it has been found convenient
to fix upon a still smaller unit of measurement, and as the millimetre is
the smallest unit of the metric system, the ten-millionth of a millimetre
(technically termed 'tenth meter') is the unit adopted for the measurement
of wave-lengths, which is equal to about the 250 millionth of an inch. Thus
the wave-lengths of the red and blue lines characteristic of hydrogen are
6563.07 and 4861.51 respectively. This excessively minute scale of
wave-lengths, once determined by the most refined measurement, is of very
great importance. Having the wave-lengths of any two lines of a spectrum so
determined, the space between them can be laid down on a diagram of any
length, and all the lines that occur in any other spectrum between these
two lines can be marked in their exact relative positions. Now, as the
visible spectrum consists of about 300,000 rays of light, each of different
wave-lengths and therefore of different refrangibilities, if it is laid
down on such a scale as to be of a length of 3000 inches (250 feet), each
wave-length will be 1/100 of an inch long, a space easily visible by the
naked eye.

The possession of an instrument of such wonderful delicacy, and with powers
which enable it to penetrate into the inner constitution of the remotest
orbs of space, rendered it possible, within the next quarter of a century,
to establish what is practically a new science--Astrophysics--often
popularly termed the New Astronomy. A brief outline of the main
achievements of this science must now be given.

The first great discovery made by Spectrum analysis, after the
interpretation of the sun's spectrum had been obtained, was, the real
nature of the fixed stars. It is true they had long been held by
astronomers to be suns, but this was only an opinion of the accuracy of
which it did not seem possible to obtain any proof. The opinion was founded
on two facts--their enormous distance from us, so great that the whole
diameter of the earth's orbit did not lead to any apparent change of their
relative positions, and their intense brilliancy which at such distances
could only be due to an actual size and splendour comparable with our sun.
The spectroscope at once proved the correctness of this opinion. As one
after another was examined, they were found to exhibit spectra of the same
general type as that of the sun--a band of colours crossed by dark lines.
The very first stars examined by Sir William Huggins showed the existence
of nine or ten of our elements. Very soon all the chief stars of the
heavens were spectroscopically examined, and it was found that they could
be classed in three or four groups. The first and largest group contains
more than half the visible stars, and a still larger proportion of the most
brilliant, such as Sirius, Vega, Regulus, and Alpha Crucis in the Southern
Hemisphere. They are characterised by a white or bluish light, rich in the
ultra-violet rays, and their spectra are distinguished by the breadth and
intensity of the four dark bands due to the absorption of hydrogen, while
the various black lines which indicate metallic vapours are comparatively
few, though hundreds of them can be discovered by careful examination.

The next group, to which Capella and Arcturus belong, is also very
numerous, and forms the solar type of stars. Their light is of a yellowish
colour, and their spectra are crossed throughout by innumerable fine dark
lines more or less closely corresponding with those in the solar spectrum.

The third group consists of red and variable stars, which are characterised
by fluted spectra. Such spectra show like a range of Doric columns seen in
perspective, the red side being that most illuminated.

The last group, consisting of few and comparatively small stars, has also
fluted spectra, but the light appears to come from the opposite direction.

These groups were established by Father Secchi, the Roman astronomer, in
1867, and have been adopted with some modifications by Vogel of the
Astrophysical Observatory at Potsdam. The exact interpretation of these
different spectra is somewhat uncertain, but there can be little doubt that
they coincide primarily with differences of temperature and with
corresponding differences in the composition and extent of the absorptive
atmospheres. Stars with fluted spectra indicate the presence of vapours of
the metalloids or of compound substances, while the reversed flutings
indicate the presence of carbon. These conclusions have been reached by
careful laboratory experiments which are now carried on at the same time as
the spectral examination of the stars and other heavenly bodies, so that
each peculiarity of their spectra, however puzzling and apparently
unmeaning, has been usually explained, by being shown to indicate certain
conditions of chemical constitution or of temperature.

But whatever difficulty there may be in explaining details, there remains
no doubt whatever of the fundamental fact that all the stars are true suns,
differing no doubt in size, and their stage of development as indicated by
the colour or intensity of their light or heat, but all alike possessing a
photosphere or light-emitting surface, and absorptive atmospheres of
various qualities and density.

Innumerable other details, such as the often contrasted colours of double
stars, the occasional variability of their spectra, their relations to the
nebulæ, the various stages of their development and other problems of equal
interest, have occupied the continued attention of astronomers,
spectroscopists, and chemists; but further reference to these difficult
questions would be out of place here. The present sketch of the nature of
spectrum-analysis applied to the stars is for the purpose of making its
principle and method of observation intelligible to every educated reader,
and to illustrate the marvellous precision and accuracy of the results
attained by it. So confident are astronomers of this accuracy that nothing
less than _perfect correspondence_ of the various bright lines in the
spectrum of an element in the laboratory with the dark lines in the
spectrum of the sun or of a star is required before the presence of that
element is accepted as proved. As Miss Clerke tersely puts
it--'Spectroscopic coincidences admit of no compromise. Either they are
absolute or they are worthless.'


MEASUREMENT OF MOTION IN THE LINE OF SIGHT

We must now describe another and quite distinct application of the
spectroscope, which is even more marvellous than that already described. It
is the method of measuring the rate of motion of any of the visible
heavenly bodies in a direction either directly towards us, or directly away
from us, technically described as 'radial motion,' or by the
expression--'in the line of sight.' And the extraordinary thing is that
this power of measurement is altogether independent of distance, so that
the rate of motion in miles per second of the remotest of the fixed stars,
if sufficiently bright to show a distinct spectrum, can be measured with as
much certainty and accuracy as in the case of a much nearer star or a
planet.

In order to understand how this is possible we have again to refer to the
wave-theory of light; and the analogy of other wave-motions will enable us
better to grasp the principle on which these calculations depend. If on a
nearly calm day we count the waves that pass each minute by an anchored
steamboat, and then travel in the direction the waves come from, we shall
find that a larger number pass us in the same time. Again, if we are
standing near a railway, and an engine comes towards us whistling, we shall
notice that it changes its tone as it passes us; and as it recedes the
sound will be in a lower key, although the engine may be at exactly the
same distance from us as when it was approaching. Yet the sound does not
change to the ear of the engine driver, the cause of the change being that
the sound-waves reach us in quicker succession as the source of the waves
is approaching us than when it is retreating from us. Now, just as the
pitch of a note depends upon the rapidity with which the successive
air-vibrations reach our ear, so does the colour of a particular part of
the spectrum depend upon the rapidity with which the ethereal waves which
produce colour reach our eyes; and as this rapidity is greater when the
source of the light is approaching than when it is receding from us, a
slight shifting of the position of the coloured bands, and therefore of the
dark lines, will occur, as compared with their position in the spectrum of
the sun or of any stationary source of light, if there is any motion
sufficient in amount to produce a perceptible shift.

That such a change of colour would occur was pointed out by Professor
Doppler of Prague in 1842, and it is hence usually spoken of as the
'Doppler principle'; but as the changes of colour were so minute as to be
impossible of measurement it was not at that time of any practical
importance in astronomy. But when the dark lines in the spectrum were
carefully mapped, and their positions determined with minute accuracy, it
was seen that a means of measuring the changes produced by motion in the
line of sight existed, since the position of any of the dark or coloured
lines in the spectra of the heavenly bodies could be compared with those of
the corresponding lines produced artificially in the laboratory. This was
first done in 1868 by Sir William Huggins, who, by the use of a very
powerful spectroscope constructed for the purpose, found that such a change
did occur in the case of many stars, and that their rate of motion towards
us or away from us--the radial motion--could be calculated. As the actual
distance of some of these stars had been measured, and their change of
position annually (their proper motion) determined, the additional factor
of the amount of motion in the direction of our line of sight completed the
data required to fix their true line of motion among the other stars. The
accuracy of this method under favourable conditions and with the best
instruments is very great, as has been proved by those cases in which we
have independent means of calculating the real motion. The motion of Venus
towards or away from us can be calculated with great accuracy for any
period, being a resultant of the combined motions of the planet and of our
earth in their respective orbits. The radial motions of Venus were
determined at the Lick Observatory in August and September 1890, by
spectroscopic observations, and also by calculation, to be as follows:--

             By Observation.                   By Calculation.

    Aug. 16th. 7.3 miles per second.       8.1 miles per second.
     "   22nd. 8.9   "   "    "            8.2    "   "     "
     "   30th. 7.3   "   "    "            8.3    "   "     "
    Sep.  3rd. 8.3   "   "    "            8.3    "   "     "
     "    4th. 8.2   "   "    "            8.3    "   "     "

showing that the maximum error was only one mile per second, while the mean
error was about a quarter of a mile. In the case of the stars the accuracy
of the method has been tested by observations of the same star at times
when the earth's motion in its orbit is towards or away from the star,
whose apparent radial velocity is, therefore, increased or diminished by a
known amount. Observations of this kind were made by Dr. Vogel, Director of
the Astrophysical Observatory at Potsdam, showing, in the case of three
stars, of which ten observations were taken, a mean error of about two
miles per second; but as the stellar motions are more rapid than those of
the planets, the proportionate error is no greater than in the example
given above.

The great importance of this mode of determining the real motion of the
stars is, that it gives us a knowledge of the scale on which such motions
are progressing; and when in the course of time we discover whether any of
their paths are rectilinear or curved, we shall be in a position to learn
something of the nature of the changes that are going on and of the laws on
which they depend.


INVISIBLE STARS AND IMPERCEPTIBLE MOTIONS

But there is another result of this power of determining radial motion
which is even more unexpected and marvellous, and which has extended our
knowledge of the stars in quite a new direction. By its means it is
possible to determine the existence of invisible stars and to measure the
rate of otherwise imperceptible motions; that is of stars which are
invisible in the most powerful modern telescopes, and whose motions have
such a limited range that no telescope can detect them.

Double or binary stars forming systems which revolve around their common
centre of gravity were discovered by Sir William Herschel, and very great
numbers are known; but in most cases their periods of revolution are long,
the shortest being about twelve years, while many extend to several hundred
years. These are, of course, all visible binaries, but many are now known
of which one star only is visible while the other is either non-luminous or
is so close to its companion that they appear as a single star in the most
powerful telescopes. Many of the variable stars belong to the former class,
a good example of which is Algol in the constellation Perseus, which
changes from the second to the fourth magnitude in about four and a half
hours, and in about four and a half hours more regains its brilliancy till
its next period of obscuration which occurs regularly every two days and
twenty-one hours. The name Algol is from the Arabic _Al Ghoul_, the
familiar 'ghoul' of the Arabian Nights, so named--'The Demon'--from its
strange and weird behaviour.

It had long been conjectured that this obscuration was due to a dark
companion which partially eclipsed the bright star at every revolution,
showing that the plane of the orbit of the pair was almost exactly directed
towards us. The application of the spectroscope made this conjecture a
certainty. At an equal time before and after the obscuration, motion in the
line of sight was shown, towards and away from us, at a rate of twenty-six
miles per second. From these scanty data and the laws of gravitation which
fix the period of revolution of planets at various distances from their
centres of revolution, Professor Pickering of the Harvard Observatory was
able to arrive at the following figures as highly probable, and they may be
considered to be certainly not far from the truth.

    Diameter of Algol,              1,061,000 miles.
    Diameter of dark companion,       830,000   "
    Distance between their centres, 3,230,000   "
    Orbital speed of Algol,              26.3 miles per sec.
    Orbital speed of companion,          55.4   "    "   "
    Mass of Algol,                        4/9 mass of our Sun.
    Mass of companion,                    2/9  "     "     "

When it is considered that these figures relate to a pair of stars only one
of which has ever been seen, that the orbital motion even of the visible
star cannot be detected in the most powerful telescopes, when, further, we
take into account the enormous distance of these objects from us, the great
results of spectroscopic observation will be better appreciated.

But besides the marvel of such a discovery by such simple means, the facts
discovered are themselves in the highest degree marvellous. All that we had
known of the stars through telescopic observation indicated that they were
at very great distances from each other however thickly they may appear
scattered over the sky. This is the case even with close telescopic double
stars, owing to their enormous remoteness from us. It is now estimated that
even stars of the first magnitude are, on a general average, about eighty
millions of millions of miles distant; while the closest double stars that
can be distinctly separated by large telescopes are about half a second
apart. These, if at the above distance, will be about 1500 millions of
miles from each other. But in the case of Algol and its companion, we have
two bodies both larger than our sun, yet with a distance of only 2-1/4
millions of miles between their surfaces, a distance not much exceeding
their combined diameters. We should not have anticipated that such huge
bodies could revolve so closely to each other, and as we now know that the
neighbourhood of our sun--and probably of all suns--is full of meteoric and
cometic matter, it would seem probable that in the case of two suns so near
together the quantity of such matter would be very great, and would lead
probably by continued collisions to increase of their bulk, and perhaps to
their final coalescence into a single giant orb. It is said that a Persian
astronomer in the tenth century calls Algol a red star, while it is now
white or somewhat yellowish. This would imply an increase of temperature
caused by collisions or friction, and increasing proximity of the pair of
stars.

A considerable number of double stars with dark companions have been
discovered by means of the spectroscope, although their motion is not
directly in the line of sight, and therefore there is no obscuration. In
order to discover such pairs the spectra of large numbers of stars are
taken on photographic plates every night and for considerable periods--for
a year or for several years. These plates are then carefully examined with
a high magnifying power to discover any periodical displacement of the
lines, and it is astonishing in how large a number of cases this has been
found to exist and the period of revolution of the pair determined.

But besides discovering double stars of which one is dark and one bright,
many pairs of bright stars have been discovered by the same means. The
method in this case is rather different. Each component star, being
luminous, will give a separate spectrum, and the best spectroscopes are so
powerful that they will separate these spectra when the stars are at their
maximum distance although no telescope in existence, or ever likely to be
made, can separate the component stars. The separation of the spectra is
usually shown by the most prominent lines becoming double and then after a
time single, indicating that the plane of revolution is more or less
obliquely towards us, so that the two stars if visible would appear to open
out and then get nearer together every revolution. Then, as each star
alternately approaches and recedes from us the radial velocity of each can
be determined, and this gives the relative mass. In this way not only
doubles, but triple and multiple systems, have been discovered. The stars
proved to be double by these two methods are so numerous that it has been
estimated by one of the best observers that about one star in every
thirteen shows inequality in its radial motion and is therefore really a
double star.


THE NEBULÆ

One other great result of spectrum-analysis, and in some respects perhaps
the greatest, is its demonstration of the fact that true nebulæ exist, and
that they are not all star-clusters so remote as to be irresolvable, as was
once supposed. They are shown to have gaseous spectra, or sometimes gaseous
and stellar spectra combined, and this, in connection with the fact that
nebulæ are frequently aggregated around nebulous stars or groups of stars,
renders it certain that the nebulæ are in no way separated in space from
the stars, but that they constitute essential parts of one vast stellar
universe. There is, indeed, good reason to believe that they are really the
material out of which stars are made, and that in their forms,
aggregations, and condensations, we can trace the very process of evolution
of stars and suns.


PHOTOGRAPHIC ASTRONOMY

But there is yet another powerful engine of research which the new
astronomy possesses, and which, either alone or in combination with the
spectroscope, had produced and will yet produce in the future an amount of
knowledge of the stellar universe which could never be attained by any
other means. It has already been stated how the discovery of new variable
and binary stars has been rendered possible by the preservation of the
photographic plates on which the spectra are self-recorded, night after
night, with every line, whether dark or coloured, in true position, so as
to bear magnification, and, by comparison with others of the series,
enabling the most minute changes to be detected and their amount accurately
measured. Without the preservation of such comparable records, which is in
no other way possible, by far the larger portion of spectroscopic
discoveries could never have been made.

But there are two other uses of photography of quite a different nature
which are equally and perhaps in their final outcome may be far more
important. The first is, that by the use of the photographic plate the
exact positions of scores, hundreds, or even thousands of stars can be
self-mapped simultaneously with extreme accuracy, while any number of
copies can be made of these star-maps. This entirely obviates the necessity
for the old method of fixing the position of each star by repeated
measurement by means of very elaborate instruments, and their registration
in laborious and expensive catalogues. So important is this now seen to be,
that specially constructed cameras are made for stellar photography, and by
means of the best kinds of equatorial mounting are made to revolve slowly
so that the image of each star remains stationary upon the plate for
several hours.

Arrangements have been now made among all the chief observatories of the
world to carry out a photographic survey of the heavens with identical
instruments, so as to produce maps of the whole star-system on the same
scale. These will serve as fixed data for future astronomers, who will thus
be able to determine the movements of stars of all magnitudes with a
certainty and accuracy hitherto unattainable.

The other important use of photography depends upon the fact that with a
longer exposure within certain limits we increase the light-collecting
power. It will surprise many persons to learn that an ordinary good
portrait-camera with a lens three or four inches in diameter, if properly
mounted so that an exposure of several hours can be made, will show stars
so minute that they are invisible even in the great Lick telescope. In this
way the camera will often reveal double-stars or small groups which can be
made visible in no other way.

Such photographs of the stars are now constantly reproduced in works on
Astronomy and in popular magazine articles, and although some of them are
very striking, many persons are disappointed with them, and cannot
understand their great value, because each star is represented by a white
circle often of considerable size and with a somewhat undefined outline,
not by a minute point of light as stars appear in a good telescope. But the
essential matter in all such photographs is not so much the smallness, as
the roundness, of the star-images, as this proves the extreme precision
with which the image of every star has been kept by the clockwork motion of
the instrument on the same point of the plate during the whole exposure.
For example, in the fine photograph of the Great Nebula in Andromeda, taken
29th December 1888, by Dr. Isaac Roberts, with an exposure of four hours,
there are probably over a thousand stars large and small to be seen, every
one represented by an almost exactly circular white dot of a size dependent
on the magnitude of the star. These round dots can be bisected by the cross
hairs of a micrometer with very great accuracy, and thus the distance
between the centres of any of the pairs, as well as the direction of the
line joining their centres, can be determined as accurately as if each was
represented by a point only. But as a minute white speck would be almost
invisible on the maps, and would convey no information as to the
approximate magnitude of the star, mistakes would be much more easily made,
and it would probably be found necessary to surround each star with a
circle to indicate its magnitude, and to enable it to be easily seen. It is
probable, therefore, that the supposed defect is really an important
advantage. The above-mentioned photograph is beautifully reproduced in
Proctor's _Old and New Astronomy_, published after his greatly lamented
death.

But besides the amount of altogether new knowledge obtained by the methods
of research here briefly explained, a great deal of light has been thrown
on the distribution of the stars as a whole, and hence on the nature and
extent of the stellar universe, by a careful study of the materials
obtained by the old methods, and by the application of the doctrine of
probabilities to the observed facts. In this way alone some very striking
results have been reached, and these have been supported and strengthened
by the newer methods, and also by the use of new instruments in the
measurement of stellar distances. Some of these results bear so closely and
directly upon the special subject of the present volume, that our next
chapter must be devoted to a consideration of them.



CHAPTER IV

THE DISTRIBUTION OF THE STARS


If we look at the heavens on a clear, moonless night in winter, and from a
position embracing the entire horizon, the scene is an inexpressibly grand
one. The intense sparkling brilliancy of Sirius, Capella, Vega, and other
stars of the first magnitude; their striking arrangement in constellations
or groups, of which Orion, the Great Bear, Cassiopeiæ, and the Pleiades,
are familiar examples; and the filling up between these by less and less
brilliant points down to the limit of vision, so as to cover the whole sky
with a scintillating tracery of minute points of light, convey together an
idea of such confused scattering and such enormous numbers, that it seems
impossible to count them or to reduce them to systematic order. Yet this
was done for all except the faintest stars by Hipparchus, 134 B.C., who
catalogued and fixed the positions of more than 1000 stars, and this is
about the number, down to the fifth magnitude, visible in the latitude of
Greece. A recent enumeration of all the stars visible to the naked eye,
under the most favourable conditions and by the best eyesight, has been
made by the American astronomer, Pickering. His numbers are--for the
Northern Hemisphere 2509, and for the Southern Hemisphere 2824, thus
showing a somewhat greater richness in the southern celestial hemisphere.
But as this difference is due entirely to a preponderance of stars between
magnitudes 5-1/2 and 6, that is, just on the limits of vision, while those
down to magnitude 5-1/2 are more numerous by 85 in the Northern Hemisphere,
Professor Newcomb is of opinion that there is no real superiority of
numbers of visible stars in one hemisphere over the other. Again, the total
number of the visible stars by the above enumeration is 5333. But this
includes stars down to 6.2 magnitude, while it is generally considered that
magnitude 6 marks the limit of visibility. On a re-examination of all the
materials, the Italian astronomer Schiaparelli concludes that the total
number of stars down to the sixth magnitude is 4303; and they seem to be
about equally divided between the northern and southern skies.


THE MILKY WAY

But besides the stars themselves, a most conspicuous object both in the
northern and southern hemisphere is that wonderful irregular belt of
faintly diffused light termed the Milky Way or Galaxy. This forms a
magnificent arch across the sky, best seen in the autumn months in our
latitude. This arch, while following the general course of a great circle
round the heavens, is extremely irregular in detail, sometimes being
single, sometimes double, sending off occasional branches or offshoots, and
also containing in its very midst dark rifts, spots, or patches, where the
black background of almost starless sky can be seen through it. When
examined through an opera-glass or small telescope quantities of stars are
seen on the luminous background, and with every increase in the size and
power of the telescope more and more stars become visible, till with the
largest and best modern instruments the whole of the Galaxy seems densely
packed with them, though still full of irregularities, wavy streams of
stars, and dark rifts and patches, but always showing a faint nebulous
background as if there remained other myriads of stars which a still higher
optical power would reveal.

The relations of this great belt of telescopic stars to the rest of the
star-system have long interested astronomers, and many have attempted its
solution. By a system of gauging, that is counting all the stars that
passed over the field of his telescope in a certain time, Sir William
Herschel was the first who made a systematic effort to determine the shape
of the stellar universe. From the fact that the number of stars increased
rapidly as the Milky Way was approached from whatever direction, while in
the Galaxy itself the numbers visible were at once more than doubled, he
formed the idea that the shape of the entire system must be that of a
highly compressed very broad mass or ring rather less dense towards the
centre where our sun was situated. Roughly speaking, the form was likened
to a flat disc or grindstone, but of irregular thickness, and split in two
on one side where it appears to be double. The immense quantity of the
stars which formed it was supposed to be due to the fact that we looked at
it edgewise through an immense depth of stars; while at right angles to its
direction when looking towards what is termed the pole of the Galaxy, and
also in a less degree when looking obliquely, we see out into space through
a much thinner stratum of stars, which thus seem on the average to be very
much farther apart.

But, in the latter part of his life, Sir William Herschel realised that
this was not the true explanation of the features presented by the Galaxy.
The brilliant spots and patches in it, the dark rifts and openings, the
narrow streams of light often bounded by equally narrow streams or rifts of
darkness, render it quite impossible to conceive that this complex luminous
ring has the form of a compressed disc extending in the direction in which
we see it to a distance many times greater than its thickness. In one very
luminous cluster Herschel thought that his telescope had penetrated to
regions twenty times as far off as the more brilliant stars forming the
nearer portions of the same object. Now, in the case of the Magellanic
clouds, which are two roundish nebular patches of large size some distance
from the Milky Way in the Southern Hemisphere and looking like detached
portions of it, Sir John Herschel himself has shown that any such
interpretation of its form is impossible; because it requires us to suppose
that in both these cases we see, not rounded masses of a roughly globular
shape, but immensely long cones or cylinders, placed in such a direction
that we see only the ends of them. He remarks that one such object so
situated would be an extraordinary coincidence, but that there should be
two or many such is altogether out of the question. But in the Milky Way
there are hundreds or even thousands of such spots or masses of
exceptional brilliancy or exceptional darkness; and, if the form of the
Galaxy is that of a disc many times broader than thick, and which we see
edgewise, then every one of these patches and clusters, and all the narrow
winding streams of bright light or intense blackness, must be really
excessively long cylinders, or tunnels, or deep curving laminæ, or narrow
fissures. And every one of these, which are to be found in every part of
this vast circle of luminosity, must be so arranged as to be exactly turned
towards our sun. The weight of this argument, which has been most forcibly
and clearly set forth by the late Mr. R.A. Proctor, in his very instructive
volume _Our Place among Infinities_, is now generally admitted by
astronomers, and the natural conclusion is that the form of the Milky Way
is that of a vast irregular ring, of which the section at any part is,
roughly speaking, circular; while the many narrow rifts or lanes or
openings where we seem to be able to see completely through it to the
darkness of outer space beyond, render it probable that in those directions
its thickness is less instead of greater than its apparent width, that is,
that we see the broader side rather than the narrow edge of it.

Before entering on the consideration of the relations which the bulk of the
stars we see scattered over the entire vault of heaven bear to this great
belt of telescopic stars, it will be advisable to give a somewhat full
description of the Galaxy itself, both because it is not often delineated
on star-maps with sufficient accuracy, or so as to show its wonderful
intricacies of structure, and also because it constitutes the fundamental
phenomenon upon which the argument set forth in this volume primarily
rests. For this purpose I shall use the description of it given by Sir John
Herschel in his _Outlines of Astronomy_, both because he, of all the
astronomers of the last century, had studied it most thoroughly, in the
northern and in the southern hemispheres, by eye-observation and with the
aid of telescopes of great power and admirable quality; and also because,
amid the throng of modern works and the exciting novelties of the last
thirty years, his instructive volume is, comparatively speaking, very
little known. This precise and careful description will also be of service
to any of my readers who may wish to form a closer personal acquaintance
with this magnificent and intensely interesting object, by examining its
peculiarities of form and beauties of structure either with the naked eye,
or with the aid of a good opera-glass, or with a small telescope of good
defining power.


A DESCRIPTION OF THE MILKY WAY

Sir John Herschel's description is as follows:--'The course of the Milky
Way as traced through the heavens by the unaided eye, neglecting occasional
deviations and following the line of its greatest brightness as well as its
varying breadth and intensity will permit, conforms, as nearly as the
indefiniteness of its boundary will allow it to be fixed, to that of a
great circle inclined at an angle of about 63° to the equinoctial, and
cutting that circle in Right Ascension 6h. 47m. and 18h. 47m., so that its
northern and southern poles respectively are situated in Right Ascension
12h. 47m., North Polar Distance 63°, and R.A. 0h. 47m., NPD. 117°.
Throughout the region where it is so remarkably subdivided, this great
circle holds an intermediate situation between the two great streams; with
a nearer approximation however to the brighter and continuous stream than
to the fainter and interrupted one. If we trace its course in order of
right ascension, we find it traversing the constellation Cassiopeiæ, its
brightest part passing about two degrees to the north of the star Delta of
that constellation. Passing thence between Gamma and Epsilon Cassiopeiæ, it
sends off a branch to the south-preceding side, towards Alpha Persei, very
conspicuous as far as that star, prolonged faintly towards Eta of the same
constellation, and possibly traceable towards the Hyades and Pleiades as
remote outliers. The main stream, however (which is here very faint),
passes on through Auriga, over the three remarkable stars, Epsilon, Zeta,
Eta, of that constellation called the Hædi, preceding Capella, between the
feet of Gemini and the horns of the Bull (where it intersects the ecliptic
nearly in the Solstitial Colure) and thence over the club of Orion to the
neck of Monoceros, intersecting the equinoctial in R.A. 6h. 54m. Up to this
point, from the offset in Perseus, its light is feeble and indefinite, but
thenceforward it receives a gradual accession of brightness, and where it
passes through the shoulder of Monoceros and over the head of Canis Major
it presents a broad, moderately bright, very uniform, and to the naked eye,
starless stream up to the point where it enters the prow of the ship Argo,
nearly on the southern tropic. Here it again subdivides (about the star _m_
Puppis), sending off a narrow and winding branch on the preceding side as
far as Gamma Argûs, where it terminates abruptly. The main stream pursues
its southward course to the 123rd parallel of NPD., where it diffuses
itself broadly and again subdivides, opening out into a wide fan-like
expanse, nearly 20° in breadth, formed of interlacing branches, which all
terminate abruptly, in a line drawn nearly through Lambda and Gamma Argûs.

'At this place the continuity of the Milky Way is interrupted by a wide
gap, and where it recommences on the opposite side it is by a somewhat
similar fan-shaped assemblage of branches which converge upon the bright
star Eta Argûs. Thence it crosses the hind feet of the Centaur, forming a
curious and sharply-defined semicircular concavity of small radius, and
enters the Cross by a very bright neck or isthmus of not more than three or
four degrees in breadth, being the narrowest portion of the Milky Way.
After this it immediately expands into a broad and bright mass, enclosing
the stars Alpha and Beta Crucis and Beta Centauri, and extending almost up
to Alpha of the latter constellation. In the midst of this bright mass,
surrounded by it on all sides, and occupying about half its breadth, occurs
a singular dark pear-shaped vacancy, so conspicuous and remarkable as to
attract the notice of the most superficial gazer and to have acquired among
the early southern navigators the uncouth but expressive appellation of the
_coal-sack_. In this vacancy, which is about 8° in length and 5° broad,
only one very small star visible to the naked eye occurs, though it is far
from devoid of telescopic stars, so that its striking blackness is simply
due to the effect of contrast with the brilliant ground with which it is on
all sides surrounded. This is the place of nearest approach of the Milky
Way to the South Pole. Throughout all this region its brightness is very
striking, and when compared with that of its more northern course already
traced, conveys strongly the impression of greater proximity, and would
almost lead to a belief that our situation as spectators is separated on
all sides by a considerable interval from the dense body of stars composing
the Galaxy, which in this view of the subject would come to be considered
as a flat ring or some other re-entering form of immense and irregular
breadth and thickness, within which we are excentrically situated, nearer
to the southern than to the northern part of its circuit.

'At Alpha Centauri the Milky Way again subdivides, sending off a great
branch of nearly half its breadth, but which thins off rapidly, at an angle
of about 20° with its general direction to Eta and _d_ Lupi, beyond which
it loses itself in a narrow and faint streamlet. The main stream passes on
increasing in breadth to Gamma Normæ, where it makes an abrupt elbow and
again subdivides into one principal and continuous stream of very irregular
breadth and brightness, and a complicated system of interlaced streaks and
masses, which covers the tail of Scorpio, and terminates in a vast and
faint effusion over the whole extensive region occupied by the preceding
leg of Ophiuchus, extending northward to the parallel of 103° NPD., beyond
which it cannot be traced; a wide interval of 14°, free from all appearance
of nebulous light, separating it from the great branch on the north side
of the equinoctial of which it is usually represented as a continuation.

'Returning to the point of separation of this great branch from the main
stream, let us now pursue the course of the latter. Making an abrupt bend
to the following side, it passes over the stars Iota Aræ, Theta and Iota
Scorpii, and Gamma Tubi to Gamma Sagittarii, where it suddenly collects
into a vivid oval mass about 6° in length and 4° in breadth, so excessively
rich in stars that a very moderate calculation makes their number exceed
100,000. Northward of this mass, this stream crosses the ecliptic in
longitude about 276°, and proceeding along the bow of Sagittarius into
Antinous has its course rippled by three deep concavities, separated from
each other by remarkable protuberances, of which the larger and brighter
forms the most conspicuous patch in the southern portion of the Milky Way
visible in our latitudes.

'Crossing the equinoctial at the 19th hour of R.A., it next runs in an
irregular, patchy, and winding stream through Aquila, Sagitta, and
Vulpecula up to Cygnus; at Epsilon of which constellation its continuity is
interrupted, and a very confused and irregular region commences, marked by
a broad dark vacuity, not unlike the southern "coal-sack," occupying the
space between Epsilon, Alpha, and Gamma Cygni, which serves as a kind of
centre for the divergence of three great streams; one, which we have
already traced; a second, the continuation of the first (across the
interval) from Alpha northward, between Lacerta and the head of Cepheus to
the point in Cassiopeiæ whence we set out, and a third branching off from
Gamma Cygni, very vivid and conspicuous, running off in a southern
direction through Beta Cygni, and _s_ Aquilæ almost to the equinoctial,
where it loses itself in a region thinly sprinkled with stars, where in
some maps the modern constellation Taurus Poniatowski is placed. This is
the branch which, if continued across the equinoctial, might be supposed to
unite with the great southern effusion in Ophiuchus already noticed. A
considerable offset, or protuberant appendage, is also thrown off by the
northern stream from the head of Cepheus directly towards the pole,
occupying the greater part of the quartile formed by Alpha, Beta, Iota, and
Delta of that constellation.'

To complete this careful, detailed description of the Milky Way, it will be
well to add a few passages from the same work as to its telescopic
appearance and structure.

'When examined with powerful telescopes, the constitution of this wonderful
zone is found to be no less various than its aspect to the naked eye is
irregular. In some regions the stars of which it is composed are scattered
with remarkable uniformity over immense tracts, while in others the
irregularity of their distribution is quite as striking, exhibiting a rapid
succession of closely clustering rich patches separated by comparatively
poor intervals, and indeed in some instances by spaces absolutely dark _and
completely void of any star_, even of the smallest telescopic magnitude. In
some places not more than 40 or 50 stars on an average occur in a
gauge-field of 15', while in others a similar average gives a result of 400
or 500. Nor is less variety observable in the character of its different
regions in respect of the magnitudes of the stars they exhibit, and the
proportional numbers of the larger and smaller magnitudes associated
together, than in respect of their aggregate numbers. In some, for
instance, extremely minute stars occur in numbers so moderate as to lead us
irresistibly to the conclusion that in these regions we see _fairly
through_ the starry stratum, since it is impossible otherwise that the
numbers of the smaller magnitudes should not go on continually increasing
ad infinitum. In such cases, moreover, the ground of the heavens is for the
most part perfectly dark, which again would not be the case if innumerable
multitudes of stars, too minute to be individually discernible, existed
beyond. In other regions we are presented with the phænomenon of an almost
uniform degree of brightness of the individual stars, accompanied with a
very even distribution of them over the ground of the heavens, both the
larger and smaller magnitudes being strikingly deficient. In such cases it
is equally impossible not to perceive that we are looking _through_ a sheet
of stars nearly of a size, and of no great thickness compared with the
distance which separates them from us. Were it otherwise we should be
driven to suppose the more distant stars uniformly the larger, so as to
compensate by their greater intrinsic brightness for their greater
distance, a supposition contrary to all probability....

'Throughout by far the larger portion of the extent of the Milky Way in
both hemispheres, the general blackness of the ground of the heavens on
which its stars are projected, and the absence of that innumerable
multitude and excessive crowding of the smallest visible magnitudes, and of
glare produced by the aggregate light of multitudes too small to affect the
eye singly, must, we think, be considered unequivocal indications that its
dimensions in _directions where these conditions obtain_ are not only not
infinite, but that the space-penetrating power of our telescopes suffices
fairly to pierce through and beyond it.'

In the above-quoted passage the italics are those of Sir John Herschel
himself, and we see that he drew the very same conclusions from the facts
he describes, and for much the same reasons, as Mr. Proctor has drawn from
the observations of Sir William Herschel; and, as we shall see, the best
astronomers to-day have arrived at a similar result, from the additional
facts at their disposal, and in some cases from fresh lines of argument.


THE STARS IN RELATION TO THE MILKY WAY

Sir John Herschel was so impressed with the form, structure, and immensity
of the Galactic Circle, as he sometimes terms it, that he says (in a
footnote p. 575, 10th ed.), 'This circle is to sidereal what the invariable
ecliptic is to planetary astronomy--a plane of ultimate reference, the
ground-plane of the sidereal system.' We have now to consider what are the
relations of the whole body of the stars to this Galactic Circle--this
plane of ultimate reference for the whole stellar universe.

If we look at the heavens on a starry night, the whole vault appears to be
thickly strewn with stars of various degrees of brightness, so that we
could hardly say that any extensive region--the north, east, south, or
west, or the portion vertically above us--is very conspicuously deficient
or superior in numbers. In every part there are to be found a fair
proportion of stars of the first two or three magnitudes, while where these
may seem deficient a crowd of smaller stars takes their place.

But an accurate survey of the visible stars shows that there is a large
amount of irregularity in their distribution, and that all magnitudes are
really more numerous in or near the Milky Way, than at a distance from it,
though not in so large a degree as to be very conspicuous to the naked eye.
The area of the whole of the Milky Way cannot be estimated at more than
one-seventh of the whole sphere, while some astronomers reckon it at only
one-tenth. If stars of any particular size were uniformly distributed, at
most one-seventh of the whole number should be found within its limits. But
Mr. Gore finds that of 32 stars brighter than the second magnitude 12 lie
upon the Milky Way, or considerably more than twice as many as there should
be if they were uniformly distributed. And in the case of the 99 stars
which are brighter than the third magnitude 33 lie upon the Milky Way, or
one-third instead of one-seventh. Mr. Gore also counted all the stars in
Heis's Atlas which lie upon the Milky Way, and finds there are 1186 out of
a total of 5356, a proportion of between a fourth and a fifth instead of a
seventh.

The late Mr. Proctor in 1871 laid down on a chart two feet diameter all the
stars down to magnitude 9-1/2 given in Agrelander's forty large charts of
the stars visible in the northern hemisphere. They were 324,198 in number,
and they distinctly showed by their greater density not only the whole
course of the Milky Way but also its more luminous portions and many of the
curious dark rifts and vacuities, which latter are almost wholly avoided by
these stars.

Later on Professor Seeliger of Munich made an investigation of the relation
of more than 135,000 stars down to the ninth magnitude to the Milky Way, by
dividing the whole of the heavens into nine regions, one and nine being
circles of 20° wide (equal to 40° diameter) at the two poles of the Galaxy;
the middle region, five, is a zone 20° wide including the Milky Way itself,
and the other six intermediate zones are each 20° wide. The following table
shows the results as given by Professor Newcomb, who has made some
alterations in the last column of 'Density of Stars' in order to correct
differences in the estimate of magnitudes by the different authorities.

    Regions.     Area in Degree.     Number of Stars.     Density.

       I.            1,398.7             4,277              2.78
      II.            3,146.9            10,185              3.03
     III.            5,126.6            19,488              3.54
      IV.            4,589.8            24,492              5.32
       V.            4,519.5            33,267              8.17
      VI.            3,971.5            23,580              6.07
     VII.            2,954.4            11,790              3.71
    VIII.            1,796.6             6,375              3.21
      IX.              468.2             1,644              3.14

     _N.B._--The inequality of the N. and S. areas is because the
     enumeration of the stars only went as far as 24° S. Decl., and
     therefore included only a part of Regions VII., VIII., and IX.

[Illustration: DIAGRAM OF STAR-DENSITY From Herschel's Gauges (as given by
Professor Newcomb, p. 251).]

Upon this table of densities Professor Newcomb remarks as follows:--'The
star-density in the several regions increases continuously from each pole
(regions I. and IX.) to the Galaxy itself (region V.). IF THE LATTER WERE
A SIMPLE RING OF STARS SURROUNDING A SPHERICAL SYSTEM OF STARS, THE
STAR-DENSITY WOULD BE ABOUT THE SAME IN REGIONS I., II., and III., and also
in VII., VIII., and IX., but would suddenly increase in IV. and VI. as the
boundary of the ring was approached. Instead of such being the case, the
numbers 2.78, 3.03, and 3.54 in the north, and 3.14, 3.21, and 3.71 in the
south, show a progressive increase from the galactic pole to the Galaxy
itself. The conclusion to be drawn is a fundamental one. The universe, or
at least the denser portion of it, is really flattened between the galactic
poles, as supposed by Herschel and Struve.'

But looking at the series of figures in the table, and again as quoted by
Professor Newcomb, they seem to me to show in some measure what he says
they do not show. I therefore drew out the above diagram from the figures
in the table, and it certainly shows that the density in regions I., II.,
and III., and in regions VII., VIII., and IX., may be said to be 'about the
same,' that is, they increase very slowly, and that they _do_ 'suddenly
increase' in IV. and VI. as the boundary of the Galaxy is approached. This
may be explained either by a flattening towards the poles of the Galaxy, or
by the thinning out of stars in that direction.

In order to show the enormous difference of star-density in the Galaxy and
at the galactic poles, Professor Newcomb gives the following table of the
Herschelian gauges, on which he only remarks that they show an enormously
increased density in the galactic region due to the Herschels having
counted so many more stars there than any other observers.

    +-----------+----+----+----+----+-----+-----+-----+-----+-----+
    |Region,   .| I. | II.|III.| IV.|  V. | VI. | VII.|VIII.| IX. |
    |           |    |    |    |    |     |     |     |     |     |
    |Density,  .|107 |154 |281 |560 |2,019| 672 | 261 | 154 | 111 |
    +-----------+----+----+----+----+-----+-----+-----+-----+-----+

[Illustration: DIAGRAM OF STAR-DENSITY From a table in _The Stars_ (p.
249).]

But an important characteristic of these figures is, that the Herschels
alone surveyed the whole of the heavens from the north to the south pole,
that they did this with instruments of the same size and quality, and that
from almost life-long experience in this particular work they were
unrivalled in their power of counting rapidly and accurately the stars that
passed over each field of view of their telescopes. Their results,
therefore, must be held to have a comparative value far above those of any
other observer or combination of observers. I have therefore thought it
advisable to draw a diagram from their figures, and it will be seen how
strikingly it agrees with the former diagram in the very slow increase of
star-richness in the first three regions north and south, the sudden
increase in regions IV. and VI. as we approach the Galaxy, while the only
marked difference is in the enormously greater richness of the Galaxy
itself, which is an undoubtedly real phenomenon, and is brought out here by
the unrivalled observing power of the two greatest astronomers in this
special department that have ever lived.

We shall find later on that Professor Newcomb himself, as the result of a
quite different inquiry arrives at a result in accordance with these
diagrams which will then be again referred to. As this is a very
interesting subject, it will be well to give another diagram from two
tables of star-density in Sir John Herschel's volume already quoted. The
tables are as follows:--

      Zones of Galactic         Average number of Star
    North Polar Distance.         per Field of 15'.

         0° to 15°                       4.32
        15° to 30°                       5.42
        30° to 45°                       8.21
        45° to 60°                      13.61
        60° to 75°                      24.09
        75° to 90°                      53.43

      Zones of Galactic          Average number of Stars
    South Polar Distance.        per Field of 15'.

         0° to 15°                      6.05
        15° to 30°                      6.62
        30° to 45°                      9.08
        45° to 60°                     13.49
        60° to 75°                     26.29
        75° to 90°                     59.06

In these tables the Milky Way itself is taken as occupying two zones of 15°
each, instead of one of 20° as in Professor Newcomb's tables, so that the
excess in the number of stars over the other zones is not so large. They
show also a slight preponderance in all the zones of the southern
hemisphere, but this is not great, and may probably be due to the clearer
atmosphere of the Cape of Good Hope as compared with that of England.

[Illustration: DIAGRAM OF STAR-DENSITY. From Table in Sir J. Herschel's
_Outlines of Astronomy_ (10th ed., pp. 577-578).]

It need only be noted here that this diagram shows the same general
features as those already given, of a continuous increase of star-density
from the poles of the Galaxy, but more rapidly as the Galaxy itself is
more nearly approached. This fact must, therefore, be accepted as
indisputable.


CLUSTERS AND NEBULÆ IN RELATION TO THE GALAXY

An important factor in the structure of the heavens is afforded by the
distribution of the two classes of objects known as clusters and nebulæ.
Although we can form an almost continuous series from double stars which
revolve round their common centre of gravity, through triple and quadruple
stars, to groups and aggregations of indefinite extent--of which the
Pleiades form a good example, since the six stars visible to the naked eye
are increased to hundreds by high telescopic powers, while photographs with
three hours' exposure show more than 2000 stars--yet none of these
correspond to the large class known as clusters, whether globular or
irregular, which are very numerous, about 600 having been recorded by Sir
John Herschel more than fifty years ago. Many of these are among the most
beautiful and striking objects in the heavens even with a very small
telescope or good opera-glass. Such is the luminous spot called Praesepe,
or the Beehive in the constellation Cancer, and another in the sword handle
of Perseus.

In the southern hemisphere there is a hazy star of about the fourth
magnitude, Omega Centauri, which with a good telescope is seen to be really
a magnificent cluster nearly two-thirds the diameter of the moon, and
described by Sir John Herschel as very gradually increasing in brightness
to the centre, and composed of innumerable stars of the thirteenth and
fifteenth magnitudes, forming the richest and largest object of the kind in
the heavens. He describes it as having rings like lace-work formed of the
larger stars. By actual count, on a good photograph, there are more than
6000 stars, while other observers consider that there are at least 10,000.
In the northern hemisphere one of the finest is that in the constellation
Hercules, known as 13 Messier. It is just visible to the naked eye or with
an opera glass as a hazy star of the sixth magnitude, but a good telescope
shows it to be a globular cluster, and the great Lick telescope resolves
even the densest central portion into distinct stars, of which Sir John
Herschel considered there were many thousands. These two fine clusters are
figured in many of the modern popular works on astronomy, and they afford
an excellent idea of these beautiful and remarkable objects, which, when
more thoroughly studied, will probably aid in elucidating some of the
obscure problems connected with the constitution and development of the
stellar universe.

But for the purpose of the present work the most interesting fact connected
with star-clusters is their remarkable distribution in the heavens. Their
special abundance in and near the Milky Way had often been noted, but the
full importance of the fact could not be appreciated till Mr. Proctor and,
later, Mr. Sidney Waters marked down, on maps of the two hemispheres, all
the star-clusters and nebulæ in the best catalogues. The result is most
interesting. The clusters are seen to be thickly strewn over the entire
course of the Milky Way, and along its margins, while in every other part
of the heavens they are thinly scattered at very distant intervals, with
the one exception of the Magellanic clouds of the southern hemisphere where
they are again densely grouped; and if anything were needed to prove the
physical connection of these clusters with the Galaxy it would be their
occurrence in these extensive nebulous patches which seem like outlying
portions of the Milky Way itself. With these two exceptions probably not
one-twentieth part of the whole number of star-clusters are found in any
part of the heavens remote from the Milky Way.

Nebulæ were for a long time confounded with star-clusters, because it was
thought that with sufficient telescopic power they could all be resolvable
into stars as in the case of the Milky Way itself. But when the
spectroscope showed that many of the nebulæ consisted wholly or mainly of
glowing gases, while neither the highest powers of the best telescopes nor
the still greater powers of the photographic plate gave any indications of
resolvability, although a few stars were often found to be, as it were,
entangled in them, and evidently forming part of them, it was seen that
they constituted a distinct stellar phenomenon, a view which was enforced
and rendered certain by their quite unique mode of distribution. A few of
the larger and irregular type, as in the case of the grand Orion nebula
visible to the naked eye, the great spiral nebula in Andromeda, and the
wonderful Keyhole nebula round Eta Argûs, are situated in or near the Milky
Way; but with these and a few other exceptions the overwhelming majority of
the smaller irresolvable nebulæ appear to avoid it, there being a space
almost wholly free from nebulæ along its borders, both in the northern and
southern hemispheres; while the great majority are spread over the sky, far
away from it in the southern hemisphere, and in the north clustering in a
very marked degree around the galactic pole. The distribution of nebulæ is
thus seen to be the exact opposite to that of the star-clusters, while both
are so distinctly related to the position of the Milky Way--the
ground-plane of the sidereal system, as Sir John Herschel termed it--that
we are compelled to include them all as connected portions of one grand
and, to some extent, symmetrical universe, whose remarkable and opposite
mode of distribution over the heavens may probably afford a clue to the
mode of development of that universe and to the changes that are even now
taking place within it. The maps referred to above are of such great
importance, and are so essential to a clear comprehension of the nature and
constitution of the vast sidereal system which surrounds us, that I have,
with the permission of the Royal Astronomical Society, reproduced them
here. (See end of volume.)

A careful examination of them will give a clearer idea of the very
remarkable facts of distribution of star-clusters and nebulæ than can be
afforded by any amount of description or of numerical statements.

The forms of many of the nebulæ are very curious. Some are quite irregular,
as the Orion nebula, the Keyhole nebula in the southern hemisphere, and
many others. Some show a decidedly spiral form, as those in Andromeda and
Canes Venatici; others again are annular or ring-shaped, as those in Lyra
and Cygnus, while a considerable number are termed planetary nebulæ, from
their exhibiting a faint circular disc like that of a planet. Many have
stars or groups of stars evidently forming parts of them, and this is
especially the case with those of the largest size. But all these are
comparatively few in number and more or less exceptional in type, the great
majority being minute cloudy specks only visible with good telescopes, and
so faint as to leave much doubt as to their exact shape and nature. Sir
John Herschel catalogued 5000 in 1864, and more than 8000 were discovered
up to 1890; while the application of the camera has so increased the
numbers that it is thought there may really be many hundreds of thousands
of them.

The spectroscope shows the larger irregular nebulæ to be gaseous, as are
the annular and planetary nebulæ as well as many very brilliant white
stars; and all these objects are most frequent in or near the Milky Way.
Their spectra show a green line not produced by any terrestrial element.
With the great Lick telescope several of the planetary nebulæ have been
found to be irregular and sometimes to be formed of compressed or looped
rings and other curious forms.

Many of the smaller nebulæ are double or triple, but whether they really
form revolving systems is not yet known. The great mass of the small nebulæ
that occupy large tracts of the heavens remote from the Galaxy are often
termed irresolvable nebulæ, because the highest powers of the largest
telescopes show no indication of their being star-clusters, while they are
too faint to give any definite indications of structure in the
spectroscope. But many of them resemble comets in their forms, and it is
thought not impossible that they may be not very dissimilar in
constitution.

       *       *       *       *       *

We have now passed in review the main features presented to us in the
heavens outside the solar system, so far as regards the numbers and
distribution of the lucid stars (those visible to the naked eye) as well as
those brought to view by the telescope; the form and chief characteristics
of the Milky Way or Galaxy; and lastly, the numbers and distribution of
those interesting objects--star-clusters and nebulæ in their special
relations to the Milky Way. This examination has brought clearly before us
the unity of the whole visible universe; that everything we can see, or
obtain any knowledge of, with all the resources of modern gigantic
telescopes, of the photographic plate, and of the even more marvellous
spectroscope, forms parts of one vast system which may be shortly and
appropriately termed the Stellar universe.

In our next chapter we shall carry the investigation a step further, by
sketching in outline what is known of the motions and distances of the
stars, and thus obtain some important information bearing upon our special
subject of inquiry.



CHAPTER V

DISTANCE OF THE STARS--THE SUN'S MOTION THROUGH SPACE


In early ages, before any approximate idea was reached of the great
distances of the stars from us, the simple conception of a crystal sphere
to which these luminous points were attached and carried round every day on
an axis near which our pole-star is situated, satisfied the demands for an
explanation of the phenomena. But when Copernicus set forth the true
arrangement of the heavenly bodies, earth and planets alike revolving round
the sun at distances of many millions of miles, and when this scheme was
enforced by the laws of Kepler and the telescopic discoveries of Galileo, a
difficulty arose which astronomers were unable satisfactorily to overcome.
If, said they, the earth revolves round the sun at a distance which cannot
be less (according to Kepler's measurement of the distance of Mars at
opposition) than 13-1/2 millions of miles, then how is it that the nearer
stars are not seen to shift their apparent places when viewed from opposite
sides of this enormous orbit? Copernicus, and after him Kepler and Galileo,
stoutly maintained that it was because the stars were at such an enormous
distance from us that the earth's orbit was a mere point in comparison.
But this seemed wholly incredible, even to the great observer Tycho Brahé,
and hence the Copernican theory was not so generally accepted as it
otherwise would have been.

Galileo always declared that the measurement would some day be made, and he
even suggested the method of effecting it which is now found to be the most
trustworthy. But the sun's distance had to be first measured with greater
accuracy, and that was only done in the latter part of the eighteenth
century by means of transits of Venus; and by later observations with more
perfect instruments it is now pretty well fixed at about 92,780,000 miles,
the limits of error being such that 92-3/4 millions may perhaps be quite as
accurate.

With such an enormous base-line as twice this distance, which is available
by making observations at intervals of about six months when the earth is
at opposite points in its orbit, it seemed certain that some parallax or
displacement of the nearer stars could be found, and many astronomers with
the best instruments devoted themselves to the work. But the difficulties
were enormous, and very few really satisfactory results were obtained till
the latter half of the nineteenth century. About forty stars have now been
measured with tolerable certainty, though of course with a considerable
margin of possible or probable error; and about thirty more, which are
found to have a parallax of one-tenth of a second or less, must be
considered to leave a very large margin of uncertainty.

The two nearest fixed stars are Alpha Centauri and 61 Cygni. The former is
one of the brightest stars in the southern hemisphere, and is about
275,000 times as far from us as the sun. The light from this star will take
4-1/4 years to reach us, and this 'light-journey,' as it is termed, is
generally used by astronomers as an easily remembered mode of recording the
distances of the fixed stars, the distance in miles--in this case about 25
millions of millions--being very cumbrous. The other star, 61 Cygni, is
only of about the fifth magnitude, yet it is the second nearest to us, with
a light-journey of about 7-1/4 years. If we had no other determinations of
distance than these two, the facts would be of the highest importance. They
teach us, first, that magnitude or brightness of a star is no proof of
nearness to us, a fact of which there is much other evidence; and in the
second place, they furnish us with a probable minimum distance of
independent suns from one another, which, in proportion to their sizes,
some being known to be many times larger than our sun, is not more than we
might expect. This remoteness may be partly due to those which were once
nearer together having coalesced under the influence of gravitation.

As this measurement of the distance of the nearer stars should be clearly
understood by every one who wishes to obtain some real comprehension of the
scale of this vast universe of which we form a part, the method now adopted
and found to be most effectual will be briefly explained.

Everyone who is acquainted with the rudiments of trigonometry or
mensuration, knows that an inaccessible distance can be accurately
determined if we can measure a base-line from both ends of which the
inaccessible object can be seen, and if we have a good instrument with
which to measure angles. The accuracy will mainly depend upon our base-line
being not excessively short in comparison with the distance to be measured.
If it is as much as half or even a quarter as long the measurement may be
as accurate as if directly performed over the ground, but if it is only
one-hundredth or one-thousandth part as long, a very small error either in
the length of the base or in the amount of the angles will produce a large
error in the result.

In measuring the distance of the moon, the earth's diameter, or a
considerable portion of it, has served as a base-line. Either two observers
at great distances from each other, or the same observer after an interval
of nine or ten hours, may examine the moon from positions six or seven
thousand miles apart, and by accurate measurements of its angular distance
from a star, or by the time of its passage over the meridian of the place
as observed with a transit instrument, the angular displacement can be
found and the distance determined with very great accuracy, although that
distance is more than thirty times the length of the base. The distance of
the planet Mars when nearest to us has been found in the same way. His
distance from us even when at his nearest point during the most favourable
oppositions is about 36 million miles, or more than four thousand times the
earth's diameter, so that it requires the most delicate observations many
times repeated and with the finest instruments to obtain a tolerably
approximate result. When this is done, by Kepler's law of the fixed
proportion between the distances of planets from the sun and their times of
revolution, the proportionate distance of all the other planets and that
of the sun can be ascertained. This method, however, is not sufficiently
accurate to satisfy astronomers, because upon the sun's distance that of
every other member of the solar system depends. Fortunately there are two
other methods by which this important measurement has been made with much
greater approach to certainty and precision.

[Illustration: Diagram illustrating the transit of Venus.]

The first of these methods is by means of the rare occasions when the
planet Venus passes across the sun's disc as seen from the earth. When this
takes place, observations of the transit, as it is termed, are made at
remote parts of the earth, the distance between which places can of course
easily be calculated from their latitudes and longitudes. The diagram here
given illustrates the simplest mode of determining the sun's distance by
this observation, and the following description from Proctor's _Old and New
Astronomy_ is so clear that I copy it verbally:--'V represents Venus
passing between the Earth E and the Sun S; and we see how an observer at E
will see Venus as at v', while an observer at E' will see her as at v. The
measurement of the distance v v', as compared with the diameter of the
sun's disc, determines the angle v V v' or E V E'; whence the distance E V
can be calculated from the known length of the base-line E E'. For
instance, it is known (from the known proportions of the Solar System as
determined from the times of revolution by Kepler's third law) that E V
bears to V v the proportion 28 to 72, or 7 to 18; whence E E' bears to v v'
the same proportion. Suppose, now, that the distance between the two
stations is known to be 7000 miles, so that v v' is 18,000 miles; and that
v v' is found by accurate measurement to be 1/48 part of the sun's
diameter. Then the sun's diameter, as determined by this observation, is 48
times 18,000 miles, or 864,000 miles; whence from his known apparent size,
which is that of a globe 107-1/3 times farther away from us than its own
diameter, his distance is found to be 92,736,000 miles.'

Of course, there being two observers, the proportion of the distance v v'
to the diameter of the sun's disc cannot be measured directly, but each of
them can measure the apparent angular distance of the planet from the sun's
upper and lower margins as it passes across the disc, and thus the angular
distance between the two lines of transit can be obtained. The distance v
v' can also be found by accurately noting the times of the upper and lower
passage of Venus, which, as the line of transit is considerably shorter in
one than the other, gives by the known properties of the circle the exact
proportion of the distance between them to the sun's diameter; and as this
is found to be the most accurate method, it is the one generally adopted.
For this purpose the stations of the observers are so chosen that the
length of the two chords, v and v', may have a considerable difference,
thus rendering the measurement more easy.

The other method of determining the sun's distance is by the direct
measurement of the velocity of light. This was first done by the French
physicist, Fizeau, in 1849, by the use of rapidly revolving mirrors, as
described in most works on physics. This method has now been brought to
such a decree of perfection that the sun's distance so determined is
considered to be equally trustworthy with that derived from the transits of
Venus. The reason that the determination of the velocity of light leads to
a determination of the sun's distance is, because the time taken by light
to pass from the sun to the earth is independently known to be 8 min.
13-1/3 sec. This was discovered so long ago as 1675 by means of the
eclipses of Jupiter's satellites. These satellites revolve round the planet
in from 1-3/4 to 16 days, and, owing to their moving very nearly in the
plane of the ecliptic and the shadow of Jupiter being so large, the three
which are nearest to the planet are eclipsed at every revolution. This
rapid revolution of the satellites and frequency of the eclipses enabled
their periods of recurrence to be determined with extreme accuracy,
especially after many years of careful observation. It was then found that
when Jupiter was at its farthest distance from the earth the eclipses of
the satellites took place a little more than eight minutes later than the
time calculated from the mean period of revolution, and when the planet was
nearest to us the eclipses occurred the same amount earlier. And when
further observation showed that there was no difference between calculation
and observation when the planet was at its mean distance from us, and that
the error arose and increased exactly in proportion to our varying
distance from it, then it became clear that the only cause adequate to
produce such an effect was, that light had not an infinite velocity but
travelled at a certain fixed rate. This however, though a highly probable
explanation, was not absolutely proved till nearly two centuries later, by
means of two very difficult measurements--that of the actual distance of
the sun from the earth, and that of the actual speed of light in miles per
second; the latter corresponding almost exactly with the speed deduced from
the eclipses of Jupiter's satellites and the sun's distance as measured by
the transits of Venus.

[Illustration: (A) 5-3/4 inches from it (accurately 5.72957795 inches)]

But this problem of measuring the sun's distance, and through it the
dimensions of the orbits of all the planets of our system, sinks into
insignificance when compared with the enormous difficulties in the way of
the determination of the distance of the stars. As a great many people,
perhaps the majority of the readers of any popular scientific book, have
little knowledge of mathematics and cannot realise what an angle of a
minute or a second really means, a little explanation and illustration of
these terms will not be out of place. An angle of one degree (1°) is the
360th part of a circle (viewed from its centre), the 90th part of a right
angle, the 60th part of either of the angles of an equilateral triangle. To
see exactly how much is an angle of one degree we draw a short line (B C)
one-tenth of an inch long, and from a point we draw straight lines to B
and C. Then the angle at A is one degree.

Now, in all astronomical work, one degree is considered to be quite a large
angle. Even before the invention of the telescope the old observers fixed
the position of the stars and planets to half or a quarter of a degree,
while Mr. Proctor thinks that Tycho Brahé's positions of the stars and
planets were correct to about one or two minutes of arc. But a minute of
arc is obtained by dividing the line B C into sixty equal parts and seeing
the distance between two of these with the naked eye from the point A. But
as very long-sighted people can see very minute objects at 10 or 12 inches
distance, we may double the distance A B, and then making the line B C one
three-hundredth part of an inch long, we shall have the angle of one minute
which Tycho Brahé was perhaps able to measure. How very large an amount a
minute is to the modern astronomer is, however, well shown by the fact that
the maximum difference between the calculated and observed positions of
Uranus, which led Adams and Leverrier to search for and discover Neptune,
was only 1-1/2 minutes, a space so small as to be almost invisible to the
average eye, so that if there had been two planets, one in the calculated,
the other in the observed place, they would have appeared as one to
unassisted vision.

In order now to realise what one second of arc really means, let us look at
the circle here shown, which is as nearly as possible one-tenth of an inch
in diameter--(one-O-tenth of an inch). If we remove this circle to a
distance of 28 feet 8 inches it will subtend an angle of one minute, and we
shall have to place it at a distance of nearly 1730 feet--almost one-third
of a mile--to reduce the angle to one second. But the very nearest to us of
the fixed stars, Alpha Centauri, has a parallax of only three-fourths of a
second; that is, the distance of the earth from the sun--about 92-3/4
millions of miles--would appear no wider, seen from the nearest star, than
does three-fourths of the above small circle at one-third of a mile
distance. To see this circle at all at that distance would require a very
good telescope with a power of at least 100, while to see any small part of
it and to measure the proportion of that part to the whole would need very
brilliant illumination and a large and powerful astronomical telescope.


WHAT IS A MILLION?

But when we have to deal with millions, and even with hundreds and
thousands of millions, there is another difficulty--that few people can
form any clear conception of what a million is. It has been suggested that
in every large school the walls of one room or hall should be devoted to
showing a million at one view. For this purpose it would be necessary to
have a hundred large sheets of paper each about 4 feet 6 inches square,
ruled in quarter inch squares. In each alternate square a round black wafer
or circle should be placed a little over-lapping the square, thus leaving
an equal amount of white space between the black spots. At each tenth spot
a double width should be left so as to separate each hundred spots (10 ×
10). Each sheet would then hold ten thousand spots, which would all be
distinctly visible from the middle of a room 20 feet wide, each horizontal
or vertical row containing a thousand. One hundred such sheets would
contain a million spots, and they would occupy a space 450 feet long in one
row, or 90 feet long in five rows, so that they would entirely cover the
walls of a room, about 30 feet square and 25 feet high, from floor to
ceiling, allowing space for doors but not for windows, the hall or gallery
being lighted from above. Such a hall would be in the highest degree
educational in a country where millions are spoken of so glibly and wasted
so recklessly; while no one can really appreciate modern science, dealing
as it does with the unimaginably great and little, unless he is enabled to
realise by actual vision, and summing up, what a vast number is comprised
in _one_ of those millions, which, in modern astronomy and physics, he has
to deal with not singly only, but by hundreds and thousands or even by
millions. In every considerable town, at all events, a hall or gallery
should have a _million_ thus shown upon its walls. It would in no way
interfere with the walls being covered when required with maps, or
ornamental hangings, or pictures; but when these were removed, the visible
and countable million would remain as a permanent lesson to all visitors;
and I believe that it would have widespread beneficial effects in almost
every department of human thought and action. On a small scale any one can
do this for himself by getting a hundred sheets of engineer's paper ruled
in small squares, and making the spots very small; and even this would be
impressive, but not so much so as on the larger scale.

In order to enable every reader of this volume at once to form some
conception of the number of units in a million, I have made an estimate of
the number of _letters_ contained in it, and I find them to amount to about
420,000--considerably less than half a million. Try and realise, when
reading it, that if every letter were a pound sterling, we waste as many
pounds as there are letters in _two_ such volumes whenever we build a
battleship.

Having thus obtained some real conception of the immensity of a million, we
can better realise what it must be to have every one of the dots above
described, or every one of the letters in two such volumes as this
lengthened out so as to be each a mile long, and even then we should have
reached little more than a hundredth part of the distance from our earth to
the sun. When, by careful consideration of these figures, we have even
partially realised this enormous distance, we may take the next step, which
is, to compare this distance with that of the nearest fixed star. We have
seen that the parallax of that star is three-fourths of a second, an amount
which implies that the star is 271,400 times as far from us as our sun is.
If after _seeing_ what a million is, and knowing that the sun is 92-3/4
times this distance from us in miles--a distance which itself is almost
inconceivable to us--we find that we have to multiply this almost
inconceivable distance 271,400 times--more than a quarter of a million
times--to reach the _nearest_ of the fixed stars, we shall begin to
realise, however imperfectly, how vast is the system of suns around us, and
on what a scale of immensity the material universe, which we see so
gloriously displayed in the starry heavens and the mysterious galaxy, is
constructed.

This somewhat lengthy preliminary discussion is thought necessary in order
that my readers may form some idea of the enormous difficulty of obtaining
any measurement whatever of such distances. I now propose to point out what
the special difficulties are, and how they have been overcome; and thus I
hope to be able to satisfy them that the figures astronomers give us of the
distances of the stars are in no way mere guesses or probabilities, but are
real measurements which, within certain not very wide limits of error, may
be trusted as giving us correct ideas of the magnitude of the visible
universe.


MEASUREMENT OF STELLAR DISTANCES

The fundamental difficulty of this measurement is, of course, that the
distances are so vast that the longest available base-line, the diameter of
the earth's orbit, only subtends an angle of little more than a second from
the nearest star, while for all the rest it is less than one second and
often only a small fraction of it. But this difficulty, great as it is, is
rendered far greater by the fact that there is no fixed point in the
heavens from which to measure, since many of the stars are known to be in
motion, and all are believed to be so in varying degrees, while the sun
itself is now known to be moving among the stars at a rate which is not yet
accurately determined, but in a direction which is fairly well known. As
the various motions of the earth while passing round the sun, though
extremely complex, are very accurately known, it was first attempted to
determine the changed position of stars by observations, many times
repeated at six months' intervals, of the moment of their passage over the
meridian and their distance from the zenith; and then by allowing for all
the known motions of the earth, such as precession of the equinoxes and
nutation of the earth's axis, as well as for refraction and for the
aberration of light, to determine what residual effect was due to the
difference of position from which the star was viewed; and a result was
thus obtained in several cases, though almost always a larger one than has
been found by later observations and by better methods. These earlier
observations, however perfect the instruments and however skilful the
observer, are liable to errors which it seems impossible to avoid. The
instruments themselves are subject in all their parts to expansion and
contraction by changes of temperature; and when these changes are sudden,
one part of the instrument may be affected more than another, and this will
often lead to minute errors which may seriously affect the amount to be
measured when that is so small. Another source of error is due to
atmospheric refraction, which is subject to changes both from hour to hour
and at different seasons. But perhaps most important of all are minute
changes in level of the foundations of the instruments even when they are
carried down to solid rock. Both changes of temperature and changes of
moisture of the soil produce minute alterations of level; while
earth-tremors and slow movements of elevation or depression are now known
to be very frequent. Owing to all these causes, actual measurements of
differences of position at different times of the year, amounting to small
fractions of a second, are found to be too uncertain for the determination
of such minute angles with the required accuracy.

But there is another method which avoids almost all these sources of error,
and this is now generally preferred and adopted for these measurements. It
is, that of measuring the distance between two stars situated apparently
very near each other, one of which has large proper motion, while the other
has none which is measurable. The proper motions of the stars was first
suspected by Halley in 1717, from finding that several stars, whose places
had been given by Hipparchus, 130 B.C., were not in the positions where
they now ought to be; and other observations by the old astronomers,
especially those of occultations of stars by the moon, led to the same
result. Since the time of Halley very accurate observations of the stars
have been made, and in many cases it is found that they move perceptibly
from year to year, while others move so slowly that it is only after forty
or fifty years that the motion can be detected. The greatest proper motions
yet determined amount to between 7" and 8" in a year, while other stars
require twenty, or even fifty or a hundred years to show an equal amount of
displacement. At first it was thought that the brightest stars would have
the largest proper motion, because it was supposed they were nearest to us,
but it was soon found that many small and quite inconspicuous stars moved
as rapidly as the most brilliant, while in many very bright stars no
proper motion at all can be detected. That which moves most rapidly is a
small star of less than the sixth magnitude.

It is a matter of common observation that the motion of things at a
distance cannot be perceived so well as when near, even though the speed
may be the same. If a man is seen on the top of a hill several miles off,
we have to observe him closely for some time before we can be sure whether
he is walking or standing still. But objects so enormously distant as we
now know that the stars are, may be moving at the rate of many miles in a
second and yet require years of observation to detect any movement at all.

The proper motions of nearly a hundred stars have now been ascertained to
be more than one second of arc annually, while a large number have less
than this, and the majority have no perceptible motion, presumably due to
their enormous distance from us. It is therefore not difficult in most
cases to find one or two motionless stars sufficiently close to a star
having a large proper motion (anything more than one-tenth of a second is
so called) to serve as fixed points of measurement. All that is then
required is, to measure with extreme accuracy the angular distance of the
moving from the fixed stars at intervals of six months. The measurements
can be made, however, on every fine night, each one being compared with one
at nearly an interval of six months from it. In this way a hundred or more
measurements of the same star may be made in a year, and the mean of the
whole, allowance being made for proper motion in the interval, will give a
much more accurate result than any single measurement. This kind of
measurement can be made with extreme accuracy when the two stars can be
seen together in the field of the telescope; either by the use of a
micrometer, or by means of an instrument called a heliometer, now often
constructed for the purpose. This is an astronomical telescope of rather
large size, the object glass of which is cut in two straight across the
centre, and the two halves made to slide upon each other by means of an
exceedingly fine and accurate screw-motion, so adjusted and tested as to
measure the angular distance of two objects with extreme accuracy. This is
done by the number of turns of the screw required to bring the two stars
into contact with each other, the image of each one being formed by one of
the halves of the object glass.

But the greatest advantage of this method of determining parallax is, as
Sir John Herschell points out, that it gets rid of all the sources of error
which render the older methods so uncertain and inaccurate. No corrections
are required for precession, nutation, or aberration, since these affect
both stars alike, as is the case also with refraction; while alterations of
level of the instrument have no prejudicial effect, since the measures of
angular distance taken by this method are quite independent of such
movements. A test of the accuracy of the determination of parallax by this
instrument is the very close agreement of different observers, and also
their agreement with the new and perhaps even superior method by
photography. This method was first adopted by Professor Pritchard of the
Oxford Observatory, with a fine reflector of thirteen inches aperture. Its
great advantage is, that all the small stars in the vicinity of the star
whose parallax is sought are shown in their exact positions upon the plate,
and the distances of all of them from it can be very accurately measured,
and by comparing plates taken at six months' intervals, each of these stars
gives a determination of parallax, so that the mean of the whole will lead
to a very accurate result. Should, however, the result from any one of
these stars differ considerably from that derived from the rest, it will be
due in all probability to that star having a proper motion of its own, and
it may therefore be rejected. To illustrate the amount of labour bestowed
by astronomers on this difficult problem, it may be mentioned that for the
photographic measurement of the star 61 Cygni, 330 separate plates were
taken in 1886-7, and on these 30,000 measurements of distances of the pairs
of star-images were made. The result agreed closely with the best previous
determination by Sir Robert Ball, using the micrometer, and the method was
at once admitted by astronomers as being of the greatest value.

Although, as a rule, stars having large proper motions are found to be
comparatively near us, there is no regular proportion between these
quantities, indicating that the rapidity of the motion of the stars varies
greatly. Among fifty stars whose distances have been fairly well
determined, the rate of actual motion varies from one or two up to more
than a hundred miles per second. Among six stars with less than a tenth of
a second of annual proper motion there is one with a parallax of nearly
half a second, and another of one-ninth of a second, so that they are
nearer to us than many stars which move several seconds a year. This may be
due to actual slowness of motion, but is almost certainly caused in part by
their motion being either towards us or away from us, and therefore only
measurable by the spectroscope; and this had not been done when the lists
of parallaxes and proper motions from which these facts are taken were
published. It is evident that the actual direction and rate of motion of a
star cannot be known till this radial movement, as it is termed--that is,
towards or away from us--has been measured; but as this element always
tends to increase the visually observed rate of motion, we cannot, through
its absence, exaggerate the actual motions of the stars.


THE SUN'S MOVEMENT THROUGH SPACE

But there is yet another important factor which affects the apparent
motions of all the stars--the movement of our sun, which, being a star
itself, has a proper motion of its own. This motion was suspected and
sought for by Sir William Herschel a century ago, and he actually
determined the direction of its motion towards a point in the constellation
Hercules, not very far removed from that fixed upon as the average of the
best observations since made. The method of determining this motion is very
simple, but at the same time very difficult. When we are travelling in a
railway carriage near objects pass rapidly out of sight behind us, while
those farther from us remain longer in view, and very distant objects
appear almost stationary for a considerable time. For the same reason, if
our sun is moving in any direction through space, the nearer stars will
appear to travel in an opposite direction to our movement, while the more
distant will remain quite stationary. This movement of the nearest stars is
detected by an examination and comparison of their proper motions, by which
it is found that in one part of the heavens there is a preponderance of the
proper motions in one direction and a deficiency in the opposite direction,
while in the directions at right angles to these the proper motions are not
on the average greater in one direction than in the opposite. But the
proper motions of the stars being themselves so minute, and also so
irregular, it is only by a most elaborate mathematical investigation of the
motions of hundreds or even of thousands of stars, that the direction of
the solar motion can be determined. Till quite recently astronomers were
agreed that the motion was towards a point in Hercules near the
outstretched arm in the figure of that constellation. But the latest
inquiries into this problem, involving the comparison of the motions of
several thousand stars in all parts of the heavens, have led to the
conclusion that the most probable direction of the 'solar apex' (as the
point towards which the sun is moving is termed), is in the adjacent
constellation Lyra, and not far from the brilliant star Vega. This is the
position which Professor Newcomb of Washington thinks most probable, though
there is still room for further investigation. To determine the rate of the
motion is very much more difficult than to fix its direction, because the
distances of so few stars have been determined, and very few indeed of
these lie in the directions best adapted to give accurate results. The best
measurements down to 1890 led to a motion of about 15 miles a second. But
more recently the American astronomer, Campbell, has determined by the
spectroscope the motion in the line of sight of a considerable number of
stars towards and away from the solar apex, and by comparing the average of
these motions, he derives a motion for the sun of about 12-1/2 miles a
second, and this is probably as near as we can yet reach towards the true
amount.


SOME NUMERICAL RESULTS OF THE ABOVE MEASUREMENTS

The measurements of distances and proper motions of a considerable number
of the stars, of the motion of our sun in space (its proper motion),
together with accurate determinations of the comparative brilliancy of the
brightest stars as compared with our sun and with each other, have led to
some very remarkable numerical results which serve as indications of the
scale of magnitude of the stellar universe.

The parallaxes of about fifty stars have now been repeatedly measured with
such consistent results that Professor Newcomb considers them to be fairly
trustworthy, and these vary from one-hundredth to three-quarters of a
second. Three more, all stars of the first magnitude--Rigel, Canopus, and
Alpha Cygni--have no measurable parallax, notwithstanding the
long-continued efforts of many astronomers, affording a striking example of
the fact that brilliancy alone is no test of proximity. Six more stars
have a parallax of only one-fiftieth of a second, and five of these are
either of the first or second magnitudes. Of these nine stars having very
small parallax or none, six are situated in or near to the Milky Way,
another indication of exceeding remoteness, which is further shown by the
fact that they all have a very small proper motion or none at all. These
facts support the conclusion, which had been already reached by astronomers
from a careful study of the distribution of the stars, that the larger
portion of the stars of all magnitudes scattered throughout the Milky Way
or along its borders really belong to the same great system, and may be
said to form a part of it. This is a conclusion of extreme importance
because it teaches us that the grandest of the suns, such as Rigel and
Betelgeuse in the constellation Orion, Antares in the Scorpion, Deneb in
the Swan (Alpha Cygni), and Canopus (Alpha Argus), are in all probability
as far removed from us as are the innumerable minute stars which give the
nebulous or milky appearance to the Galaxy.

It is well to consider for a moment what these facts mean. Professor S.
Newcomb, one of the highest authorities on these problems, tells us that
the long series of measurements to discover the parallax of Canopus, the
brightest star in the southern hemisphere, would have shown a parallax of
one-hundredth of a second, had such existed. Yet the results always seemed
to converge to a mean of 0".000! Suppose, then, we assume the parallax of
this star to be somewhat less than the hundredth of a second--let us say
1/125 of a second. At the distance this gives, light would take almost
exactly 400 years to reach us, so that if we suppose this very brilliant
star to be situated a little on this side of the Galaxy, we must give to
that great luminous circle of stars a distance of about 500 light years. We
shall now perceive the advantage of being able to realise what a million
really is. A person who had once seen a wall-space more than 100 feet long
and 20 feet high completely covered with quarter-inch spots a quarter of an
inch apart; and then tried to imagine every spot to be a mile long and to
be placed end to end in one row, would form a very different conception of
a million miles than those who almost daily _read_ of millions, but are
quite unable to visualise even one of them. Having really seen one million,
we can partially realise the velocity of light, which travels over this
million miles in a little less than 5-1/2 seconds; and yet light takes more
than 4-1/3 years at this inconceivable speed to come to us from the very
_nearest_ of the stars. To realise this still more impressively, let us
take the _distance_ of this nearest star, which is 26 _millions_ of
_millions_ of miles. Let us look in imagination at this large and lofty
hall covered from floor to ceiling with quarter-inch spots--only _one_
million. Let all these be imagined as miles. Then repeat this number of
miles in a straight line, one after the other, as many times as there are
spots in this hall; and even then you have reached only one twenty-sixth
part of the distance to the nearest fixed star! This _million_ times a
_million_ miles has to be repeated twenty-six times to reach the _nearest_
fixed star; and it seems probable that this gives us a good indication of
the distance from each other of at least all the stars down to the sixth
magnitude, perhaps even of a large number of the telescopic stars. But as
we have found that the bright stars of the Milky Way must be at least one
hundred times farther from us than these nearest stars, we have found what
may be termed a minimum distance for that vast star-ring. It may be
immensely farther, but it is hardly possible that it should be anything
less.


THE PROBABLE SIZE OF THE STARS

Having thus obtained an inferior limit for the distance of several stars of
the first magnitude, and their actual brilliancy or light-emission as
compared with our sun having been carefully measured, we have afforded us
some indication of size though perhaps an uncertain one. By these means it
has been found that Rigel gives out about ten thousand times as much light
as our sun, so that if its surface is of the same brightness, it must be a
hundred times the diameter of the sun. But as it is one of the white or
Sirian type of stars it is probably very much more luminous, but even if it
were twenty times brighter it would still have to be twenty-two and a half
times the diameter of the sun; and as the stars of this type are probably
wholly gaseous and much less dense than our sun, this enormous size may not
be far from the truth. It is believed that the Sirian stars generally have
a greater surface brilliancy than our sun. Beta Aurigæ, a star of the
second magnitude but of the Sirian type, is one of the double stars whose
distance has been measured, and this has enabled Mr. Gore to find the mass
of the binary system to be five times that of the sun, and their light one
hundred and seventeen times greater. Even if the density is much less than
the sun's, the intrinsic brilliancy of the surface will be considerably
greater. Another double star, Gamma Leonis, has been found to be three
hundred times more brilliant than the sun if of the same density, but it
would require to be seven times rarer than air to have the extent of
surface needed to give the same amount of light if its surface emitted no
more light than our sun from equal areas.

It is clear, therefore, that many of the stars are much larger than our sun
as well as more luminous; but there are also large numbers of small stars
whose large proper motions, as well as the actual measurement of some,
prove them to be comparatively near to us which yet are only about
one-fiftieth part as bright as the sun. These must, therefore, be either
comparatively small, or if large must be but slightly luminous. In the case
of some double stars it has been proved that the latter is the case; but it
seems probable that others are very much smaller than the average. Up to
the present time no means of determining the size of a star by actual
measurement has been discovered, since their distances are so enormous that
the most powerful telescopes show only a point of light. But now that we
have really measured the distance of a good many stars we are able to
determine an upper limit for their actual dimensions. As the nearest fixed
star, Alpha Centauri, has a parallax of 0".75, this means that if this star
has a diameter as great as our distance from the sun (which is not much
more than a hundred times the sun's diameter) it would be seen to have a
distinct disc about as large as that of Jupiter's first satellite. If it
were even one-tenth of the size supposed it would probably be seen as a
disc in our best modern telescopes. The late Mr. Ranyard remarks that if
the Nebular Hypothesis is true, and our sun once extended as far as the
orbit of Neptune, then, among the millions of visible suns there ought to
be some now to be found in every stage of development. But any sun having a
diameter at all approaching this size, and situated as far off as a hundred
times the distance of Alpha Centauri, would be seen by the Lick telescope
to have a disc half a second in diameter. Hence the fact that there are no
stars with visible discs proves that there are no suns of the required
size, and adds another argument, though not perhaps a strong one, against
the acceptance of the Nebular Hypothesis.



CHAPTER VI

THE UNITY AND EVOLUTION OF THE STAR SYSTEM


The very condensed sketch now given of such of the discoveries of recent
Astronomy as relate to the subject we are discussing will, it is hoped,
give some idea both of the work already done and of the number of
interesting problems yet remaining to be solved. The most eminent
astronomers in every part of the world look forward to the solution of
these problems not, perhaps, as of any great value in themselves, but as
steps towards a more complete knowledge of our universe as a whole. Their
aim is to do for the star-system what Darwin did for the organic world, to
discover the processes of change that are at work in the heavens, and to
learn how the mysterious nebulæ, the various types of stars, and the
clusters and systems of stars are related to each other. As Darwin solved
the problem of the origin of organic species from other species, and thus
enabled us to understand how the whole of the existing forms of life have
been developed out of pre-existing forms, so astronomers hope to be able to
solve the problem of the evolution of suns from some earlier stellar types,
so as to be able, ultimately, to form some intelligible conception of how
the whole stellar universe has come to be what it is. Volumes have already
been written on this subject, and many ingenious suggestions and hypotheses
have been advanced. But the difficulties are very great; the facts to be
co-ordinated are excessively numerous, and they are necessarily only a
fragment of an unknown whole. Yet certain definite conclusions have been
reached; and the agreement of many independent observers and thinkers on
the fundamental principles of stellar evolution seems to assure us that we
are progressing, if slowly yet with some established basis of truth,
towards the solution of this, the most stupendous scientific problem with
which the human intellect has ever attempted to grapple.


THE UNITY OF THE STELLAR UNIVERSE

During the latter half of the nineteenth century the opinion of astronomers
has been tending more and more to the conception that the whole of the
visible universe of stars and nebulæ constitutes one complete and
closely-related system; and during the last thirty years especially the
vast body of facts accumulated by stellar research has so firmly
established this view that it is now hardly questioned by any competent
authority.

The idea that the nebulæ were far more remote from us than the stars long
held sway, even after it had been given up by its chief supporter. When Sir
William Herschel, by means of his then unapproached telescopic power,
resolved the Milky Way more or less completely into stars, and showed that
numerous objects which had been classed as nebulæ were really clusters of
stars, it was natural to suppose that those which still retained their
cloudy appearance under the highest telescopic powers were also clusters or
systems of stars, which only needed still higher powers to show their true
nature. This idea was supported by the fact that several nebulæ were found
to be more or less ring-shaped, thus corresponding on a smaller scale to
the form of the Milky Way; so that when Herschel discovered thousands of
telescopic nebulæ, he was accustomed to speak of them as so many distinct
universes scattered through the immeasurable depths of space.

Now, although any real conception of the immensity of the one stellar
universe, of which the Milky Way with its associated stars is the
fundamental feature, is, as I have shown, almost unattainable, the idea of
an unlimited number of other universes, almost infinitely remote from our
own and yet distinctly visible in the heavens, so seized upon the
imagination that it became almost a commonplace of popular astronomy and
was not easily given up even by astronomers themselves. And this was in a
large part due to the fact that Sir William Herschel's voluminous writings,
being almost all in the Philosophical Transactions of the Royal Society,
were very little read, and that he only indicated his change of view by a
few brief sentences which might easily be overlooked. The late Mr. Proctor
appears to have been the first astronomer to make a thorough study of the
whole of Herschel's papers, and he tells us that he read them all over five
times before he was able thoroughly to grasp the writer's views at
different periods.

But the first person to point out the real teaching of the facts as to the
distribution of the nebulæ was not an astronomer, but our greatest
philosophical student of science in general, Herbert Spencer. In a
remarkable essay on 'The Nebular Hypothesis' in the _Westminster Review_ of
July, 1858, he maintained that the nebulæ really formed a part of our own
Galaxy and of our own stellar universe. A single passage from his paper
will indicate his line of argument, which, it may be added, had already
been partially set forth by Sir John Herschel in his _Outlines of
Astronomy_.

'If there were but one nebula, it would be a curious coincidence were this
one nebula so placed in the distant regions of space as to agree in
direction with a starless spot in our own sidereal system. If there were
but two nebulæ, and both were so placed, the coincidence would be
excessively strange. What, then, shall we say on finding that there are
thousands of nebulæ so placed? Shall we believe that in thousands of cases
these far-removed galaxies happen to agree in their visible positions with
the thin places in our own galaxy? Such a belief is impossible.'

He then applies the same argument to the distribution of the nebulæ as a
whole:--'In that zone of celestial space where stars are excessively
abundant, nebulæ are rare, while in the two opposite celestial spaces that
are farthest removed from this zone, nebulæ are abundant. Scarcely any
nebulæ lie near the galactic circle (or plane of the Milky Way); and the
great mass of them lie round the galactic poles. Can this also be mere
coincidence?' And he concludes, from the whole mass of the evidence, that
'the proofs of a physical connection become overwhelming.'

Nothing could be more clear or more forcible; but Spencer not being an
astronomer, and writing in a comparatively little read periodical, the
astronomical world hardly noticed him; and it was from ten to fifteen years
later, when Mr. R.A. Proctor, by his laborious charts and his various
papers read before the Royal and Royal Astronomical Societies from 1869 to
1875, compelled the attention of the scientific world, and thus did more
perhaps than any other man to establish firmly the grand and far-reaching
principle of the essential unity of the stellar universe, which is now
accepted by almost every astronomical writer of eminence in the civilised
world.


THE EVOLUTION OF THE STELLAR UNIVERSE

Amid the enormous mass of observations and of suggestive speculation upon
this great and most interesting problem, it is difficult to select what is
most important and most trustworthy. But the attempt must be made, because,
unless my readers have some knowledge of the most important facts bearing
upon it (besides those already set forth), and also learn something of the
difficulties that meet the inquirer into causes at every step of his way,
and of the various ideas and suggestions which have been put forth to
account for the facts and to overcome the difficulties, they will not be in
a position to estimate, however imperfectly, the grandeur, the marvel, and
the mystery of the vast and highly complex universe in which we live and of
which we are an important, perhaps the most important, if not the only
permanent outcome.


THE SUN A TYPICAL STAR

It being now a recognised fact that the stars are suns, some knowledge of
our own sun is an essential preliminary to an inquiry into their nature,
and into the probable changes they have undergone.

The fact that the sun's density is only one-fourth that of the earth, or
less than one and a half times that of water, demonstrates that it cannot
be solid, since the force of gravity at its surface being twenty-six and a
half times that at the earth's surface, the materials of a solid globe
would be so compressed that the resulting density would be at least twenty
times greater instead of four times less than that of the earth. All the
evidence goes to show that the body of the sun is really gaseous, but so
compressed by its gravitative force as to behave more like a liquid. A few
figures as to the vast dimensions of the sun and the amount of light and
heat emitted by it will enable us better to understand the phenomena it
presents, and the interpretation of those phenomena.

Proctor estimated that each square inch of the sun's surface emitted as
much light as twenty-five electric arcs; and Professor Langley has shown by
experiment that the sun is 5300 times brighter, and eighty-seven times
hotter than the white-hot metal in a Bessemer converter. The actual amount
of solar heat received by the earth is sufficient, if wholly utilised, to
keep a three-horse-power engine continually at work on every square yard of
the surface of our globe. The size of the sun is such, that if the earth
were at its centre, not only would there be ample space for the moon's
orbit, but sufficient for another satellite 190,000 miles beyond the moon,
all revolving inside the sun. The mass of matter in the sun is 745 times
greater than that of all the planets combined; hence the powerful
gravitative force by which they are retained in their distant orbits.

What we see as the sun's surface is the photosphere or outer layer of
gaseous or partially liquid matter kept at a definite level by the power of
gravitation. The photosphere has a granular texture implying some diversity
of surface or of luminosity; although the even contour of the sun's margin
shows that these irregularities are not on a very large scale. This surface
is apparently rent asunder by what are termed sun-spots, which were long
supposed to be cavities, showing a dark interior; but are now thought to be
due to downpours of cooled materials driven out from the sun, and forming
the prominences seen during solar eclipses. They appear to be black, but
around their margin is a shaded border or penumbra formed of elongated
shining patches crossing and over-lapping, something like heaps of straw.
Sometimes brilliant portions overhang the dark spots, and often completely
bridge them over; and similar patches, called faculæ, accompany spots, and
in some cases almost surround them.

Sun-spots are sometimes numerous on the sun's disc, sometimes very few,
and they are of such enormous size that when present they can easily be
seen with the naked eye, protected by a piece of smoked glass; or, better
still, with an ordinary opera-glass similarly protected. They are found to
increase in number for several years, and then to decrease; the maxima
recurring after an average period of eleven years, but with no exactness,
since the interval between two maxima or minima is sometimes only nine and
sometimes as much as thirteen years; while the minima do not occur midway
between two maxima, but much nearer to the succeeding than to the preceding
one. What is more interesting is, that variations in terrestrial magnetism
follow them with great accuracy; while violent commotions in the sun,
indicated by the sudden appearance of faculæ, sun-spots, or prominences on
the sun's limb, are always accompanied by magnetic disturbances on the
earth.


WHAT SURROUNDS THE SUN

It has been well said that what we commonly term the sun is really the
bright spherical nucleus of a nebulous body. This nucleus consists of
matter in the gaseous state, but so compressed as to resemble a liquid or
even a viscous fluid. About forty of the elements have been detected in the
sun by means of the dark lines in its spectrum, but it is almost certain
that all the elements, in some form or other, exist there. This semi-liquid
glowing surface is termed the photosphere, since from it are given out the
light and heat which reach our earth.

Immediately above this luminous surface is what is termed the 'reversing
layer' or absorbing layer, consisting of dense metallic vapours only a few
hundred miles thick, and, though glowing, somewhat cooler than the surface
of the photosphere. Its spectrum, taken, at the moment when the sun is
totally darkened, through a slit which is directed tangentially to the
sun's limb, shows a mass of bright lines corresponding in a large degree to
the dark lines in the ordinary solar spectrum. It is thus shown to be a
vaporous stratum which absorbs the special rays emitted by each element and
forming its characteristic coloured lines, changing them into black lines.
But as coloured lines are not found in this layer corresponding to all the
black lines in the solar spectrum, it is now held that special absorption
must also occur in the chromosphere and perhaps in the corona itself. Sir
Norman Lockyer, in his volume on _Inorganic Evolution_, even goes so far as
to say, that the true 'reversing layer' of the sun--that which by its
absorption produced the dark lines in the solar spectrum--is now shown to
be _not_ the chromosphere itself but a layer above it, of lower
temperature.

Above the reversing layer comes the chromosphere, a vast mass of rosy or
scarlet emanations surrounding the sun to a depth of about 4000 miles. When
seen during eclipses it shows a serrated waving outline, but subject to
great changes of form, producing the prominences already mentioned. These
are of two kinds: the 'quiescent,' which are something like clouds of
enormous extent, and which keep their forms for a considerable time; and
the 'eruptive,' which shoot out in towering tree-like flames or
geyser-like eruptions, and while doing so have been proved to reach
velocities of over 300 miles a second, and subside again with almost equal
rapidity. The chromosphere and its quiescent prominences appear to be truly
gaseous, consisting of hydrogen, helium, and coronium, while the eruptive
prominences always show the presence of metallic vapours, especially of
calcium. Prominences increase in size and number in close accordance with
the increase of sun-spots. Beyond the red chromosphere and prominences is
the marvellous white glory of the corona, which extends to an enormous
distance round the sun. Like the prominences of the chromosphere, it is
subject to periodical changes in form and size, corresponding to the
sun-spot period, but in inverse order, a minimum of sun-spots going with a
maximum extension of the corona. At the total eclipse of July 1878, when
the sun's surface was almost wholly clear, a pair of enormous equatorial
streamers stretched east and west of the sun to a distance of ten millions
of miles, and less extensions of the corona occurred at the poles. At the
eclipses of 1882 and 1883, on the other hand, when sun-spots were at a
maximum, the corona was regularly stellate with no great extensions, but of
high brilliancy. This correspondence has been noted at every eclipse, and
there is therefore an undoubted connection between the two phenomena.

The light of the corona is believed to be derived from three sources--from
incandescent solid or liquid particles thrown out from the sun, from
sunlight reflected from these particles, and from gaseous emissions. Its
spectrum possesses a green ray, which is peculiar to it, and is supposed to
indicate a gas named 'coronium'; in other respects the spectrum is more
like that of reflected sunlight. The enormous extensions of the corona into
great angular streamers seem to indicate electrical repulsive forces
analogous to those which produce the tails of comets.

Connected with the sun's corona is that strange phenomenon, the zodiacal
light. This is a delicate nebulosity, which is often seen after sunset in
spring and before sunrise in autumn, tapering upwards from the sun's
direction along the plane of the ecliptic. Under very favourable conditions
it has been traced in the eastern sky in spring to 180° from the sun's
position, indicating that it extends beyond the earth's orbit.
Long-continued observations from the summit of the Pic du Midi show that
this is really the case, and that it lies almost exactly in the plane of
the sun's equator. It is therefore held to be produced by the minute
particles thrown off the sun, through those coronal wings and streamers
which are visible only during solar eclipses.

The careful study of the solar phenomena has very clearly established the
fact that none of the sun's envelopes, from the reversing layer to the
corona itself, is in any sense an atmosphere. The combination of enormous
gravitative force with an amount of heat which turns all the elements into
the liquid or gaseous state, leads to consequences which it is difficult
for us to follow or comprehend. There is evidently constant internal
movement or circulation in the interior of the sun, resulting in the
faculæ, the sun-spots, the intensely luminous photosphere, and the
chromosphere with its vast flaming coruscations and eruptive protuberances.
But it seems impossible that this incessant and violent movement can be
kept up without some great and periodical or continuous inrush of fresh
materials to renew the heat, keep up the internal circulation, and supply
the waste. Perhaps the movement of the sun through space may bring him into
contact with sufficiently large masses of matter to continually excite that
internal movement without which the exterior surface would rapidly become
cool and all planetary life cease. The various solar envelopes are the
result of this internal agitation, uprushes, and explosions, while the vast
white corona is probably of little more density than comets' tails,
probably even of less density, since comets not unfrequently rush through
its midst without suffering any loss of velocity. The fact that none of the
solar envelopes are visible to us until the light of the photosphere is
completely shut off, and that they all vanish the very instant the first
gleam of direct sunlight reaches us, is another proof of their extreme
tenuity, as is also the sharply defined edge of the sun's disc. The
envelopes therefore consist partly of liquid or vaporous matter, in a very
finely divided state, driven off by explosions or by electrical forces, and
this matter, rapidly cooling, becomes solidified into minutest particles,
or even physical molecules. Much of this matter continually falls back on
the sun's surface, but a certain quantity of the very finest dust is
continually driven away by electrical repulsion, so as to form the corona
and the zodiacal light. The vast coronal streamers and the still more
extensive ring of the zodiacal light are therefore in all probability due
to the same causes, and have a similar physical constitution with the tails
of comets.

As the whole of our sunlight must pass through both the reversing layer and
the red chromosphere, its colour must be somewhat modified by them. Hence
it is believed that, if they were absent, not only would the light and heat
of the sun be considerably greater, but its colour would be a purer white,
tending towards bluish rather than towards the yellowish tinge it actually
possesses.


THE NEBULAR AND METEORITIC HYPOTHESES

As the constitution of the sun, and its agency in producing magnetism and
electricity in the matter and orbs around it, afford us our best guide to
the constitution of the stars and nebulæ, and to their possible action on
each other, and even upon our earth, so the mode of evolution of the sun
and solar system, from some pre-existing condition, is likely to help us
towards gaining some knowledge of the constitution of the stellar universe
and the processes of change going on there.

At the very commencement of the nineteenth century the great mathematician
Laplace published his Nebular Theory of the Origin of the Solar System; and
although he put it forth merely as a suggestion, and did not support it
with any numerical or physical data, or by any mathematical processes, his
great reputation, and its apparent probability and simplicity, caused it to
be almost universally accepted, and to be extended so as to apply to the
evolution of the stellar universe. This theory, very briefly stated, is,
that the whole of the matter of the solar system once formed a globular or
spheroidal mass of intensely heated gases, extending beyond the orbit of
the outermost planet, and having a slow motion of revolution about an axis.
As it cooled and contracted, its rate of revolution increased, and this
became so great that at successive epochs it threw off rings, which, owing
to slight irregularities, broke up, and, gravitating together, formed the
planets. The contraction continuing, the sun, as we now see it, was the
result.

For about half a century this nebular hypothesis was generally accepted,
but during the last thirty years so many objections and difficulties have
been suggested, that it has been felt impossible to retain it even as a
working hypothesis. At the same time another hypothesis has been put forth
which seems more in accordance with the facts of nature as we find them in
our own solar system, and which is not open to any of the objections
against the nebular theory, even if it introduces a few new ones.

A fundamental objection to Laplace's theory is, that in a gas of such
extreme tenuity as the solar nebula must have been, even when it extended
only to Saturn or Uranus, it could not possibly have had any cohesion, and
therefore could not have given off whole rings at distant intervals, but
only small fragments continuously as condensation went on, and these,
rapidly cooling, would form solid particles, a kind of meteoric dust, which
might aggregate into numerous small planets, or might persist for
indefinite periods, like the rings of Saturn or the great ring of the
Asteroids.

Another equally vital objection is, that, as the nebula when extending
beyond the orbit of Neptune could have had a mean density of only about the
two-hundred millionth of our air at sea level, it must have been many
hundred times less dense than this at and near its outer surface, and would
there be exposed to the cold of stellar space--a cold that would solidify
hydrogen. It is thus evident that the gases of all the metallic and other
solid elements could not possibly exist as such, but would rapidly, perhaps
almost instantaneously, become first liquid and then solid, forming
meteoric dust even before contraction had gone far enough to produce such
increased rotation as would throw off any portion of the gaseous matter.

Here we have the foundations of the meteoritic hypothesis which is now
steadily making its way. It is supported by the fact that we everywhere
find proofs of such solid matter in the planetary spaces around us. It
falls continually upon the earth. It can be collected on the Arctic and
Alpine snows. It occurs everywhere in the deepest abysses of the ocean
where there are not sufficient organic deposits to mask it. It constitutes,
as has now been demonstrated, the rings of Saturn. Thousands of vast rings
of solid particles circulate around the sun, and when our earth crosses any
of these rings, and their particles enter our atmosphere with planetary
velocity, the friction ignites them and we see falling stars. Comets'
tails, the sun's corona, and the zodiacal light are three strange
phenomena, which, though wholly insoluble on any theory of gaseous
formation, receive their intelligible explanation by means of excessively
minute solid particles--microscopic cosmic dust--driven outward by the
tremendous electrical repulsions that emanate from the sun.

Having these and other proofs that solid matter, ranging in size, perhaps,
from the majestic orbs of Jupiter and Saturn down to the inconceivably
minute particles driven millions of miles into space to form a comet's
tail, does actually exist everywhere around us, and by collisions between
the particles or with planetary atmospheres can produce heat and light and
gaseous emanations, we find a basis of fact and observation for the
meteoritic hypothesis which Laplace's nebular, and essentially gaseous,
theory does not possess.

During the latter half of the nineteenth century several writers suggested
this idea of the possible formation of the Solar System, but so far as I am
aware, the late R.A. Proctor was the first to discuss it in any detail, and
to show that it explained many of the peculiarities in the size and
arrangement of the planets and their satellites which the nebular
hypothesis did not explain. This he does at some length in the chapter on
meteors and comets in his _Other Worlds than Ours_, published in 1870. He
assumed, instead of the fire-mist of Laplace, that the space now occupied
by the solar system, and for an unknown distance around it, was occupied by
vast quantities of solid particles of all the kinds of matter which we now
find in the earth, sun, and stars. This matter was dispersed somewhat
irregularly, as we see that all the matter of the universe is now
distributed; and he further assumed that it was all in motion, as we now
know that all the stars and other cosmical masses are, and must be, in
motion towards or around some centre.

Under these conditions, wherever the matter was most aggregated, there
would be a centre of attraction through gravitation, which would
necessarily lead to further aggregation, and the continual impacts of such
aggregating matter would produce heat. In course of time, if the supply of
cosmic matter was ample (as the result shows that it must have been,
whatever theory we adopt), our sun, thus formed, would approximate to its
present mass and acquire sufficient heat by collision and gravitation to
convert its whole body into the liquid or gaseous condition. While this was
going on, subordinate centres of aggregation might form, which would
capture a certain proportion of the matter flowing in under the attraction
of the central mass, while, owing to the nearly uniform direction and
velocity with which the whole system was revolving, each subordinate centre
would revolve around the central mass, in somewhat different planes, but
all in the same direction.

Mr. Proctor shows the probability that the largest outside aggregation
would be at a great distance from the central mass, and this having once
been formed, any centres farther away from the sun would be both smaller
and very remote, while those inside the first would, as a rule, become
smaller as they were nearer the centre. The heated condition of the earth's
interior would thus be due, not to the primitive heat of matter in a
gaseous state out of which it was formed--a condition physically
impossible--but would be acquired in the process of aggregation by the
collisions of meteoric masses falling on it, and by its own gravitative
force producing continuous condensation and heat.

On this view Jupiter would probably be formed first, and after him at very
great distances, Saturn, Uranus, and Neptune; while the inner aggregations
would be smaller, as the much greater attractive power of the sun would
give them comparatively little opportunity of capturing the meteoric matter
that was continuously flowing towards him.


THE METEORITIC NATURE OF THE NEBULÆ

Having thus reached the conclusion that wherever apparently nebulous matter
exists within the limits of the solar system it is not gaseous but consists
of solid particles, or, if heated gases are associated with the solid
matter they can be accounted for by the heat due to collisions either with
other solid particles or with accumulations of gases at a low temperature,
as when meteorites enter our atmosphere, it was an easy step to consider
whether the cosmic nebulæ and stars may not have had a similar origin.

From this point of view the nebulæ are supposed to be vast aggregations of
meteorites or cosmic dust, or of the more persistent gases, revolving with
circular or spiral motions, or in irregular streams, and so sparsely
scattered that the separate particles of dust may be miles--perhaps
hundreds of miles--apart; yet even those nebulæ, only visible by the
telescope, may contain as much matter as the whole solar system. From this
simple origin, by steps which can be observed in the skies, almost all the
forms of suns and systems can be traced by means of the known laws of
motion, of heat-production, and of chemical action. The chief English
advocate of this view at the present time is Sir Norman Lockyer, who, in
numerous papers, and in his works on _The Meteoritic Hypothesis_ and
_Inorganic Evolution_, has developed it in detail, as the result of many
years' continuous research, aided by the contributory work of continental
and American astronomers. These views are gradually spreading among
astronomers and mathematicians, as will be seen by the very brief outline
which will now be given of the explanations they afford of the main groups
of phenomena presented by the stellar universe.


DR. ROBERTS ON SPIRAL NEBULÆ

Dr. Isaac Roberts, who possesses one of the finest telescopes constructed
for photographing stars and nebulæ, has given his views on stellar
evolution, in _Knowledge_ of February 1897, illustrated by four beautiful
photographs of spiral nebulæ. These curious forms were at first thought to
be rare, but are now found to be really very numerous when details are
brought out by the camera. Many of the very large and apparently quite
irregular nebulæ, like the Magellanic Clouds, are found to have faint
indications of spiral structure. As more than ten thousand nebulæ are now
known, and new ones are continually being discovered, it will be a long
time before these can all be carefully studied and photographed, but
present indications seem to show that a considerable proportion of them
will exhibit spiral forms.

Dr. Roberts tells us that all the spiral nebulæ he has photographed are
characterised by having a nucleus surrounded by dense nebulosity, most of
them being also studded with stars. These stars are always arranged more or
less symmetrically, following the curves of the spiral, while outside the
visible nebula are other stars arranged in curves strongly suggesting a
former greater extension of the nebulous matter. This is so marked a
feature that it at once leads to a possible explanation of the numerous
slightly curved lines of stars found in every part of the heavens, as being
the result of their origin from spiral nebulæ whose material substance has
been absorbed by them.

Dr. Roberts proposes several problems in relation to these bodies: Of what
materials are spiral nebulæ composed? Whence comes the vortical motion
which has produced their forms? The material he finds in those faint clouds
of nebulous matter, often of vast extent, that exist in many parts of the
sky, and these are so numerous that Sir William Herschel alone recorded the
positions of fifty-two such regions, many of which have been confirmed by
recent photographs. Dr. Roberts considers these to be either gaseous or
with discrete solid particles intermixed. He also enumerates smaller
nebulous masses undergoing condensation and segregation into more regular
forms; spiral nebulæ in various stages of condensation and of aggregation;
elliptic nebulæ; and globular nebulæ. In the last three classes there is
clear evidence, on every photograph that has been taken, that condensation
into stars or star like forms is now going on.

He adopts Sir Norman Lockyer's view that collisions of meteorites within
each swarm or cloud would produce luminous nebulosity; so also would
collisions between separate swarms of meteorites produce the conditions
required to account for the vortical motions and the peculiar distribution
of the nebulosity in the spiral nebulæ. Almost any collision between
unequal masses of diffused matter would, in the absence of any massive
central body round which they would be forced to revolve, lead to spiral
motions. It is to be noted that, although the stars formed in the spiral
convolutions of the nebulæ follow those curves, and retain them after the
nebulous matter has been all absorbed by them, yet, whenever such a nebula
is seen by us edgewise, the convolutions with their enclosed stars will
appear as straight lines; and thus not only numbers of star groups arranged
in curves, but also those which form almost perfect straight lines, may
possibly be traced back to an origin from spiral nebulæ.

Motion being a necessary result of gravitation, we know that every star,
planet, comet, or nebula must be in motion through space, and these
motions--except in systems physically connected or which have had a common
origin--are, apparently, in all directions. How these motions originated
and are now regulated we do not know; but there they are, and they furnish
the motive power of the collisions, which, when affecting large bodies or
masses of diffused matter, lead to the formation of the various kinds of
permanent stars; while when smaller masses of matter are concerned those
temporary stars are formed which have interested astronomers in all ages.
It must be noted that although the motions of the single stars appear to be
in straight lines, yet the spaces through which they have been observed to
move are so small that they may really be moving in curved orbits around
some central body, or the centre of gravity of some aggregation of stars
bright and dark, which may itself be comparatively at rest. There may be
thousands of such centres around us, and this may sufficiently explain the
apparent motions of stars in all directions.


A SUGGESTION AS TO THE FORMATION OF SPIRAL NEBULÆ

In a remarkable paper in the Astrophysical Journal (July 1901), Mr. T.C.
Chamberlin suggests an origin for the spiral nebulæ, as well as of swarms
of meteorites and comets, which seems likely to be a true, although perhaps
not the only one.

There is a well-known principle which shows that when two bodies in space,
of stellar size, pass within a certain distance of each other, the smaller
one will be liable to be torn into fragments by the differential attraction
of the larger and denser body. This was originally proved in the case of
gaseous and liquid bodies, and the distance within which the smaller one
will be disrupted (termed the Roche limit) is calculated on the supposition
that the disrupted body is a liquid mass. Mr. Chamberlin shows, however,
that a solid body will also be disrupted at a lesser distance dependent on
its size and cohesive strength; but, as the size of the two bodies
increases, the distance at which disruption will occur increases also, till
with very large bodies, such as suns, it becomes almost as large as in the
case of liquids or gases.

The disruption occurs from the well-known law of differential gravitation
on the two sides of a body leading to tidal deformation in a liquid, and to
unequal strain in a solid. When the changes of gravitative force take place
slowly, and are also small in amount, the tides in liquids or strains in
solids are very small, as in the case of our earth when acted on by the sun
and moon, the result is a small tide in the ocean and atmosphere, and no
doubt also in the molten interior, to which the comparatively thin crust
may partially adjust itself. But if we suppose two dark or luminous suns
whose proper motions are in such a direction as to bring them near each
other, then, as they approach, each will be deflected towards the other,
and will pass round their common centre of gravity with immense velocity,
perhaps hundreds of miles in a second. At a considerable distance they will
begin to produce tidal elongation towards and away from each other, but
when the disruptive limit is nearly reached, the gravitative forces will be
increasing so rapidly that even a liquid mass could not adjust its shape
with sufficient quickness and the tremendous internal strains would produce
the effects of an explosion, tearing the whole mass (of the smaller of the
two) into fragments and dust.

But it is also shown that, during the entire process, the two elongated
portions of the originally spherical mass would be so acted upon by
gravity as to produce increasing rotation, which as the crisis approached
would extend the elongation, and aid in the explosive result. This rapid
rotation of the elongated mass would, when the disruption occurred,
necessarily give to the fragments a whirling or spiral motion, and thus
initiate a spiral nebula of a size and character dependent on the size and
constitution of the two masses, and on the amount of the explosive forces
set up by their approach.

There is one very suggestive phenomenon which seems to prove that this _is_
one of the modes of formation of spiral nebulæ. When the explosive
disruption occurs the two protuberances or elongations of the body will fly
apart, and having also a rapid rotatory movement, the resulting spiral will
necessarily be a double one. Now, it is the fact that almost all the
well-developed spiral nebulæ have two such arms opposite to each other, as
beautifully shown in M. 100 Comæ, M. 51 Canum, and others photographed by
Dr. I. Roberts. It does not seem likely that any other origin of these
nebulæ should give rise to a double rather than to a single spiral.


THE EVOLUTION OF DOUBLE STARS

The advance in knowledge of double and multiple stars has been wonderfully
rapid, numerous observers having devoted themselves to this special branch.
Many thousands were discovered during the first half of the nineteenth
century, and as telescopic power increased new ones continued to flow in by
hundreds and thousands, and there has been recently published by the
Yerkes Observatory a catalogue of 1290 such stars, discovered between 1871
and 1899 by one observer, Mr. S.W. Burnham. All these have been found by
the use of the telescope, but during the last quarter of a century the
spectroscope has opened up a new world of double stars of enormous extent
and the highest interest.

The telescopic binaries which have been observed for a sufficient time to
determine their orbits, range from periods of about eleven years as a
minimum up to hundreds and even more than a thousand years. But the
spectroscope reveals the fact that the many thousands of telescopic
binaries form only a very small part of the binary systems in existence.
The overwhelming importance of this discovery is, that it carries the times
of revolution from the minimum of the telescopic doubles downward in
unbroken series through periods of a few years, to those reckoned by
months, by days, and even by hours. And with this reduction of period there
necessarily follows a corresponding reduction of distance, so that
sometimes the two stars must be in contact, and thus the actual birth or
origin of a double star has been observed to occur, even though not
actually seen. This mode of origin was indeed anticipated by Dr. Lee of
Chicago in 1892, and it has been confirmed by observation in the short
space of ten years.

In a remarkable communication to _Nature_ (September 12th, 1901) Mr.
Alexander W. Roberts of Lovedale, South Africa, gives some of the main
results of this branch of inquiry. Of course all the variable stars are to
be found among the spectroscopic binaries. They consist of that portion of
the class in which the plane of the orbit is directed towards us, so that
during their revolution one of the pair either wholly or partially eclipses
the other. In some of these cases there are irregularities, such as double
maxima and minima of unequal lengths, which may be due to triple systems or
to other causes not yet explained, but as they all have short periods and
always appear as one star in the most powerful telescopes, they form a
special division of the spectroscopic binary systems.

There are known at present twenty-two variables of the Algol type, that is,
stars having each a dark companion very close to it which obscures it
either wholly or partially during every revolution. In these cases the
density of the systems can be approximately determined, and they are found
to be, on the average, only one-fifth that of water, or one-eighth that of
our sun. But as many of them are as large as our sun, or even considerably
larger, it is evident that they must be wholly gaseous, and, even if very
hot, of a less complex constitution than our luminary. Mr. A.W. Roberts
tells us that five out of these twenty-two variables revolve _in absolute
contact_ forming systems of the shape of a dumb-bell. The periods vary from
twelve days to less than nine hours; and, starting from these, we now have
a continuous series of lengthening periods up to the twin stars of Castor
which require more than a thousand years to complete their revolution.

During his observations of the above five stars, Mr. Roberts states that
one, X Carinæ, was found to have parted company, so that instead of being
actually united to its companion the two are now at a distance apart equal
to one-tenth of their diameters, and he may thus be said to have been
almost a witness of the birth of a stellar system.

A year later we find the record (in _Knowledge_, October 1902) of Professor
Campbell's researches at the Lick Observatory. He states that, out of 350
stars observed spectroscopically, one in eight is a spectroscopic binary;
and so impressed is he with their abundance that, as accuracy of
measurement increases, he believes that _the star that is not a
spectroscopic binary will prove to be the rare exception_! Professor G.
Darwin had already shown that the 'dumb-bell' was a figure of equilibrium
in a rotating mass of fluid; and we now find proofs that such figures
exist, and that they form the starting-point for the enormous and
ever-increasing quantities of spectroscopic binary star-systems that are
now known. The origin of these binary stars is also of especial interest as
giving support to Professor Darwin's well-known explanation of the origin
of the moon by disruption from the earth, owing to the very rapid rotation
of the parent planet. It now appears that suns often subdivide in the same
manner, but, owing perhaps to their intensely heated gaseous state they
seem usually to form nearly equal globes. The evolution of this special
form of star-system is therefore now an observed fact; though it by no
means follows that all double stars have had the same mode of origin.


CLUSTERS OF STARS AND VARIABLES

The clusters of stars, which are tolerably abundant in the heavens and
offer so many strange and beautiful forms to the telescopist, are yet
among the most puzzling phenomena the philosophic astronomer has to deal
with.

Many of these clusters which are not very crowded and of irregular forms,
strongly suggest an origin from the equally irregular and fantastic forms
of nebulæ by a process of aggregation like that which Dr. Roberts describes
as developing within the spiral nebulæ. But the dense globular clusters
which form such beautiful telescopic objects, and in some of which more
than six thousand stars have been counted besides considerable numbers so
crowded in the centre as to be uncountable, are more difficult to explain.
One of the problems suggested by these clusters is as to their stability.
Professor Simon Newcomb remarks on this point as follows: 'Where thousands
of stars are condensed into a space so small, what prevents them from all
falling together into one confused mass? Are they really doing so, and will
they ultimately form a single body? These are questions which can be
satisfactorily answered only by centuries of observation; they must
therefore be left to the astronomers of the future.'

There are, however, some remarkable features in these clusters which afford
possible indications of their origin and essential constitution. When
closely examined most of them are seen to be less regular than they at
first appear. Vacant spaces can be noted in them; even rifts of definite
forms. In some there is a radiated structure; in others there are curved
appendages; while some have fainter centres. These features are so exactly
like what are found, in a more pronounced form, in the larger nebulæ, that
we can hardly help thinking that in these clusters we have the result of
the condensation of very large nebulæ, which have first aggregated towards
numerous centres, while these agglomerations have been slowly drawn towards
the common centre of gravity of the whole mass. It is suggestive of this
origin that while the smaller telescopic nebulæ are far removed from the
Milky Way, the larger ones are most abundant near its borders; while the
star-clusters are excessively abundant on and near the Milky Way, but very
scarce elsewhere, except in or near vast nebulæ like the Magellanic Clouds.
We thus see that the two phenomena may be complementary to each other, the
condensation of nebulæ having gone on most rapidly where material was most
abundant, resulting in numerous star-clusters where there are now few
nebulæ.

There is one striking feature of the globular clusters which calls for
notice; the presence in some of them of enormous quantities of variable
stars, while in others few or none can be found. The Harvard Observatory
has for several years devoted much time to this class of observations, and
the results are given in Professor Newcomb's recent volume on 'The Stars.'
It appears that twenty-three clusters have been observed spectroscopically,
the number of stars examined in each cluster varying from 145 up to 3000,
the total number of stars thus minutely tested being 19,050. Out of this
total number 509 were found to be variable; but the curious fact is, the
extreme divergence in the proportion of variables to the whole number
examined in the several clusters. In two clusters, though 1279 stars were
examined, not a single variable was found. In three others the proportion
was from one in 1050 to one in 500. Five more ranged up to one in 100, and
the remainder showed from that proportion up to one in seven, 900 stars
being examined in the last mentioned cluster of which 132 were variable!

When we consider that variable stars form only a portion, and necessarily a
very small proportion, of binary systems of stars, it follows that in all
the clusters which show a large proportion of variables, a very much larger
proportion--in some cases perhaps all, must be double or multiple stars
revolving round each other. With this remarkable evidence, in addition to
that adduced for the prevalence of double stars and variables among the
stars in general, we can understand Professor Newcomb adding his testimony
to that of Professor Campbell already quoted, that 'it is probable that
among the stars in general, single stars are the exception rather than the
rule. If such be the case, the rule should hold yet more strongly among the
stars of a condensed cluster.'


THE EVOLUTION OF THE STARS

So long as astronomers were limited to the use of the telescope only, or
even the still greater powers of the photographic plate, nothing could be
learnt of the actual constitution of the stars or of the process of their
evolution. Their apparent magnitudes, their movements, and even the
distances of a few could be determined; while the diversity of their
colours offered the only clue (a very imperfect one) even to their
temperature. But the discovery of spectrum analysis has furnished the means
of obtaining some definite knowledge of the physics and chemistry of the
stars, and has thus established a new branch ofscience--Astrophysics--which
has already attained large proportions, and which furnishes the materials
for a periodical and some important volumes. This branch of the subject
is very complex, and as it is not directly connected with our present
inquiry, it is only referred to again in order to introduce such of its
results as bear upon the question of the classification and evolution
of the stars.

By a long series of laboratory experiments it has been shown that numerous
changes occur in the spectra of the elements when subjected to different
temperatures, ranging upwards to the highest attainable by means of a
battery producing an electric spark several feet long. These changes are
not in the relative position of the bands or dark lines, but in their
number, breadth, and intensity. Other changes are due to the density of the
medium in which the elements are heated, and to their chemical condition as
to purity; and from these various modifications and their comparison with
the solar spectrum and those of its appendages, it has become possible to
determine, from the spectrum of a star, not only its temperature as
compared with that of the electric spark and of the sun, but also its place
in a developmental series.

The first general result obtained by this research is, that the bluish
white or pure white stars, having a spectrum extending far towards the
violet end, and which exhibits the coloured bands of gases only, usually
hydrogen and helium, are the hottest. Next come those with a shorter
spectrum not extending so far towards the violet end, and whose light is
therefore more yellow in tint. To this group our sun belongs; and they are
all characterised like it by dark lines due to absorption, and by the
presence of metals, especially iron, in a gaseous state. The third group
have the shortest spectra and are of a red colour, while their spectra
contain lines denoting the presence of carbon. These three groups are often
spoken of as 'gaseous stars,' 'metallic stars,' and 'carbon stars.' Other
astronomers call the first group 'Sirian stars,' because Sirius, though not
the hottest, is a characteristic type; the second being termed 'solar
stars'; others again speak of them as stars of Class I., Class II., etc.,
according to the system of classification they have adopted. It was soon
perceived, however, that neither the colour nor the temperature of stars
gave much information as to their nature and state of development, because,
unless we supposed the stars to begin their lives already intensely hot
(and all the evidence is against this), there must be a period during which
heat increases, then one of maximum heat, followed by one of cooling and
final loss of light altogether. The meteoritic theory of the origin of all
luminous bodies in the heavens, now very widely adopted, has been used, as
we have seen, to explain the development of stars from nebulæ, and its
chief exponent in this country, Sir Norman Lockyer, has propounded a
complete scheme of stellar evolution and decay which may be here briefly
outlined:

Beginning with nebulæ, we pass on to stars having banded or fluted
spectra, indicating comparatively low temperatures and showing bands or
lines of iron, manganese, calcium, and other metals. They are more or less
red in colour, Antares in the Scorpion being one of the most brilliant red
stars known. These stars are supposed to be in the process of aggregation,
to be continually increasing in size and heat, and thus to be subject to
great disturbances. Alpha Cygni has a similar spectrum but with more
hydrogen, and is much hotter. The increase of heat goes on through Rigel
and Beta Crucis, in which we find mainly hydrogen, helium, oxygen,
nitrogen, and also carbon, but only faint traces of metals. Reaching the
hottest of all--Epsilon Orionis and two stars in Argo--hydrogen is
predominant, with traces of a few metals and carbon. The cooling series is
indicated by thicker lines of hydrogen and thinner lines of the metallic
elements, through Sirius, to Arcturus and our sun, thence to 19 Piscium,
which shows chiefly flutings of carbon, with a few faint metallic lines.
The process of further cooling brings us to the dark stars.

We have here a complete scheme of evolution, carrying us from those
ill-defined but enormously diffused masses of gas and cosmic dust we know
as nebulæ, through planetary nebulæ, nebulous stars, variable and
double-stars, to red and white stars and on to those exhibiting the most
intense blue-white lustre. We must remember, however, that the most
brilliant of these stars, showing a gaseous spectrum and forming the
culminating point of the ascending series, are not necessarily hotter than,
or even so hot as, some of those far down on the descending scale; since
it is one of the apparent paradoxes of physics that a body may become
hotter during the very process of contraction through loss of heat. The
reason is that by cooling it contracts and thus becomes denser, that a
portion of its mass falls towards its centre, and in doing so produces an
amount of heat which, though absolutely less than the heat lost in cooling,
will under certain conditions cause the reduced surface to become hotter.
The essential point is, that the body in question must be wholly gaseous,
allowing of free circulation from surface to centre. The law, as given by
Professor S. Newcomb, is as follows:--

'_When a spherical mass of incandescent gas contracts through the loss of
its heat by radiation into space, its temperature continually becomes
higher as long as the gaseous condition is retained._'

To put it in another way: if the compression was caused by external force
and no heat was lost, the globe would get hotter by a calculable amount for
each unit of contraction. But the heat lost in causing a similar amount of
contraction is so little more than the increase of heat produced by
contraction, that the slightly diminished total heat in a smaller bulk
causes the temperature of the mass to increase.

But if, as there is reason to believe, the various types of stars differ
also in chemical constitution, some consisting mainly of the more permanent
gases, while in others the various metallic and non-metallic elements are
present in very different proportions, there should really be a
classification by constitution as well as by temperature, and the course of
evolution of the differently constituted groups may be to some extent
dissimilar.

With this limitation the process of evolution and decay of sun through a
cycle of increasing and decreasing temperature, as suggested by Sir Norman
Lockyer, is clear and suggestive. During the ascending series the star is
growing both in mass and heat, by the continual accretion of meteoritic
matter either drawn to it by gravitation or falling towards it through the
proper motions of independent masses. This goes on till all the matter for
some distance around the star has been utilised, and a maximum of size,
heat, and brilliancy attained. Then the loss of heat by radiation is no
longer compensated by the influx of fresh matter, and a slow contraction
occurs accompanied by a slightly increased temperature. But owing to the
more stable conditions continuous envelopes of metals in the gaseous state
are formed, which check the loss of heat and reduce the brilliancy of
colour; whence it follows that bodies like our sun may be really hotter
than the most brilliant white stars, though not giving out quite so much
heat. The loss of heat is therefore reduced; and this may serve to account
for the undoubted fact that during the enormous epochs of geological time
there has been very little diminution in the amount of heat we have
received from the sun.

On the general question of the meteoritic hypothesis one of our first
mathematicians, Professor George Darwin, has thus expressed his views: 'The
conception of the growth of the planetary bodies by the aggregation of
meteorites is a good one, and perhaps seems more probable than the
hypothesis that the whole solar system was gaseous.' I may add, that one of
the chief objections made to it, that meteorites are too complex to be
supposed to be the primitive matter out of which suns and worlds have been
made, does not seem to me valid. The primitive matter, whatever it was, may
have been used up again and again, and if collisions of large solid globes
ever occur--and it is assumed by most astronomers that they must sometimes
occur--then meteoric particles of all sizes would be produced which might
exhibit any complexity of mineral constitution. The material universe has
probably been in existence long enough for all the primitive elements to
have been again and again combined into the minerals found upon the earth
and many others. It cannot be too often repeated that no explanation--no
theory--can ever take us to the beginning of things, but only one or two
steps at a time into the dim past, which may enable us to comprehend,
however imperfectly, the processes by which the world, or the universe, as
it is, has been developed out of some earlier and simpler condition.



CHAPTER VII

ARE THE STARS INFINITE IN NUMBER?


Most of the critics of my first short discussion of this subject laid great
stress upon the impossibility of proving that the universe, a part of which
we see, is not infinite; and a well-known astronomer declared that unless
it can be demonstrated that our universe is finite the entire argument
founded upon our position within it fall to the ground. I had laid myself
open to this objection by rather incautiously admitting that if the
preponderance of evidence pointed in this direction any inquiry as to our
place in the universe would be useless, because as regards infinity there
can be no difference of position. But this statement is by no means exact,
and even in an infinite universe of matter containing an infinite number of
stars, such as those we see, there might well be such infinite diversities
of distribution and arrangement as would give to certain positions all the
advantages which I submit we actually possess. Supposing, for example, that
beyond the vast ring of the Milky Way the stars rapidly decrease in number
in all directions for a distance of a hundred or a thousand times the
diameter of that ring, and that then for an equal distance they slowly
increase again and become aggregated into systems or universes totally
distinct from ours in form and structure, and so remote that they can
influence us in no way whatever. Then, I maintain, our position within our
own stellar universe might have exactly the same importance, and be equally
suggestive, as if ours were the only material universe in existence--as if
the apparent diminution in the number of stars (which is an observed fact)
indicated a continuous diminution, leading at some unknown distance to
entire absence of luminous--that is, of active, energy-emitting
aggregations of matter.[1] As to whether there are such other material
universes or not I offer no opinion, and have no belief one way or the
other. I consider all speculations as to what may or may not exist in
infinite space to be utterly valueless. I have limited my inquiries
strictly to the evidence accumulated by modern astronomers, and to direct
inferences and logical deductions from that evidence. Yet, to my great
surprise, my chief critic declares that 'Dr. Wallace's underlying error is,
indeed, that he has reasoned from the area which we can embrace with our
limited perceptions to the infinite beyond our mental or intellectual
grasp.' I have distinctly _not_ done this, but many astronomers have done
so. The late Richard Proctor not only continually discussed the question of
infinite matter as well as infinite space, but also argued, from the
supposed attributes of the Deity, for the necessity of holding this
material universe to be infinite, and the last chapter of his _Other Worlds
than Ours_ is mainly devoted to such speculations. In a later work, _Our
Place among Infinities_, he says that 'the teachings of science bring us
into the presence of the unquestionable infinities of time and of space,
and the presumable infinities of matter and of operation--hence therefore
into the presence of infinity of energy. But science teaches us nothing
about these infinities as such. They remain none the less inconceivable,
however clearly we may be taught to recognise their reality.' All this is
very reasonable, and the last sentence is particularly important.
Nevertheless, many writers allow their reasonings from facts to be
influenced by these ideas of infinity. In Proctor's posthumous work, _Old
and New Astronomy_, the late Mr. Ranyard, who edited it, writes: 'If we
reject as abhorrent to our minds the supposition that the universe is not
infinite, we are thrown back on one of two alternatives--either the ether
which transmits the light of the stars to us is not perfectly elastic, or a
large proportion of the light of the stars is obliterated by dark bodies.'
Here we have a well-informed astronomer allowing his abhorrence of the idea
of a finite universe to affect his reasoning on the actual phenomena we can
observe--doing in fact exactly what my critic erroneously accuses me of
doing. But setting aside all ideas and prepossessions of the kind here
indicated, let us see what are the actual facts revealed by the best
instruments of modern astronomy, and what are the natural and logical
inferences from those facts.


ARE THE STARS INFINITE IN NUMBER?

The views of those astronomers who have paid attention to this subject are,
on the whole, in favour of the view that the stellar universe is limited in
extent and the stars therefore limited in number. A few quotations will
best exhibit their opinions on this question, with some of the facts and
observations on which they are founded.

Miss A.M. Clerke, in her admirable volume, _The System of the Stars_, says:
'The sidereal world presents us, to all appearance, with a finite
system.... The probability amounts almost to certainty that star-strewn
space is of measurable dimensions. For from innumerable stars a limitless
sum-total of radiations should be derived, by which darkness would be
banished from our skies; and the "intense inane," glowing with the mingled
beams of suns individually indistinguishable, would bewilder our feeble
senses with its monotonous splendour.... Unless, that is to say, light
suffer some degree of enfeeblement in space.... But there is not a particle
of evidence that any such toll is exacted; contrary indications are strong;
and the assertion that its payment is inevitable depends upon analogies
which may be wholly visionary. We are then, for the present, entitled to
disregard the problematical effect of a more than dubious cause.'

Professor Simon Newcomb, one of the first of American mathematicians and
astronomers, arrives at a similar conclusion in his most recent volume,
_The Stars_ (1902). He says, in his conclusions at the end of the work:
'That collection of stars which we call the universe is limited in extent.
The smallest stars that we see with the most powerful telescopes are not,
for the most part, more distant than those a grade brighter, but are mostly
stars of less luminosity situate in the same regions' (p. 319). And on page
229 of the same work he gives reasons for this conclusion, as follows:
'There is a law of optics which throws some light on the question. Suppose
the stars to be scattered through infinite space so that every great
portion of space is, in the general average, equally rich in stars. Then at
some great distance we describe a sphere having its centre in our sun.
Outside this sphere describe another one of a greater radius, and beyond
this other spheres at equal distances apart indefinitely. Thus we shall
have an endless succession of spherical shells, each of the same thickness.
The volume of each of these shells will be nearly proportional to the
squares of the diameters of the spheres which bound it. Hence each of the
regions will contain a number of stars increasing as the square of the
radius of the region. Since the amount of light we receive from each star
is as the inverse square of its distance, it follows that the sum total of
the light received from each of these spherical shells will be equal. Thus
as we add sphere after sphere we add equal amounts of light without limit.
The result would be that if the system of stars extended out indefinitely
the whole heavens would be filled with a blaze of light as bright as the
sun.'

But the whole light given us by the stars is variously estimated at from
one-fortieth to one-twentieth or, as an extreme limit, to one-tenth of
moonlight, while the sun gives as much light as 300,000 full moons, so
that starlight is only equivalent at a fair estimate to the six-millionth
part of sunlight. Keeping this in mind, the possible causes of the
extinction of almost the whole of the light of the stars (if they are
infinite in number and distributed, on the average, as thickly beyond the
Milky Way as they are up to its outer boundary) are absurdly inadequate.
These causes are (1) the loss of light in passing through the ether, and
(2) the stoppage of light by dark stars or diffused meteoritic dust. As to
the first, it is generally admitted that there is not a particle of
evidence of its existence. There is, however, some distinct evidence that,
if it exists, it is so very small in amount that it would not produce a
perceptible effect for any distances less remote than hundreds or perhaps
thousands of times as far as the farthest limits of the Milky Way are from
us. This is indicated by the fact that the brightest stars are _not_
always, or even generally, the nearest to us, as is shown both by their
small proper motions and the absence of measurable parallax. Mr. Gore
states that out of twenty-five stars, with proper motions of more than two
seconds annually, only two are above the third magnitude. Many first
magnitude stars, including Canopus, the second brightest star in the
heavens, are so remote that no parallax can be found, notwithstanding
repeated efforts. They must therefore be much farther off than many small
and telescopic stars, and perhaps as far as the Milky Way, in which so many
brilliant stars are found; whereas if any considerable amount of light were
lost in passing that distance we should find but few stars of the first
two or three magnitudes that were very remote from us. Of the twenty-three
stars of the first magnitude, only ten have been found to have parallaxes
of more than one-twentieth of a second, while five range from that small
amount down to one or two hundredths of a second, and there are two with no
ascertainable parallax. Again, there are 309 stars brighter than magnitude
3.5, yet only thirty-one of these have proper motions of more than 100" a
century, and of these only eighteen have parallaxes of more than
one-twentieth of a second. These figures are from tables given in Professor
Newcomb's book, and they have very great significance, since they indicate
that the brightest stars are _not_ the nearest to us. More than this, they
show that out of the seventy-two stars whose distance has been measured
with some approach to certainty, only twenty-three (having a parallax of
more than one-fiftieth of a second) are of greater magnitudes than 3.5,
while no less than forty-nine are smaller stars down to the eighth or ninth
magnitude, and these are on the average much nearer to us than the brighter
stars!

Taking the whole of the stars whose parallaxes are given by Professor
Newcomb, we find that the average parallax of the thirty-one bright stars
(from 3.5 magnitude up to Sirius) is 0.11 seconds; while that of the
forty-one stars below 3.5 magnitude down to about 9.5, is 0.21 seconds,
showing that they are, on the average, only half as far from us as the
brighter stars. The same conclusion was reached by Mr. Thomas Lewis of the
Greenwich Observatory in 1895, namely, that the stars from 2.70 magnitude
down to about 8.40 magnitude have, on the average, double the parallaxes
of the brighter stars. This very curious and unexpected fact, however it
may be accounted for, is directly opposed to the idea of there being any
loss of light by the more distant as compared with the nearer stars; for if
there should be such a loss it would render the above phenomenon still more
difficult of explanation, because it would tend to exaggerate it. The
bright stars being on the whole farther away from us than the less bright
down to the eighth and ninth magnitudes, it follows, if there is any loss
of light, that the bright stars are really brighter than they appear to us,
because, owing to their enormous distance some of their light has been lost
before it reached us. Of course it may be said that this does not
_demonstrate_ that no light is lost in passing through space; but, on the
other hand, it is exactly the opposite of what we should expect if the more
distant stars were perceptibly dimmed by this cause, and it may be
considered to prove that if there is any loss it is exceedingly small, and
will not affect the question of the limits of our stellar system, which is
all that we are dealing with.

This remarkable fact of the enormous remoteness of the majority of the
brighter stars is equally effective as an argument against the loss of
light by dark stars or cosmic dust, because, if the light is not
appreciably diminished for stars which have less than the fiftieth of a
second of parallax, it cannot greatly interfere with our estimates of the
limits of our universe.

Both Mr. E.W. Maunder of the Greenwich Observatory and Professor W.W.
Turner of Oxford lay great stress on these dark bodies, and the former
quotes Sir Robert Ball as saying, 'the dark stars are incomparably more
numerous than those that we can see ... and to attempt to number the stars
of our universe by those whose transitory brightness we can perceive would
be like estimating the number of horseshoes in England by those which are
red-hot.' But the proportion of dark stars (or nebulæ) to bright ones
cannot be determined _a priori_, since it must depend upon the causes that
heat the stars, and how frequently those causes come into action as
compared with the life of a bright star. We do know, both from the
stability of the light of the stars during the historic period and much
more precisely by the enormous epochs during which our sun has supported
life upon this earth--yet which must have been 'incomparably' less than its
whole existence as a light-giver--that the life of most stars must be
counted by hundreds or perhaps by thousands of millions of years. But we
have no knowledge whatever of the rate at which true stars are born. The
so-called 'new stars' which occasionally appear evidently belong to a
different category. They blaze out suddenly and almost as suddenly fade
away into obscurity or total invisibility. But the true stars probably go
through their stages of origin, growth, maturity, and decay, with extreme
slowness, so that it is not as yet possible for us to determine by
observation when they are born or when they die. In this respect they
correspond to species in the organic world. They would probably first be
known to us as stars or minute nebulæ: at the extreme limit of telescopic
vision or of photographic sensitiveness, and the growth of their
luminosity might be so gradual as to require hundreds, perhaps thousands of
years to be distinctly recognisable. Hence the argument derived from the
fact that we have never witnessed the birth of a true permanent star, and
that, therefore, such occurrences are very rare, is valueless. New stars
may arise every year or every day without our recognising them; and if this
is the case, the reservoir of dark bodies, whether in the form of large
masses or of clouds of cosmic dust, so far from being incomparably greater
than the whole of the visible stars and nebulæ, may quite possibly be only
equal to it, or at most a few times greater; and in that case, considering
the enormous distances that separate the stars (or star-systems) from each
other, they would have no appreciable effect in shutting out from our view
any considerable proportion of the luminous bodies constituting our stellar
universe. It follows, that Professor Newcomb's argument as to the very
small total light given by the stars has not been even weakened by any of
the facts or arguments adduced against it.

Mr. W.H.S. Monck, in a letter to _Knowledge_ (May 1903), puts the case
very strongly so as to support my view. He says:--'The highest estimate
that I have seen of the total light of the full moon is 1/300000 of that of
the sun. Suppose that the dark bodies were a hundred and fifty thousand
times as numerous as the bright ones. Then the whole sky ought to be as
bright as the illuminated portion of the moon. Every one knows that this is
not so. But it is said that the stars, though infinite, may only extend to
infinity in particular directions, _e.g._ in that of the Galaxy. Be it so.
Where, in the very brightest portion of the Galaxy, will we find a part
equal in angular magnitude to the moon which affords us the same quantity
of light? In the very brightest spot, the light probably does not amount to
one hundredth part that of the full moon.' It follows that, even if dark
stars were fifteen million times as numerous as the bright ones, Professor
Newcomb's argument would still apply against an infinite universe of stars
of the same average density as the portion we see.


TELESCOPIC EVIDENCE AS TO THE LIMITS OF THE STAR SYSTEM

Throughout the earlier portion of the nineteenth century every increase of
power and of light-giving qualities of telescopes added so greatly to the
number of the stars which became visible, that it was generally assumed
that this increase would go on indefinitely, and that the stars were really
infinite in number and could not be exhausted. But of late years it has
been found that the increase in the number of stars visible in the larger
telescopes was not so great as might be expected, while in many parts of
the heavens a longer exposure of the photographic plate adds comparatively
little to the number of stars obtained by a shorter exposure with the same
instrument.

Mr. J.E. Gore's testimony on this point is very clear. He says:--'Those who
do not give the subject sufficient consideration, seem to think that the
number of the stars is practically infinite, or at least, that the number
is so great that it cannot be estimated. But this idea is totally
incorrect, and due to complete ignorance of telescopic revelations. It is
certainly true that, to a certain extent, the larger the telescope used in
the examination of the heavens, the more the number of the stars seems to
increase; but we now know that there is a limit to this increase of
telescopic vision. And the evidence clearly shows that we are rapidly
approaching this limit. Although the number of stars visible in the
Pleiades rapidly increases at first with increase in the size of the
telescope used, and although photography has still further increased the
number of stars in this remarkable cluster, it has recently been found that
an increased length of exposure--beyond three hours--adds very few stars to
the number visible on the photograph taken at the Paris Observatory in
1885, on which over two thousand stars can be counted. Even with this great
number on so small an area of the heavens, comparatively large vacant
places are visible between the stars, and a glance at the original
photograph is sufficient to show that there would be ample room for many
times the number actually visible. I find that if the whole heavens were as
rich in stars as the Pleiades, there would be only thirty-three millions in
both hemispheres.'

Again, referring to the fact that Celoria, with a telescope showing stars
down to the eleventh magnitude, could see almost exactly the same number of
stars near the north pole of the Galaxy as Sir William Herschel found with
his much larger and more powerful telescope, he remarks: 'Their absence,
therefore, seems certain proof that very faint stars do _not_ exist in that
direction, and that here, at least, the sidereal universe is limited in
extent.'

Sir John Herschel notes the same phenomena, stating that even in the Milky
Way there are found 'spaces absolutely dark _and completely void of any
star_, even of the smallest telescopic magnitude'; while in other parts
'extremely minute stars, though never altogether wanting, occur in numbers
so moderate as to lead us irresistibly to the conclusion that in these
regions we see _fairly through_ the starry stratum, since it is impossible
otherwise (supposing their light not intercepted) that the numbers of the
smaller magnitudes should not go on continually increasing ad infinitum. In
such cases, moreover, the ground of the heavens, as seen between the stars,
is for the most part perfectly dark, which again would not be the case if
innumerable multitudes of stars, too minute to be individually discernible,
existed beyond.' And again he sums up as follows:--'Throughout by far the
larger portion of the extent of the Milky Way in both hemispheres, the
general blackness of the ground of the heavens on which its stars are
projected, and the absence of that innumerable multitude and excessive
crowding of the smallest visible magnitudes, and of glare produced by the
aggregate light of multitudes too small to affect the eye singly, which the
contrary supposition would appear to necessitate, must, we think, be
considered unequivocal indications that its dimensions _in directions where
these conditions obtain_, are not only not infinite, but that the
space-penetrating power of our telescopes suffices fairly to pierce
through and beyond it.'[2]

This expression of opinion by the astronomer who, probably beyond any now
living, was the most competent authority on this question, to which he
devoted a long life of observation and study extending over the whole
heavens, cannot be lightly set aside by the opinions or conjectures of
those who seem to assume that we must believe in an infinity of stars if
the contrary cannot be absolutely proved. But as not a particle of evidence
can be adduced to prove infinity, and as all the facts and indications
point, as here shown, in a directly opposite direction, we must, if we are
to trust to evidence at all in this matter, arrive at the conclusion that
the universe of stars is limited in extent.

Dr. Isaac Roberts gives similar evidence as regards the use of
photographic plates. He writes:--'Eleven years ago photographs of the
Great Nebula in _Andromeda_ were taken with the 20-inch reflector, and
exposures of the plates during intervals up to four hours; and upon some
of them were depicted stars to the faintness of 17th to 18th magnitude,
and nebulosity to an equal degree of faintness. The films of the plates
obtainable in those days were less sensitive than those which have been
available during the past five years, and during this period photographs
of the nebula with exposures up to four hours have been taken with the
20-inch reflector. No extensions of the nebulosity, however, nor
increase in the number of the stars can be seen on the later rapid
plates than were depicted upon the earlier slower ones, though the
star-images and the nebulosity have greater density on the later
plates.'

Exactly similar facts are recorded in the cases of the Great Nebula in
_Orion_, and the group of the Pleiades. In the case of the Milky Way in
_Cygnus_ photographs have been taken with the same instrument, but with
exposures varying from one hour to two hours and a half, but no fainter
stars could be found on one than on the other; and this fact has been
confirmed by similar photographs of other areas in the sky.


THE LAW OF DIMINISHING NUMBERS OF STARS

We will now consider another kind of evidence equally weighty with the two
already adduced. This is what may be termed the law of diminishing numbers
beyond a certain magnitude, as observed by larger and larger telescopes.

For some years past star-magnitudes have been determined very accurately by
means of careful photometric comparisons. Down to the sixth magnitude stars
are visible to the naked eye, and are hence termed lucid stars. All fainter
stars are telescopic, and continuing the magnitudes in a series in which
the difference in luminosity between each successive magnitude is equal,
the seventeenth magnitude is reached and indicates the range of visibility
in the largest telescopes now in existence. By the scale now used a star of
any magnitude gives nearly two and a half times as much light as one of the
next lower magnitude, and for accurate comparison the apparent brightness
of each star is given to the tenth of a magnitude which can easily be
observed. Of course, owing to differences in the colour of stars, these
determinations cannot be made with perfect accuracy, but no important error
is due to this cause. According to this scale a sixth magnitude star gives
about one-hundredth part of the light of an average first magnitude star.
Sirius is so exceptionally bright that it gives nine times as much light as
a standard or average first magnitude star.

Now it is found that from the first to the sixth magnitude the stars
increase in number at the rate of about three and a half times those of the
preceding magnitudes. The total number of stars down to the sixth magnitude
is given by Professor Newcomb as 7647. For higher magnitudes the numbers
are so great that precision and uniformity are more difficult of
attainment; yet there is a wonderful continuance of the same law of
increase down to the tenth magnitude, which is estimated to include
2,311,000 stars, thus conforming very nearly with the ratio of 3.5 as
determined by the lucid stars.

But when we pass beyond the tenth magnitude to those vast numbers of faint
stars only to be seen in the best or the largest telescopes, there appears
to be a sudden change in the ratio of increased numbers per magnitude. The
numbers of these stars are so great that it is impossible to count the
whole as with the higher magnitude stars, but numerous counts have been
made by many astronomers in small measured areas in different parts of the
heavens, so that a fair average has been obtained, and it is possible to
make a near approximation to the total number visible down to the
seventeenth magnitude. The estimate of these by astronomers who have made a
special study of this subject is, that the total number of visible stars
does not exceed one hundred millions.[3]

But if we take the number of stars down to the ninth magnitude, which are
known with considerable accuracy, and find the numbers in each succeeding
magnitude down to the seventeenth, according to the same ratio of increase
which has been found to correspond very nearly in the case of the higher
magnitudes, Mr. J.E. Gore finds that the total number should be about 1400
millions. Of course neither of these estimates makes any pretence to exact
accuracy, but they are founded on all the facts at present available, and
are generally accepted by astronomers as being the nearest approach that
can be made to the true numbers. The discrepancy is, however, so enormous
that probably no careful observer of the heavens with very large telescopes
doubts that there is a very real and very rapid diminution in the numbers
of the fainter as compared with the brighter stars.

There is, however, yet one more indication of the decreasing numbers of the
faint telescopic stars, which is almost conclusive on this question, and,
so far as I am aware, has not yet been used in this relation. I will
therefore briefly state it.


THE LIGHT RATIO AS INDICATING THE NUMBER OF FAINT STARS

Professor Newcomb points out a remarkable result depending on the fact
that, while the average light of successively lower magnitudes diminishes
in a ratio of 2.5, their numbers increase at nearly a ratio of 3.5. From
this it follows that, so long as this law of increase continues, the total
of starlight goes on increasing by about forty per cent. for each
successive magnitude, and he gives the following table to illustrate it:--

    Mag. 1    Total Light  =  1
     "   2         "       =  1.4
     "   3         "       =  2.0
     "   4         "       =  2.8
     "   5         "       =  4.0
     "   6         "       =  5.7
     "   7         "       =  8.0
     "   8         "       = 11.3
     "   9         "       = 16.0
     "  10         "       = 22.6
                         --------
    Total light to Mag. 10 = 74.8
                         --------

Thus the total amount of the light given by all stars down to the tenth
magnitude is seventy-four times as great as that from the few first
magnitude stars. We also see that the light given by the stars of any
magnitude is twice as much as that of the stars two magnitudes higher in
the scale, so that we can easily calculate what additional light we ought
to receive from each additional magnitude if they continue to increase in
numbers below the tenth as they do above that magnitude. Now it has been
calculated as the result of careful observations, that the total light
given by stars down to nine and a half magnitude is one-eightieth of full
moonlight, though some make it much more. But if we continue the table of
light-ratios from this low starting-point down to magnitude seventeen and a
half, we shall find, if the numbers of the stars go on increasing at the
same rate as before, that the light of all combined should be at least
seven times as great as moonlight; whereas the photometric measurements
make it actually about one-twentieth. And as the calculation from
light-ratios only includes stars just visible in the largest telescopes,
and does not include all those proved to exist by photography, we have in
this case a demonstration that the numbers of the stars below the tenth and
down to the seventeenth magnitude diminish rapidly.

We must remember that the minuter telescopic stars preponderate enormously
in and near the Milky Way. At a distance from it they diminish rapidly,
till near its poles they are almost entirely absent. This is shown by the
fact (already referred to at p. 146) that Professor Celoria of Milan, with
a telescope of less than three inches aperture, counted almost as many
stars in that region as did Herschel with his eighteen-inch reflector. But
if the stellar universe extends without limit we can hardly suppose it to
do so in one plane only; hence the absence of the minuter stars and of
diffused milky light over the larger part of the heavens is now held to
prove that the myriads of very minute stars in the Milky Way really belong
to it, and not to the depths of space far beyond.

It seems to me that here we have a fairly direct proof that the stars of
our universe are really limited in number.

There are thus four distinct lines of argument all pointing with more or
less force to the conclusion that the stellar universe we see around us, so
far from being infinite, is strictly limited in extent and of a definite
form and constitution. They may be briefly summarised as follows:--

(1) Professor Newcomb shows that, if the stars were infinite in number, and
if those we see were approximately a fair sample of the whole, and further,
if there were not sufficient dark bodies to shut out almost the whole of
their light, then we should receive from them an amount of light
theoretically greater than that of sunlight. I have shown, at some length,
that neither of these causes of loss of light will account for the enormous
disproportion between the theoretical and the actual light received from
the stars; and therefore Professor Newcomb's argument must be held to be a
valid one against the infinite extent of our universe. Of course, this does
not imply that there may not be any number of other universes in space, but
as we know absolutely nothing of them--even whether they are material or
non-material--all speculation as to their existence is worse than useless.

(2) The next argument depends on the fact that all over the heavens, even
in the Milky Way itself, there are areas of considerable extent, besides
rifts, lanes, and circular patches, where stars are either quite absent or
very faint and few in number. In many of these areas the largest telescopes
show no more stars than those of moderate size, while the few stars seen
are projected on an intensely dark background. Sir William Herschel,
Humboldt, Sir John Herschel, R.A. Proctor, and many living astronomers hold
that, in these dark areas, rifts, and patches, we see completely through
our stellar universe into the starless depths of space beyond.

(3) Then we have the remarkable fact that the steady increase in the number
of stars, down to the ninth or tenth magnitudes, following one constant
ratio either gradually or suddenly changes, so that the total number from
the tenth down to the seventeenth magnitudes is only about one-tenth of
what it would have been had the same ratio of increase continued. The
conclusion to be drawn from this fact clearly is, that these faint stars
are becoming more and more thinly scattered in space, while the dark
background on which they are usually seen shows that, except in the region
of the Milky Way, there are _not_ multitudes of still smaller invisible
stars beyond them.

(4) The last indication of a limited stellar universe--the estimate of
numbers by the light-ratio of each successive magnitude--powerfully
supports the three preceding arguments.

The four distinct classes of evidence now adduced must be held to
constitute, as nearly as the circumstances permit, a satisfactory proof
that the stellar universe, of which our solar system forms a part, has
definite limits; and that a full knowledge of its form, structure, and
extent, is not beyond the possibility of attainment by the astronomers of
the future.

FOOTNOTES:

[1] In a letter to _Knowledge_, June 1903, Mr. W.H.T. Monck puts the same
point in a mathematical form.

[2] _Outlines of Astronomy_, pp. 578-9. In the passages quoted the italics
are Sor John Herschel's.

[3] Mr. J.E. Gore in _Concise Knowledge Astronomy_, pp. 541-2.



CHAPTER VIII

OUR RELATION TO THE MILKY WAY


We now approach what may be termed the very heart of the subject of our
inquiry, the determination of how we are actually situated within this vast
but finite universe, and how that position is likely to affect our globe as
being the theatre of the development of life up to its highest forms.

We begin with our relation to the Milky Way (which we have fully described
in our fourth chapter), because it is by far the most important feature in
the whole heavens. Sir John Herschel termed it 'the ground-plane of the
sidereal system'; and the more it is studied the more we become convinced
that the whole of the stellar universe--stars, clusters of stars, and
nebulæ--are in some way connected with it, and are probably dependent on it
or controlled by it. Not only does it contain a greater number of stars of
the higher magnitudes than any other part of the heavens of equal extent,
but it also comprises a great preponderance of star-clusters, and a great
extent of diffused nebulous matter, besides the innumerable myriads of
minute stars which produce its characteristic cloud-like appearance. It is
also the region of those strange outbursts forming new stars; while gaseous
stars of enormous bulk--some probably a thousand or even ten thousand
times that of our sun, and of intense heat and brilliancy--are more
abundant there than in any other part of the heavens. It is now almost
certain that these enormous stars and the myriads of minute stars just
visible with the largest telescopes, are actually intermingled, and
together constitute its essential features; in which case the fainter stars
are really small and cannot be far apart, forming, as it were, the first
aggregations of the nebulous substratum, and perhaps supplying the fuel
which keeps up the intense brilliancy of the giant suns. If this is so,
then the Galaxy must be the theatre of operation of vast forces, and of
continuous combinations of matter, which escape our notice owing to its
enormous distance from us. Among its millions of minute telescopic stars,
hundreds or thousands may appear or disappear yearly without being
perceived by us, till the photographic charts are completed and can be
minutely scrutinised at short intervals. As undoubted changes have occurred
in many of the larger nebulæ during the last fifty years, we may anticipate
that analogous changes will soon be noted in the stars and the nebulous
masses of the Milky Way. Dr. Isaac Roberts has even observed changes in
nebulæ after such a short interval as eight years.


THE MILKY WAY A GREAT CIRCLE

Notwithstanding all its irregularities, its divisions, and its diverging
branches, astronomers are generally agreed that the Milky Way forms a great
circle in the heavens. Sir John Herschel, whose knowledge of it was
unrivalled, stated that its course 'conforms, as nearly as the
indefiniteness of its boundary will allow it to be fixed, to that of a
great circle'; and he gives the Right Ascension and Declination of the
points where it crosses the equinoctial, in figures which define those
points as being exactly opposite each other. He also defines its northern
and southern poles by other figures, so as to show that they are the poles
of a great circle. And after referring to Struve's view that it was _not_ a
great circle, he says, 'I retain my own opinion.' Professor Newcomb says
that its position 'is nearly always near a great circle of the sphere'; and
again he says: 'that we are in the galactic plane itself seems to be shown
in two ways: (1) the equality in the counts of stars on the two sides of
this plane all the way to its poles; and (2) the fact that the central line
of the Galaxy is a great circle, which it would not be if we viewed it from
one side of its central plane' (_The Stars_, p. 317). Miss Clerke, in her
_History of Astronomy_, speaks of 'our situation _in_ the galactic plane'
as one of the undisputed facts of astronomy; while Sir Norman Lockyer, in a
lecture delivered in 1899, said, 'the middle line of the Milky Way is
really not distinguishable from a great circle,' and again in the same
lecture--'but the recent work, chiefly of Gould in Argentina, has shown
that it practically is a great circle.'[4]

About this fact, then, there can be no dispute. A great circle is a circle
dividing the celestial sphere into two equal portions, as seen from the
earth, and therefore the plane of this circle must pass through the earth.
Of course the whole thing is on such a vast scale, the Milky Way varying
from ten to thirty degrees wide, that the plane of its circular course
cannot be determined with minute accuracy. But this is of little
importance. When carefully laid down on a chart, as in that of Mr. Sidney
Waters (see end of volume), we can see that its central line does follow a
very even circular course, conforming 'as nearly as may be' to a great
circle. We are therefore certainly well within the space that would be
enclosed if its northern and southern margins were connected together
across the vast intervening abyss, and in all probability not far from the
central plane of that enclosed space.


THE FORM OF THE MILKY WAY AND OUR POSITION ON ITS PLANE

Although the Galaxy forms a great circle in the heavens from our point of
view, it by no means follows that it is circular in plan. Being unequal in
width and irregular in outline, it might be elliptic or even angular in
shape without being at all obviously so to us. If we were standing in an
open plain or field two or three miles in diameter, and bounded in every
direction by woods of very irregular height and density and great diversity
of tint, we should find it difficult to judge of the shape of the field,
which might be either a true circle, an oval, a hexagon, or quite irregular
in outline, without our being able to detect the exact shape unless some
parts were very much nearer to us than others. Again, just as the woods
bounding the field might be either a narrow belt of nearly uniform width,
or might in some places be only a few yards wide and in others stretch out
for miles, so there have been many opinions as to the width of the Milky
Way in the direction of its plane, that is, in the direction in which we
look towards it. Lately, however, as the result of long-continued
observation and study, astronomers are fairly well agreed as to its general
form and extent, as will be seen by the following statements of fact and
reasoning.

Miss Clerke, after giving the various views of many astronomers--and as the
historian of modern astronomy her opinion has much weight--considers that
the most probable view of it is, that it is really very much what it seems
to us--an immense ring with streaming appendages extending from the main
body in all directions, producing the very complex effect we see. The
belief seems to be now spreading that the whole universe of stars is
spherical or spheroidal, the Milky Way being its equator, and therefore in
all probability circular or nearly so in plan; and it is also held that it
must be rotating--perhaps very slowly--as nothing else can be supposed to
have led to the formation of such a vast ring, or can preserve it when
formed.

Professor Newcomb considers, from the numbers of the stars in all
directions towards the Milky Way being approximately equal, that there
cannot be much difference in our distance from it in various directions. It
would follow that its plan is approximately circular or broadly elliptic.
The existence of ring-nebulæ may be held to render such a form probable.

Sir Norman Lockyer gives facts which tend in the same direction. In an
article in _Nature_ of November 8th, 1900, he says: 'We find that the
gaseous stars are not only confined to the Milky Way, but they are the most
remote in every direction, in every galactic longitude; all of them have
the smallest proper motion.' And again, referring to the hottest stars
being equally remote on all sides of us, he says: 'It is because we are in
the centre, because the solar system is in the centre, that the observed
effect arises.' He also considers that the ring-nebula in Lyra nearly
represents the form of our whole system; and he adds: 'We practically know
that in our system the centre is the region of least disturbance, and
therefore cooler conditions.'

These various facts and conclusions of some of the most eminent astronomers
all point to one definite inference, that our position, or that of the
solar system, is not very far from the centre of the vast ring of stars
constituting the Milky Way, while the same facts imply a nearly circular
form to this ring. Here, more than as regards our position in the plane of
the Galaxy, there is no possibility of precise determination; but it is
quite certain that if we were situated very far away from the centre, say,
for instance, one-fourth of its diameter from one side of it and
three-fourths from the other, the appearances would not be what they are,
and we should easily detect the excentricity of our position. Even if we
were one-third the diameter from one side and two-thirds from the other, it
will, I think, be admitted that this also would have been ascertained by
the various methods of research now available. We must, therefore, be
somewhere between the actual centre and a circle whose radius is one-third
of the distance to the Milky Way. But if we are about midway between these
two positions, we shall only be one-sixth of the radius or one-twelfth of
the diameter of the Milky Way from its exact centre; and if we form part of
a cluster or group of stars slowly revolving around that centre, we should
probably obtain all the advantages, if any, that may arise from a nearly
central position in the entire star-system.

This question of our situation within the great circle of the Milky Way is
of considerable importance from the point of view I am here suggesting, so
that every fact bearing upon it should be noted; and there is one which has
not, I think, been given the full weight due to it. It is generally
admitted that the greater brilliancy of some parts of the Milky Way is no
indication of nearness, because surfaces possess equal brilliancy from
whatever distance they are seen. Thus each planet has its special
brilliancy or reflective power, technically termed its 'albedo,' and this
remains the same at all distances if the other conditions are similar. But
notwithstanding this well-known fact, Sir John Herschel's remark that the
greater brightness of the southern Milky Way 'conveys strongly the
impression of greater proximity,' and therefore, that we are excentrically
placed in its plane, has been adopted by many writers as if it were the
statement of a fact, or at least a clearly expressed opinion, instead of
being a mere 'impression,' and really a misleading one. I therefore wish to
adduce a phenomenon which has a real bearing on the question. It is evident
that, if the Milky Way were actually of uniform width throughout, then
differences of apparent width would indicate differences of distance. In
the parts nearer to us it would appear wider, where more remote, narrower;
but in these opposite directions there would not necessarily be any
differences in brightness. We should, however, expect that in the parts
nearer to us the lucid stars, as well as those within any definite limits
of magnitude, would be either more numerous or more wide apart on the
average. No such difference as this, however, has been recorded; but there
_is_ a peculiar correspondence in the opposite portions of the Galaxy which
is very suggestive. In the beautiful charts of the Nebulæ and Star Clusters
by the late Mr. Sidney Waters, published by the Royal Astronomical Society
and here reproduced by their permission (see end of volume), the Milky Way
is delineated in its whole extent with great detail and from the best
authorities. These charts show us that, in both hemispheres, it reaches its
maximum extension on the right and left margins of the charts, where it is
almost equal in extent; while in the centre of each chart, that is at its
nearest points to the north and south poles respectively, it is at its
narrowest portion; and, although this part in the southern hemisphere is
brightest and most strongly defined, yet the actual extent, including the
fainter portions, is, again, not very unequal in the opposite segments.
Here we have a remarkable and significant symmetry in the proportions of
the Milky Way, which, taken in connection with the nearly symmetrical
scattering of the stars in all parts of the vast ring, is strongly
suggestive of a nearly circular form and of our nearly central position
within its plane. There is one other feature in this delineation of the
Milky Way which is worthy of notice. It has been the universal practice to
speak of it as being double through a considerable portion of its extent,
and all the usual star-maps show the division greatly exaggerated,
especially in the northern hemisphere; and this division was considered so
important as to lead to the cloven-disc theory of its form, or that it
consisted of two separate irregular rings, the nearer one partly hiding the
more distant; while various spiral combinations were held by others to be
the best way of explaining its complex appearance. But this newer map,
reduced from a large one by Lord Rosse's astronomer, Dr. Boeddicker, who
devoted five years to its delineation, shows us that there is no actual
division in any portion of it in the northern hemisphere, but that
everywhere, throughout its whole width, it consists of numerous
intermingled streams and branches, varying greatly in luminosity, and with
many faint or barely distinguishable extensions along its margins, yet
forming one unmistakable nebulous belt; and the same general character
applies to it in the southern hemisphere as delineated by Dr. Gould.

Another feature, which is well shown to the eye by these more accurate
maps, is the regular curvature of the central line of the Milky Way. We can
judge of this almost sufficiently by the eye; but if, with a pair of
compasses, we find the proper radius and centre of curvature, we shall see
that the true circular curve is always in the very centre of the nebulous
mass, and the same radius applied in the same manner to the opposite
hemisphere gives a similar result. It will be noted that as the Milky Way
is obliquely situated on these charts, the centre of the curve will be
about in R.A. 0h. 40m. in the map of the southern hemisphere, and in R.A.
12h. 40m. in that of the northern hemisphere; while the radius of curvature
will be about the length of the chord of eight hours of R.A. as measured on
the margin of the maps. This great regularity of curve of the central line
of the Galaxy strongly suggests rotation as the only means by which it
could have originated and be maintained.


THE SOLAR CLUSTER

Astronomers are now generally agreed that there is a cluster of stars of
which our sun forms a part, though its exact dimensions, form, and limits
are still under discussion. Sir William Herschel long ago arrived at the
conclusion that the Milky Way 'consists of stars very differently scattered
from those immediately around us.' Dr. Gould believed that there were about
five hundred bright stars much nearer to us than the Milky Way, which he
termed the solar cluster. And Miss Clerke observes that the actual
existence of such a cluster is indicated by the fact that 'an enumeration
of the stars in photometric order discloses a systematic excess of stars
brighter than the 4th magnitude, making it certain that there is an actual
condensation in the neighbourhood of the sun--that the average allowance of
cubical space per star is smaller within a sphere enclosing him with a
radius, say, of 140 light-years, than further away.'[5]

But the most interesting inquiry into this subject is that by Professor
Kapteyn of Gröningen, one of the most painstaking students of the
distribution of the stars. He founds his conclusions mainly on the proper
motions of the stars, this being the best general indication of distance in
the absence of actual determination of parallax. He made use of the proper
motions and the spectra of more than two thousand stars, and he finds that
a considerable body of stars having large proper motions, and also
presenting the solar type of spectra, surround our sun in all directions,
and show no increased density, as the more distant stars do, towards the
Milky Way. He finds also that towards the centre of this cluster stars are
far closer together than near its outer limits (he says there are
ninety-eight times as many), that it is roughly spherical in shape, and
that the maximum compression is, as nearly as can be ascertained, at the
centre of the circle of the Milky Way, while the sun is at some distance
away from this central point.[6]

It is a very suggestive fact that most of the stars belonging to this
cluster have spectra of the solar type, which indicates that they are of
the same general chemical constitution as our sun, and are also at about
the same stage of evolution; and this may well have arisen from their
origin in a great nebulous mass situated at or near the centre of the
galactic plane, and probably revolving round their common centre of
gravity.

As Kapteyn's result was based on materials which were not so full or
reliable as those now available, Professor S. Newcomb has examined the
question himself, using two recent lists of stars, one limited to those
having proper motions of 10" a century, of which there are 295, and the
other of nearly 1500 stars with 'appreciable proper motions.' They are
situated in two zones, each about 5° in breadth and cutting across the
Milky Way in different parts of its course. They afford, therefore, a good
test of the distribution of these nearer stars with regard to the Galaxy.
The result is, that on the average these stars are not more numerous in or
near the Milky Way than elsewhere; and Professor Newcomb expresses himself
on this point as follows:--'The conclusion is interesting and important. If
we should blot out from the sky all the stars having no proper motion large
enough to be detected, we should find remaining stars of all magnitudes;
but they would be scattered almost uniformly over the sky, and show little
or no tendency to crowd towards the Galaxy, unless, perhaps, in the region
near 19h. of Right Ascension.'[7]

A little consideration will show that, as the stars of all magnitudes which
are, on the average, nearest to us are spread over the sky in 'all
directions' and 'almost uniformly,' this necessarily implies that they
form a cluster or group, and that our sun is somewhere not very far from
the centre of this group. Again, Professor Newcomb refers to 'the
remarkable equality in the number of stars in opposite directions from us.
We do not detect any marked difference between the numbers lying round the
opposite poles of the Galaxy, nor, so far as known, between the
star-density in different regions at equal distances from the Milky Way'
(_The Stars_, p. 315). And again he refers to the same question at p. 317,
where he says: 'So far as we can judge from the enumeration of the stars in
all directions, and from the aspect of the Milky Way, our system is near
the centre of the stellar universe.'

It will, I think, now be clear to my readers that the four main
astronomical propositions stated in my article which appeared in the New
York _Independent_ and in the _Fortnightly Review_, and which were either
denied or declared to be unproved by my astronomical critics, have been
shown to be supported by so many converging lines of evidence, that it is
no longer possible to deny that they are, at least provisionally, fairly
well established. These facts are, (1) that the stellar universe is not of
infinite extent; (2) that our sun is situated in the central plane of the
Milky Way; (3) that it is also situated near to the centre of that plane;
(4) that we are surrounded by a group or cluster of stars of unknown
extent, which occupy a place not far removed from the centre of the
galactic plane, and therefore, near to the centre of our universe of stars.

Not only are these four propositions each supported by converging lines of
evidence, including some which I believe have not before been adduced in
their support, but a number of astronomers, admittedly of the first rank,
have arrived at the same conclusions as to the bearing of the evidence, and
have expressed their convictions in the clearest manner, as quoted by me.
It is _their_ conclusions which I appeal to and adopt; yet my two chief
astronomical critics positively deny that there is any valid evidence of
the finiteness of the stellar universe, which one of them terms 'a myth,'
and he even accuses _me_ of having started it. Both of them, however, agree
in stating very strongly one objection to my main thesis--that our central
position (not necessarily at the precise centre) in the stellar universe
has a meaning and a purpose, in connection with the development of life and
of man upon this earth, and, _so far as we know_, here only. With this one
objection, the only one that in my opinion has the slightest weight, I will
now proceed to deal.


THE SUN'S MOTION THROUGH SPACE

The two astronomers who did me the honour to criticise my original article
laid the greatest stress on the fact, that even if I had proved that the
sun now occupied a nearly central position in the great star-system, it was
really of no importance whatever, because, at the rate the sun was
travelling, 'five million years ago we were deep in the actual stream of
the Milky Way; five million years hence we shall have completely crossed
the gulf which it encircles, and again be a member of one of its
constituent groups, but on the opposite side. And ten million years are
regarded by geologists and biologists as but a trifle on account to meet
their demands upon the bank of Time.' Thus speaks one of my critics. The
other is equally crushing. He says:--'If there is a centre to the visible
universe, and if we occupy it to-day, we certainly did not do so yesterday,
and shall not do so to-morrow. The Solar System is known to be moving among
the stars with a velocity which would carry us to Sirius within 100,000
years, if we happened to be travelling in his direction, as we are not. In
the 50 or 100 million years during which, according to geologists, this
earth has been a habitable globe, we must have passed by thousands of stars
on the right hand and on the left.... In his eagerness to limit the
universe in space, Dr. Wallace has surely forgotten that it is equally
important, for his purpose, to limit it in time; but incomparably more
difficult in the face of ascertained facts.... Indeed, so far from our
having tranquilly enjoyed a central position in unbroken continuity for
scores or perhaps hundreds of millions of years, we should in that time
have traversed the universe from boundary to boundary.'[8]

Now the average reader of these two criticisms, taking account of the high
official position of both writers, would accept their statements of the
case as being demonstrated facts, requiring no qualification whatever, and
would conclude that my whole argument had been thereby rendered worthless,
and all that I founded upon it a fantastic dream. But if, on the other
hand, I can show that their stated facts as to the sun's motion are by no
means demonstrated, because founded upon assumptions which may be quite
erroneous; and further, that if the facts should turn out to be
substantially correct, they have both omitted to state well-known and
admitted qualifications which render the conclusions they derive from the
facts very doubtful, then the average reader will learn the valuable lesson
that official advocacy, whether in medicine, law, or science is never to be
accepted till the other side of the case has been heard. Let us see,
therefore, what the facts really are.

Professor Simon Newcomb calculates that, if there are one hundred million
stars in the stellar universe each five times the mass of our sun, and
spread over a space which light would require thirty thousand years to
cross, then any mass traversing such a system with a velocity of more than
twenty-five miles a second, would fly off into infinite space never to
return. Now as there are many stars which have, apparently, very much more
than this velocity, it would follow that the visible universe is unstable.
It also implies that these great velocities were not acquired in the system
itself, but that the bodies which possess them must have entered it from
without, thus requiring other universes as the feeders of our universe.

For the accuracy of the above statement the authority of Professor Newcomb
is an ample guarantee; but there may be modifications required in the data
on which it is founded, and these may greatly alter the result. If I do not
mistake, the estimate of a hundred million stars is founded on actual
counts or estimates of stars of successive magnitudes in different parts of
the heavens, and it does not include either those of the denser star
clusters nor the countless millions just beyond the reach of telescopes in
the Milky Way. Neither does it make allowance for the dark stars supposed
by some astronomers to be many times more numerous than the bright ones,
nor for the vast number of the nebulæ, great and small, in calculating the
total mass of the stellar system.[9] In his latest work Professor Newcomb
says, 'The total number of stars is to be counted by hundreds of millions';
and hence the controlling power of the system on bodies within it will be
many times greater than that given above, and might even be ample to retain
within its bounds such a rapidly moving star as Arcturus, which is believed
to be travelling at the rate of more than three hundred miles a second. But
there is another very important limitation to the conclusions to be drawn
from Professor Newcomb's calculation. It assumes the stars to be nearly
uniformly distributed through the whole of the space to which the system
extends. But the facts are very different. The existence of clusters, some
of which comprise many thousands of stars, is one example of irregularity
of distribution, and anyone of these larger clusters would probably be able
to change the course of even the swiftest stars passing near it. The larger
nebulæ might have the same effect, since the late Mr. Ranyard, taking all
his data so as to produce a minimum result, calculated the probable mass
of the Orion nebula to be four and a half million times that of the sun,
and there may be many other nebulæ equally large. But far more important is
the fact of the vast ring of the Milky Way, which is now universally held
by astronomers to be, not only apparently but really, more densely crowded
with stars and also with vast masses of nebulous matter than any other part
of the heavens, so that it may possibly comprise within itself a very large
proportion of the whole of the matter of the visible universe. This is
rendered more probable by the fact that the great majority of star-clusters
lie along its course, most of the huge gaseous stars belong to it, while
the occurrence there only of 'new stars' is evidence of a superabundance of
matter in various forms leading to frequent heat-producing collisions, just
as the frequent occurrence of meteoric showers on our earth is evidence of
the superabundance of meteoric matter in the solar system.

It is recognised by mathematicians that within any great system of bodies
subject to the law of gravitation there can be no such thing as motion of
any of them in a straight line; neither can any amount of motion arise
within such a system through the action of gravitation alone capable of
carrying any of its masses out of the system. The ultimate tendency must be
towards concentration rather than towards dispersal.

It seems, therefore, only reasonable to consider whatever motions and
whatever velocities we find among the stars, as having been produced by the
gravitative power of the larger aggregations, modified perhaps by
electrical repulsive forces, by collisions, and by the results of those
collisions; and we may look to the changes now visibly going on in some of
the nebulæ and clusters as indications of the forces that have probably
brought about the actual condition of the whole stellar universe.

If we examine the beautiful photographs of nebulæ by Dr. Roberts and other
observers, we find that they are of many forms. Some are extremely
irregular and almost like patches of cirrus clouds, but a large number are
either distinctly spiral in form, or show indications of becoming spiral,
and this has been found to be the case even with some of the large
irregular nebula. Then again we have numerous ring-formed nebulæ, usually
with a star involved in dense nebulosity in the centre, separated by a dark
space of various widths from the outer ring. All these kinds of nebulæ have
stars involved in them, and apparently forming part of their structure,
while others which do not differ in appearance from ordinary stars are
believed by Dr. Roberts to lie between us and the nebula. In the case of
many of the spiral nebulæ, stars are often strung along the coils of the
spiral, while other curved lines of stars are seen just outside the nebula,
so that it is impossible to avoid the conclusion that both are really
connected with it, the outer lines of stars indicating a former greater
extension of the nebula whose material has been used up in the growth of
these stars. Some of these spiral nebulæ show beautifully regular
convolutions, and these usually have a large central star like mass, as in
M. 100 Comæ and I. 84 Comæ, in Vol. II. Pl. 14 of Dr. Roberts's
photographs. The straight white streaks across the nebula of the Pleiades
and some others are believed by Dr. Roberts to be indications of spiral
nebulæ seen edgewise. In other cases, clusters of stars are more or less
nebulous, and the arrangement of the stars seems to indicate their
development from a spiral nebula. It is to be noted that many of the
objects classed as planetary nebulæ by Sir John Herschel are shown by the
best photographs to be really of the ring-type, though often with a very
narrow division between the ring and the central mass. This form may
therefore be of frequent occurrence.

But if this annular form with some kind of central nucleus, often very
large, is produced under certain conditions by the action of the ordinary
laws of motion upon more or less extensive masses of discrete matter, why
may not the same laws acting upon similar matter once dispersed over the
whole extent of the existing stellar universe, or even beyond what are now
its farthest limits, have led to the aggregation of the vast annular
formation of the Milky Way, with all the subordinate centres of
concentration or dispersal to be found within or around it? And if this is
a reasonable conception, may we not hope that by a concentration of
attention upon a few of the best marked and most favourably situated
annular and spiral systems, sufficient knowledge of their internal motions
may be obtained which may serve as a guide to the kind of motion we may
expect to find in the great galactic ring and its subordinate stars? We may
then perhaps discover which now seem so erratic, are really all parts of a
series of orbital movements limited and controlled by the forces of the
great system to which they belong, so that, if not mathematically stable,
they may yet be sufficiently so to endure for some thousand millions of
years.

It is a suggestive fact that the calculated position of the 'solar
apex'--the point towards which our sun appears to move--is now found to be
much more nearly in the plane of the Milky Way than the position first
assigned to it, and Professor Newcomb adopts, as most likely to be
accurate, a point near the bright star Vega in the constellation Lyra.
Other calculators have placed it still farther east, while Rancken and Otto
Stumpe assign it a position actually in the Milky Way; and Mr. G.C. Bompas
concludes that the sun's plane of motion nearly coincides with that of the
Galaxy. M. Rancken found that 106 stars near the Milky Way showed, in their
very small proper motions, a drift along it in a direction from Cassiopeiæ
towards Orion, and this, it is supposed, may be partly due to our sun's
motion in an opposite direction.

In many other parts of the heavens there are groups of stars which have
almost identical proper motions--a phenomenon which the late R.A. Proctor
termed 'star-drift'; and he especially pointed out that five of the stars
of the Great Bear were all drifting in the same direction; and although
this has been denied by later writers, Professor Newcomb, in his recent
book on _The Stars_, declares that Proctor was right, and explains that the
error of his critics was due to not making allowance for the divergence of
the circles of right ascension. The Pleiades are another group, the stars
of which drift in the same direction, and it is a most suggestive fact that
photographs now show this cluster to be embedded in a vast nebula, which,
therefore, has also a proper motion; but some of the smaller stars do not
partake of it. Three stars in Cassiopeiæ also move together, and no doubt
many other similarly connected groups remain to be discovered.

These facts have a very important bearing on the question of the motion of
our sun in space. For this motion has been determined by comparing the
motions of large numbers of stars which are assumed to be wholly
independent of each other, and to move, as it were, at random. Miss A.M.
Clerke, in her _System of the Stars_, puts this point very clearly, as
follows: 'For the assumption that the absolute movements of the stars have
no preference for one direction over another, forms the basis of all
investigations hitherto conducted into the translatory advance of the solar
system. The little fabric of laboriously acquired knowledge regarding it at
once crumbles if that basis has to be removed. In all investigations of the
sun's movement, the movements of the stars have been regarded as casual
irregularities; should they prove to be in any visible degree systematic,
the mode of treatment adopted (and there is no other at present open to us)
becomes invalid, and its results null and void. The point is then of
singular interest, and the evidence bearing upon it deserves our utmost
attention.'

Mr. W.H.S. Monck, a well-known astronomer, takes the same view. He says:
'The proof of this motion rests on the assumption that if we take a
sufficient number of stars, their real motions in all directions will be
equal, and that therefore the apparent preponderances which we observe in
particular directions result from the real motion of the sun. But there is
no impossibility in a systematic motion of the majority of the stars used
in these researches which might reconcile the observed facts with a
motionless sun. And, in the second place, if the sun is not in the exact
centre of gravity of the universe, we might expect him to be moving in an
orbit around this centre of gravity, and our observations on his actual
motion are not sufficiently numerous or accurate to enable us to affirm
that he is moving in a right line rather than such an orbit.'

Now this 'systematic motion,' which would render all calculations as to the
sun's motion inaccurate or even altogether worthless, is by many
astronomers held to be an observed reality. The star-drift, first pointed
out by Proctor, has been shown to exist in many other groups of stars,
while the curious arrangements of stars all over the heavens in straight
lines, or regular curves, or spirals, strongly suggests a wide extension of
the same kind of relation. But even more extensive systematic movements
have been observed or suggested by astronomers. Sir D. Gill, by an
extensive research, believes that he has found indications of a rotation of
the brighter fixed stars as a whole in regard to the fainter fixed stars as
a whole. Mr. Maxwell Hall has also found indications of a movement of a
large group of stars, including our sun, around a common centre, situated
in a direction towards Epsilon Andromedæ, and at a distance of about 490
years of light-travel. These last two motions are not yet established; but
they seem to prove two important facts--(_a_) that eminent astronomers
believe that _some_ systematic motions must exist among the stars, or they
would not devote so much labour to the search for them; and (_b_) that
extensive systematic motions of some kind do exist, or even these results
would not have been obtained.

Mr. W.W. Campbell, of the Lick Observatory, thus remarks on the uncertainty
of determinations of the sun's motions: 'The motion of the solar system is
a purely relative quantity. It refers to specified groups of stars. The
results for various groups may differ widely, and all be correct. It would
be easy to select a group of stars with reference to which the solar motion
would be reversed 180° from the values assigned above' (_Astrophysical
Journal_, vol. xiii. p. 87. 1901).

It must be remembered that, within a uniform cluster of stars, each moving
round the common centre of gravity of the whole cluster, Kepler's laws do
not prevail, the law being that the angular velocities are all identical,
so that the more distant stars move faster than those nearer the centre,
subject to modifications, however, due to the varying density of the
cluster. But if the cluster is nearly globular, there must be stars moving
round the centre in every plane, and this would lead to apparent motions in
many directions as viewed by us, although those which were moving in the
same plane as ourselves would, when compared with remote stars outside the
cluster, appear to be all moving in the same direction and at the same
rate, forming, in fact, one of those drifting systems of stars already
referred to. Again, if in the process of formation of our cluster, smaller
aggregations already having a rotatory motion were drawn into it, this
might lead to their revolving in an opposite direction to those which were
formed from the original nebula, thus increasing the diversities of
apparent motion.

The evidence now briefly set forth fully justifies, I submit, the remarks
as to the statements of my astronomical critics at the beginning of this
section. They have both given the accepted views as to direction and rate
of movement of our sun without any qualification whatever, as if they were
astronomical facts of the same certainty and the same degree of accuracy as
the sun's distance from the earth; and they will assuredly have been so
understood by the great body of non-mathematical readers. It appears,
however, if the authorities I have quoted are right, that the whole
calculation rests upon certain assumptions, which are certainly to some
extent, and may be to a very large extent, erroneous. This is my reply to
one part of their criticism.

In the next place, they both assert, or imply, not only that the sun's
motion is now in a straight line, but that it has been in a straight line
from some enormously remote period when it first entered the stellar system
on one side, and will so continue to move till it reaches the utmost bounds
of that system on the other side. And this is stated by them both, not as a
possibility, but as a certainty. They use such terms as 'must' and 'will
be,' leaving no room for any doubt whatever. But such a result implies the
abrogation of the law of gravitation, since under its action motion in a
straight line in the midst of thousands or millions of suns of various
sizes is an absolute impossibility; while it also implies that the sun must
have been started on its course from some other system outside the Milky
Way, with such a precise determination of direction as not to collide with,
or even make a near approach to, any one of the suns or clusters of suns,
or vast nebulous masses, during its passage through the very midst of the
stellar universe.

This is my reply to the main point of their criticism, and I think I am
justified in saying that nothing in my whole article is so demonstrably
baseless as the statements I have now examined.

       *       *       *       *       *

Considering then the whole bearing of the evidence, I refuse to accept the
unsupported dicta of those who would have us believe that our admitted
position not far from the centre of the stellar universe is a mere
temporary coincidence of no significance whatever; or that our sun and
hosts of other similar orbs near to us have come together by an accident,
and are being dispersed into surrounding space, never to meet again. Until
this is proved by indisputable evidence, it seems to me far more probable
that we are moving in an orbit of some kind around the centre of gravity of
a vast cluster, as determined by the investigations of Kapteyn, Newcomb,
and other astronomers; and, consequently, that the nearly central position
we now occupy may be a permanent one. For even if our sun's orbit should
have a diameter a thousand times that of Neptune, it would be but a small
fraction of the diameter of the Milky Way; while so vast is the scale of
our universe, that it might be even a hundred thousand times as great and
still leave us deeply immersed in the solar cluster, and very much nearer
to the dense central portion than to its more diffused outer regions.

Here the subject may be left for the present. After having studied the
evidence afforded by the essential conditions of life-development on the
earth, and the numerous indications that these conditions do not exist on
any of the other planets of the solar system, it may be again touched upon
in a general review of the conclusions arrived at.

FOOTNOTES:

[4] _Nature_, October 26, 1899.

[5] _The System of the Stars_, p. 385.

[6] This account of Professor Kapteyn's research is taken from an article
by Miss A.M. Clerke in _Knowledge_, April 1893.

[7] _The Stars_, p. 256. The region here referred to is that where the
Milky Way has its greatest width (though nearly as wide in the part exactly
opposite), and where it may perhaps extend somewhat in our direction. Miss
A.M. Clerke informs me that in April 1901 Kapteyn withdrew the conclusions
arrived at in 1893, as being founded on illegitimate reasoning as to the
relation of parallaxes to proper motions. But as this relation is still
accepted, under certain limitations, by Professor Newcomb and other
astronomers, who have arrived independently at very similar results, it
seems not improbable that, after all, Professor Kapteyn's conclusions may
not require very much modification. Professor Newcomb also tells us (_The
Stars_, p. 214, footnote) that he has seen the latest of Professor
Kapteyn's papers, down to 1901; but he does not therefore express any doubt
as to his own conclusions as here referred to.

[8] See _Knowledge_ and _The Fortnightly Review_ of April 1903.

[9] Sir R. Ball in an article in _Good Words_ (April 1903) says that
luminosity is an exceptional phenomenon in nature, and that luminous stars
are but the glow-worms and fire-flies of the universe, as compared with the
myriads of other animals.



CHAPTER IX

THE UNIFORMITY OF MATTER AND ITS LAWS THROUGHOUT THE STELLAR UNIVERSE


I have shown in the second chapter of this work that none of the previous
writers on the question of the habitability of the other planets have
really dealt with the subject in any adequate manner, since not only do
they appear to be quite unaware of the delicate balance of conditions which
alone renders organic life possible on any planet, but they have altogether
omitted any reference to the fact that not only must the conditions be such
as to render life possible _now_, but these conditions must have persisted
during the long geological epochs needed for the slow development of life
from its most rudimentary forms. It will therefore be necessary to enter
into some details both as to the physical and chemical essentials for a
continuous development of organic life, and also into the combination of
mechanical and physical conditions which are required on any planet to
render such life possible.


THE UNIFORMITY OF MATTER

One of the most important and far-reaching of the discoveries due to the
spectroscope is that of the wonderful identity of the elements and
material compounds in earth and sun, stars and nebulæ, and also of the
identity of the physical and chemical laws that determine the states and
forms assumed by matter. More than half the total number of the known
elements have been already detected in the sun, including all those which
compose the bulk of the earth's solid material, with the one exception of
oxygen. This is a very large proportion when we consider the very peculiar
conditions which enable us to detect them. For we can only recognise an
element in the sun when it exists at its surface in an incandescent state,
and also above its surface in the form of a somewhat cooler gas. Many of
the elements may rarely or never be brought to the surface of so vast a
body, or if they do sometimes appear there, it may not be in sufficient
quantity or in sufficient purity to produce any bands in the spectroscope,
while the cooler gas or vapour may either not be present, or be so
dispersed as not to produce sufficient absorption to render its spectral
lines visible. Again, it is believed that many elements are dissociated by
the intense heat of the sun, and may not be recognisable by us, or they may
only exist at its surface in a compound form unknown on the earth; and in
some such way those lines of the solar spectrum which remain still
unrecognised may have been produced. One of these unknown lines was that of
Helium, a gas found soon afterwards in the rare mineral 'Cleveite,' and
since detected frequently in many stars. Some of the stars have spectra
very closely resembling that of the sun. The dark lines are almost as
numerous, and most of them correspond accurately with solar lines, so that
we cannot doubt their having almost exactly the same chemical constitution,
and being also in the same condition as regards heat and stage of
development. Other stars, as we have already stated, exhibit mainly lines
of hydrogen, sometimes combined with fine metallic lines. Of the spectra of
the nebulæ comparatively little is known, but many are decidedly gaseous,
while others show a continuous spectrum indicating a more complex
constitution.

But we also obtain considerable knowledge of the matter of non-terrestrial
bodies by the analysis of the numerous meteorites which fall upon the
earth. Most of these belong to some of the many meteoric streams which
circulate round the sun, and which may be supposed to give us samples of
planetary matter. But as it is now believed that many of them are produced
by the debris of comets, and the orbits of some of these indicate that they
have come from stellar space and have been drawn into our system by the
attractive power of the larger planets, it is almost certain that the
meteoric stones not infrequently bring us matter from the remoter regions
of space, and probably afford us samples of the solid constituents of
nebula; or the cooler stars. It is, therefore, a most suggestive fact that
none of these meteorites have been found to contain a single
non-terrestrial element, although no less than twenty-four elements have
been found in them, and it will be of interest to give the list of these,
as follows:--_Oxygen_, Hydrogen, _Chlorine_, _Sulphur_, _Phosphorus_,
Carbon, Silicon, Iron, Nickel, Cobalt, Magnesium, Chromium, Manganese,
Copper, Tin, _Antimony_, Aluminium, Calcium, Potassium, Sodium, _Lithium_,
Titanium, _Arsenic_, and Vanadium. Seven of the above, printed in italics,
have not yet been found in the sun, such as Oxygen, Chlorine, Sulphur, and
Phosphorus, which form the constituents of many widespread minerals, and
they supply important gaps in the series of solar and stellar elements. It
may be noted that although meteorites have supplied no new elements, they
have furnished examples of some new combinations of these elements forming
minerals distinct from any found in our rocks.

The fact of the occurrence in meteorites not only of minerals which are
peculiar to them or are found on the earth, but also of structures
resembling our breccias, veins, and even slicken-side surfaces, has been
held to be opposed to the meteoritic theory of the origin of suns and
planets, because meteorites seem to be thus proved to be the fragments of
suns or worlds, not their primary constituents. But these cases are
exceptional, and Mr. Sorby, who made a special study of meteorites,
concluded that their materials have usually been in a state of fusion or
even of vapour, as they now exist in the sun, and that they became
condensed into minute globular particles, which afterwards collected into
larger masses, and may have been broken up by mutual impact, and again and
again become aggregated together--thus presenting features which are
completely in accordance with the meteoritic theory.

But, quite recently, Mr. T.C. Chamberlin has applied the theory of tidal
distortion to showing how solid bodies in space, without ever coming into
actual contact, must sometimes be torn apart or disrupted into numerous
fragments by passing near to each other. Especially when a small body
passes near a much larger one, there is a certain distance of approach
(termed the Roche limit) when the increasing differential force of gravity
will be sufficient to tear asunder the smaller body and cause the fragments
either to circulate around it or to be dispersed in space.[10] In this way,
therefore, those larger meteorites which exhibit planetary structure may
have been produced. Of course they would rarely have been true planets
attached to a sun, but more frequently some of the smaller dark suns, which
may possess many of the physical characteristics of planets, and of which
there may be myriads in the stellar spaces.

On the whole, then, we have positive knowledge of the existence, in the
sun, stars, and planetary and stellar spaces, of such a large proportion of
the elements of our globe, and so few indications of any not forming part
of it, that we are justified in the statement, that the whole stellar
universe is, broadly speaking, constructed of the same series of elementary
substances as those we can study upon our earth, and of which the whole
realm of nature, animal, vegetable, and mineral, is composed. The evidence
of this identity of substance is really far more complete than we could
expect, considering the very limited means of inquiry that we possess; and
we shall, therefore, not be justified in assuming that any important
difference exists.

When we pass from the elements of matter to the laws which govern it, we
also find the clearest proofs of identity. That the fundamental law of
gravitation extends to the whole physical universe is rendered almost
certain by the fact that double stars move round their common centre of
gravity in elliptical orbits which correspond well with both observation
and calculation. That the laws of light are the same both here and in
inter-planetary space is indicated by the fact that the actual measurement
of the velocity of light on the earth's surface gives a result so
completely identical with that prevailing to the limits of the solar
system, that the measurement of the sun's distance, by means of the
eclipses of Jupiter's satellites combined with the measured velocity of
light, agrees almost exactly with that obtained by means of the transits of
Venus, or through our nearest approach to the planets Mars or Eros.

Again, the more recondite laws of light are found to be identical in sun
and stars with those observed within the narrow bounds of laboratory
experiments. The minute change of position of spectral lines caused by the
source of light moving towards or away from us enables us to determine this
kind of motion in the most distant stars, in the planets, or in the moon,
and these results can be tested by the motion of the earth either in its
orbit or in its rotation; and these latter tests agree with the theoretical
determination of what must occur, dependent on the wave-lengths of the
different dark lines of the solar spectrum determined by measurements in
the laboratory.

In like manner, minute changes in the widening or narrowing of spectral
lines, their splitting up, their increase or decrease in number, and their
arrangement so as to form flutings, can all be interpreted by experiments
in the laboratory, showing that such phenomena are due to alterations of
temperature, of pressure, or of the magnetic field, thus proving that the
very same physical and chemical laws act in the same way here and in the
remotest depths of space.

These various discoveries give us the certain conviction that the whole
material universe is essentially one, both as regards the action of
physical and chemical laws, and also in its mechanical relations of form
and structure. It consists throughout of the very same elements with which
we are so familiar on our earth; the same ether whose vibrations bring us
light and heat, electricity and magnetism, and a whole host of other
mysterious and as yet imperfectly known forces; gravitation acts throughout
its vast extent; and in whatever direction and by whatever means we obtain
a knowledge of the stellar universe, we find the same mechanical, physical,
and chemical laws prevailing as upon our earth, so that we have in some
cases been actually enabled to reproduce in our laboratories phenomena with
which we had first become acquainted in the sun or among the stars.

We may therefore feel it to be an almost certain conclusion that--the
elements being the same, the laws which act upon, and combine, and modify
those elements being the same--organised living beings wherever they may
exist in this universe must be, fundamentally, and in essential nature, the
same also. The outward forms of life, if they exist elsewhere, may vary
almost infinitely, as they do vary on the earth; but, throughout all this
variety of form--from fungus or moss to rose-bush, palm or oak; from
mollusc, worm, or butterfly to humming-bird, elephant, or man--the
biologist recognises a fundamental unity of substance and of structure,
dependent on the absolute requirements of the growing, moving, developing,
living organism, built up of the same elements, combined in the same
proportions, and subject to the same laws. We do not say that organic life
_could_ not exist under altogether diverse conditions from those which we
know or can conceive, conditions which may prevail in other universes
constructed quite differently from ours, where other substances replace the
matter and ether of our universe, and where other laws prevail. But,
_within_ the universe we know, there is not the slightest reason to suppose
organic life to be possible, except under the same general conditions and
laws which prevail here. We will, therefore, now proceed to describe, very
generally, what are the conditions essential to the existence and the
continuous development of vegetable and animal life.

FOOTNOTE:

[10] _The Astrophysical Journal_, vol. xiv., July 1901, p. 17.



CHAPTER X

THE ESSENTIAL CHARACTERS OF THE LIVING ORGANISM


Before trying to comprehend the physical conditions on any planet which are
essential for the development and maintenance of a varied and complex
system of organic life comparable to that of our earth, we must obtain some
knowledge of what life is, and of the fundamental nature and properties of
the living organism.

Physiologists and philosophers have made many attempts to define 'life,'
but in most cases in aiming at absolute generality they have been vague and
uninstructive. Thus De Blainville defined it as 'The twofold internal
movement of composition and decomposition, at once general and continuous';
while Herbert Spencer's latest definition was 'Life is the continuous
adjustment of internal relations to external relations.' But neither of
these is sufficiently precise, explanatory, or distinctive, and they might
almost be applied to the changes occurring in a sun or planet, or to the
elevation and gradual formation of a continent. One of the oldest
definitions, that of Aristotle, seems to come nearer the mark: 'Life is the
assemblage of the operations of nutrition, growth, and destruction.' But
these definitions of 'life' are unsatisfactory, because they apply to an
abstract idea rather than to the actual living organism. The marvel and
mystery of life, as we know it, resides in the body which manifests it, and
this living body the definitions ignore.

The essential points in the living body, as seen in its higher
developments, are, first, that it consists throughout of highly complex but
very unstable forms of matter, every particle of which is in a continual
state of growth or decay; that it absorbs or appropriates dead matter from
without; takes this matter into the interior of its body; acts upon it
mechanically and chemically, rejecting what is useless or hurtful; and so
transforming the remainder as to renew every atom of its own structure
internal and external, at the same time throwing off, particle by particle,
all the worn-out or dead portions of its own substance. Secondly, in order
to be able to do all this, its whole body is permeated throughout by
branching vessels or porous tissues, by which liquids and gases can reach
every part and carry on the various processes of nutrition and excretion
above referred to. As Professor Burdon Sanderson well puts it: 'The most
distinctive peculiarity of living matter as compared with non-living is,
that it is ever changing while ever the same.' And these changes are the
more remarkable because they are accompanied, and even produced, by a very
large amount of mechanical work--in animals by means of their normal
activities in search of food, in assimilating that food, in continually
renewing and building up their whole organism, and in many other ways; in
plants by building up their structure, which often involves raising tons of
material high into the air, as in forest trees. As a recent writer puts
it: 'The most prominent, and perhaps the most fundamental, phenomenon of
life is what may be described as the _Energy Traffic_ or the function of
_trading in energy_. The chief physical function of living matter seems to
consist in absorbing energy, storing it in a higher potential state, and
afterwards partially expending it in the kinetic or active form.'[11]

Thirdly--and perhaps most marvellous of all--all living organisms have the
power of reproduction or increase, in the lowest forms by a process of
self-division or 'fission,' as it is termed, in the higher by means of
reproductive cells, which, though in their earliest stage quite
indistinguishable physically or chemically in very different species, yet
possess the mysterious power of developing a perfect organism, identical
with its parents in all its parts, shapes, and organs, and so wonderfully
resembling them, that the minutest distinctive details of size, form, and
colour, in hair or feathers, in teeth or claws, in scales, spines, or
crests, are reproduced with very close accuracy, though often involving
metamorphic changes during growth of so strange a nature that, if they were
not familiar to us but were narrated as occurring only in some distant and
almost inaccessible region, would be treated as travellers' tales,
incredible and impossible as those of Sindbad the Sailor.

In order that the substance of living bodies should be able to undergo
these constant changes while preserving the same form and structure in
minute details--that they should be, as it were, in a constant state of
flux while remaining sensibly unchanged, it is necessary that the molecules
of which they are built up should be so combined as to be easily separated
and as easily united--be, as it is termed, _labile_ or flowing; and this is
brought about by their chemical composition, which, while consisting of few
elements, is yet highly complex in structure, a large number of chemical
atoms being combined in an endless variety of ways.

The physical basis of life, as Huxley termed it, is protoplasm, a substance
which consists essentially of only four common elements, the three gases,
nitrogen, hydrogen, and oxygen, with the non-metallic solid, carbon; hence
all the special products of plants and animals are termed carbon-compounds,
and their study constitutes one of the most extensive and intricate
branches of modern chemistry. Their complexity is indicated by the fact
that the molecule of sugar contains 45, and that of stearine no less than
173, constituent atoms. The chemical compounds of carbon are far more
numerous than those of all the other chemical elements combined; and it is
this wonderful variety and the complexity of its possible combinations
which explain the fact, that all the various animal tissues--skin, horn,
hair, nails, teeth, muscle, nerve, etc., consist of the same four elements
(with occasionally minute quantities of sulphur, phosphorus, lime, or
silica, in some of them), as proved by the marvellous fact that these
tissues are all produced as well by the grass-eating sheep or ox as by the
fish or flesh-eating seal or tiger. And the marvel is still further
increased when we consider that the innumerable diverse substances produced
by plants and animals are all formed out of the same three or four
elements. Such are the endless variety of organic acids, from prussic acid
to those of the various fruits; the many kinds of sugars, gums, and
starches; the number of different kinds of oil, wax, etc.; the variety of
essential oils which are mostly forms of turpentines, with such substances
as camphor, resins, caoutchouc, and gutta-percha; and the extensive series
of vegetable alkaloids, such as nicotine from tobacco, morphine from opium,
strychnine, curarine, and other poisons; quinine, belladonna, and similar
medicinal alkaloids; together with the essential principles of our
refreshing drinks, tea, coffee, and cocoa, and others too numerous to be
named here--all alike consisting solely of the four common elements from
which almost our whole organism is built up. If this were not indisputably
proved, it would scarcely be credited.

Professor F.J. Allen considers that the most important element in
protoplasm, and that which confers upon it its most essential properties in
the living organism--its extreme mobility and transposibility--is nitrogen.
This element, though inert in itself, readily enters into compounds when
energy is supplied to it, the most striking illustration of which is the
formation of ammonia, a compound of nitrogen and hydrogen, produced by
electric discharges through the atmosphere. Ammonia, and certain oxides of
nitrogen produced in the atmosphere in the same way, are the chief sources
of the nitrogen assimilated by plants, and through them by animals; for
although plants are continually in contact with the free nitrogen of the
atmosphere, they are unable to absorb it. By their leaves they absorb
oxygen and carbon-dioxide to build up their woody tissues, while by their
roots they absorb water in which ammonia and oxides of nitrogen are
dissolved, and from these they produce the protoplasm which builds up the
whole substance of the animal world. The energy required to produce these
nitrogen-compounds is given up by them when undergoing further changes, and
thus the production of ammonia by electricity in the atmosphere, and its
being carried by rain into the soil, constitute the first steps in that
long series of operations which culminates in the production of the higher
forms of life.

But the remarkable transformations and combinations continually going on in
every living body, which are, in fact, the essential conditions of its
life, are themselves dependent on certain physical conditions which must be
always present. Professor Allen remarks: 'The sensitiveness of nitrogen,
its proneness to change its state of combination and energy, appear to
depend on certain conditions of temperature, pressure, etc., which exist at
the surface of this earth. Most vital phenomena occur between the
temperature of freezing water and 104° F. If the general temperature of the
earth's surface rose or fell 72° F. (a small amount relatively), the whole
course of life would be changed, even perchance to extinction.'

Another important, and even more essential fact, in connection with life,
is the existence in the atmosphere of a small but nearly constant
proportion of carbonic acid gas, this being the source from which the whole
of the carbon in the vegetable and animal kingdoms is primarily derived.
The leaves of plants absorb carbonic acid gas from the atmosphere, and the
peculiar substance, chlorophyll, from which they derive their green colour,
has the power, under the influence of sunlight, to decompose it, using the
carbon to build up its own structure and giving out the oxygen. In the
laboratory the carbon can only be separated from the oxygen by the
application of heat, under which certain metals burn by combining with the
oxygen, thus setting free the carbon. Chlorophyll has a highly complex
chemical structure very imperfectly known, but it is said to be only
produced when there is iron in the soil.

The leaves of plants, so often looked upon as mere ornamental appendages,
are among the most marvellous structures in living organisms, since in
decomposing carbonic acid at ordinary temperatures they do what no other
agency in nature can perform. In doing this they utilise a special group of
ether-waves which alone appear to have this power. The complexity of the
processes going on in leaves is well indicated in the following
quotation:--

'We have seen how green leaves are supplied with gases, water, and
dissolved salts, and how they can trap special ether-waves. The active
energy of these waves is used to transmute the simple inorganic compounds
into complex organic ones, which in the process of respiration are reduced
to simpler substances again, and the potential energy transformed into
kinetic. These metabolic changes take place in living cells full of intense
activities. Currents course through the protoplasm and cell-sap in every
direction, and between the cells which are also united by strands of
protoplasm. The gases used and given off in respiration and assimilation
are floated in and out, and each protoplasm particle burned or unburned is
the centre of an area of disturbance. Pure protoplasm is influenced equally
by all rays: that associated with chlorophyll is affected by certain red
and violet rays in particular. These, especially the red ones, bring about
the dissociation of the elements of the carbonic acid, the assimilation of
the carbon, and the excretion of the oxygen.'[12]

It is this vigorous life-activity ever at work in the leaves, the roots,
and the sap-cells, that builds up the plant, in all its wondrous beauty of
bud and foliage, flower and fruit; and at the same time produces, either as
useful or waste-products, all that wealth of odours and flavours, of
colours and textures, of fibres and varied woods, of roots and tubers, of
gums and oils and resins innumerable, that, taken altogether, render the
world of vegetable life perhaps more varied, more beautiful, more
enjoyable, more indispensable to our higher nature than even that of
animals. But there is really no comparison between them. We _could_ have
plants without animals; we could _not_ have animals without plants. And all
this marvel and mystery of vegetable life, a mystery which we rarely ponder
over because its effects are so familiar, is usually held to be
sufficiently explained by the statement that it is all due to the special
properties of protoplasm. Well might Huxley say, that protoplasm is not
only a substance but a structure or mechanism, a mechanism kept at work by
solar heat and light, and capable of producing a thousand times more varied
and marvellous results than all the human mechanism ever invented.

But besides absorbing carbonic acid from the atmosphere, separating and
utilising the carbon and giving out the oxygen, plants as well as animals
continually absorb oxygen from the atmosphere, and this is so universally
the case that oxygen is said to be the food of protoplasm, without which it
cannot continue to live; and it is the peculiar but quite invisible
structure of the protoplasm which enables it to do this, and also in plants
to absorb an enormous amount of water as well.

But although protoplasm is so complex chemically as to defy exact analysis,
being an elaborate structure of atoms built up into a molecule in which
each atom must occupy its true place (like every carved stone in a Gothic
cathedral), yet it is, as it were, only the starting-point or material out
of which the infinitely varied structures of living bodies are formed. The
extreme mobility and changeability of the structure of these molecules
enables the protoplasm to be continually modified both in constitution and
form, and, by the substitution or addition of other elements, to serve
special purposes. Thus when sulphur in small quantities is absorbed and
built into the molecular structure, proteids are formed. These are most
abundant in animal structures, and give the nourishing properties to meat,
cheese, eggs, and other animal foods; but they are also found in the
vegetable kingdom, especially in nuts and seeds such as grain, peas, etc.
These are generally known as nitrogenous foods, and are very nutritious,
but not so easily digestible as meat. Proteids exist in very varied forms
and often contain phosphorus as well as sulphur, but their main
characteristic is the large proportion of nitrogen they contain, while many
other animal and vegetable products, as most roots, tubers, and grains, and
even fats and oils, are mainly composed of starch and sugar. In its
chemical and physiological aspects protein is thus described by Professor
W.D. Haliburton:--'Proteids are produced only in the living laboratory of
animals and plants; proteid matter is the all-important material present in
protoplasm. This molecule is the most complex that is known; it always
contains five and often six or even seven elements. The task of thoroughly
understanding its composition is necessarily vast, and advance slow. But,
little by little, the puzzle is being solved, and this final conquest of
organic chemistry, when it does arrive, will furnish physiologists with new
light on many of the dark places of physiological science.'[13]

What makes protoplasm and its modifications still more marvellous is the
power it possesses of absorbing and moulding a number of other elements in
various parts of living organisms for special uses. Such are silica in the
stems of the grass family, lime and magnesia in the bones of animals, iron
in blood, and many others. Besides the four elements constituting
protoplasm, most animals and plants contain also in some parts of their
structure sulphur, phosphorus, chlorine, silicon, sodium, potassium,
calcium, magnesium, and iron; while, less frequently, fluorine, iodine,
bromine, lithium, copper, manganese, and aluminium are also found in
special organs or structures; and the molecules of all these are carried
by the protoplasmic fluids to the places where they are required and built
into the living structure, with the same precision and for similar ends as
brick and stone, iron, slate, wood, and glass are each utilised in their
proper places in any large building.[14] The organism, however, is not
built, but grows. Every organ, every fibre, cell, or tissue is formed from
diverse materials, which are first decomposed into their elementary
molecules, transformed by the protoplasm or by special solvents formed from
it, carried to the places where they are needed by the vital fluids, and
there built up atom by atom or molecule by molecule into the special
structures of which they are to form a part.

But even this marvel of growth and repair of every individual organism is
far surpassed by the greater marvel of reproduction. Every living thing of
the higher orders arises from a single microscopic cell, when fertilised,
as it is termed, by the absorption of another microscopic cell derived from
a different individual. These cells are often, even under the highest
powers of the microscope, hardly distinguishable from other cells which
occur in all animals and plants and of which their structure is built up;
yet these special cells begin to grow in a totally different manner, and
instead of forming one particular part of the organism, develop inevitably
into a complete living thing with all the organs, powers, and peculiarities
of its parents, so as to be recognisably of the same species. If the
simple growth of the fully formed organism is a mystery, what of this
growth of thousands of complex organisms each with all its special
peculiarities, yet all arising from minute germs or cells the diverse
natures of which are wholly indistinguishable by the highest powers of the
microscope? This, too, is said to be the work of protoplasm under the
influence of heat and moisture, and modern physiologists hope some day to
learn 'how it is done.' It may be well here to give the views of a modern
writer on this point. Referring to a difficulty which had been stated by
Clerk-Maxwell twenty-five years ago, that there was not room in the
reproductive cell for the millions of molecules needed to serve as the
units of growth for all the different structures in the body of the higher
animals, Professor M'Kendrick says:--'But to-day, it is reasonable from
existing data to suppose that the germinal vesicle might contain a million
of millions of organic molecules. Complex arrangements of these molecules
suited for the development of all the parts of a highly complicated
organism, might satisfy all the demands of the theory of heredity.
Doubtless the germ was a material system through and through. The
conception of the physicist was, that molecules were in various states of
movement; and the thinkers were striving toward a kinetic theory of
molecules and of atoms of solid matter, which might be as fruitful as the
kinetic theory of gases. There were motions atomic and molecular. It was
conceivable that the peculiarities of vital action might be determined by
the kind of motion that took place in the molecules of what we call living
matter. It might be different in kind from some of the motions dealt with
by physicists. Life is continually being created from non-living
material--such, at least, is the existing view of growth by the
assimilation of food. The creation of living matter out of non-living may
be the transmission to the dead matter of molecular motions which are _sui
generis_ in form.' This is the modern physiological view of 'how it may be
done,' and it seems hardly more intelligible than the very old theory of
the origin of stone axes, given by Adrianus Tollius in 1649, and quoted by
Mr. E.B. Tylor, who says:--'He gives drawings of some ordinary stone axes
and hammers and tells how naturalists say that they are generated in the
sky by a fulgureous exhalation conglobed in a cloud by the circumfixed
humour, and are, as it were, baked hard by intense heat, and the weapon
becomes pointed by the damp mixed with it flying from the dry part, and
leaving the other end denser, but the exhalations press it so hard that it
breaks through the cloud and makes thunder and lightning. But--he says--if
this is really the way in which they are generated, it is odd they are not
round, and that they have holes through them. It is hardly to be believed,
he thinks.'[15] And so, when the physiologists, determined to avoid the
assumption of anything beyond matter and motion in the germ, impute the
whole development and growth of the elephant or of man from minute cells
internally alike, by means of 'kinds of motion' and the 'transmission of
motions which are _sui generis_ in form,' many of us will be inclined to
say with the old author--'It is hardly to be believed, I think.'

This brief statement of the conclusions arrived at by chemists and
physiologists as to the composition and structure of organised living
things has been thought advisable, because the non-scientific reader has
often no conception of the incomparable marvel and mystery of the
life-processes he has always seen going on, silently and almost unnoticed,
in the world around him. And this is still more the case now that
two-thirds of our population are crowded into cities where, removed from
all the occupations, the charms, and the interests of country life, they
are driven to seek occupation and excitement in the theatre, the
music-hall, or the tavern. How little do these know what they lose by being
thus shut out from all quiet intercourse with nature; its soothing sights
and sounds; its exquisite beauties of form and colour; its endless
mysteries of birth, and life, and death. Most people give scientific men
credit for much greater knowledge than they possess in these matters; and
many educated readers will, I feel sure, be surprised to find that even
such apparently simple phenomena as the rise of the sap in trees are not
yet completely explained. As to the deeper problems of life, and growth,
and reproduction, though our physiologists have learned an infinite amount
of curious or instructive facts, they can give us no intelligible
explanation of them.

The endless complexities and confusing amount of detail in all treatises on
the physiology of animals and plants are such, that the average reader is
overwhelmed with the mass of knowledge presented to him, and concludes
that after such elaborate researches everything must be known, and that the
almost universal protests against the need of any causes but the
mechanical, physical, and chemical laws and forces are well founded. I
have, therefore, thought it advisable to present a kind of bird's-eye view
of the subject, and to show, in the words of the greatest living
authorities on these matters, both how complex are the phenomena and how
far our teachers are from being able to give us any adequate explanation of
them.

I venture to hope that the very brief sketch of the subject I have been
able to give will enable my readers to form some faint general conception
of the infinite complexity of life and the various problems connected with
it; and that they will thus be the better enabled to appreciate the extreme
delicacy of those adjustments, those forces, and those complex conditions
of the environment, that alone render life, and above all the grand
age-long panorama of the development of life, in any way possible. It is to
these conditions, as they prevail in the world around us, that we will now
direct our attention.

FOOTNOTES:

[11] Professor F.J. Allen: _What is Life?_

[12] Art. 'Vegetable Physiology' in _Chambers's Encyclopædia_.

[13] Address to the British Association, 1902, Section Physiology.

[14] This enumeration of the elements that enter into the structure of
plants and animals is taken from Professor F.J. Allen's paper already
referred to.

[15] _Early History of Mankind_, 2nd ed. p. 227.



CHAPTER XI

THE PHYSICAL CONDITIONS ESSENTIAL FOR ORGANIC LIFE


The physical conditions on the surface of our earth which appear to be
necessary for the development and maintenance of living organisms may be
dealt with under the following headings:--

1. Regularity of heat-supply, resulting in a limited range of temperature.

2. A sufficient amount of solar light and heat.

3. Water in great abundance, and universally distributed.

4. An atmosphere of sufficient density, and consisting of the gases which
are essential for vegetable and animal life. These are Oxygen,
Carbonic-acid gas, Aqueous vapour, Nitrogen, and Ammonia. These must all be
present in suitable proportions.

5. Alternations of day and night.


SMALL RANGE OF TEMPERATURE REQUIRED FOR GROWTH AND DEVELOPMENT

Vital phenomena for the most part occur between the temperatures of
freezing water and 104° Fahr., and this is supposed to be due mainly to the
properties of nitrogen and its compounds, which between these temperatures
only can maintain those peculiarities which are essential to life--extreme
sensitiveness and lability; facility of change as regards chemical
combination and energy; and other properties which alone render nutrition,
growth, and continual repair possible. A very small increase or decrease of
temperature beyond these limits, if continued for any considerable time,
would certainly destroy most existing forms of life, and would not
improbably render any further development of life impossible except in some
of its lowest forms.

As one example of the direct effects of increased temperature, we may
adduce the coagulation of albumen. This substance is one of the proteids,
and plays an important part in the vital phenomena of both plants and
animals, and its fluidity and power of easy combination and change of form
are lost by any degree of coagulation which takes place at about 160° Fahr.

The extreme importance to all the higher organisms of a moderate
temperature is strikingly shown by the complex and successful arrangements
for maintaining a uniform degree of heat in the interior of the body. The
normal blood-heat in a man is 98° Fahr., and this is constantly maintained
within one or two degrees though the external temperature may be more than
fifty degrees below the freezing-point. High temperatures upon the earth's
surface do not range so far from the mean as do the low. In the greater
part of the tropics the air-temperature seldom reaches 96° Fahr., though in
arid districts and deserts, which occur chiefly along the margins of the
northern and southern tropics, it not unfrequently surpasses 110° Fahr.,
and even occasionally rises to 115° or 120° in Australia and Central India.
Yet with suitable food and moderate care the blood-temperature of a healthy
man would not rise or fall more than one or at most two degrees. The great
importance of this uniformity of temperature in all the vital organs is
distinctly shown by the fact that when, during fevers, the temperature of
the patient rises six degrees above the normal amount, his condition is
critical, while an increase of seven or eight degrees is an almost certain
indication of a fatal result. Even in the vegetable kingdom seeds will not
germinate under a temperature of four or five degrees above the
freezing-point.

Now this extreme sensibility to variations of internal temperature is quite
intelligible when we consider the complexity and instability of protoplasm,
and of all the proteids in the living organism, and how important it is
that the processes of nutrition and growth, involving constant motion of
fluids and incessant molecular decompositions and recombinations, should be
effected with the greatest regularity. And though a few of the higher
animals, including man, are so perfectly organised that they can adapt or
protect themselves so as to be able to live under very extreme conditions
as regards temperature, yet this is not the case with the great majority,
or with the lower types, as evidenced by the almost complete absence of
reptiles from the arctic regions.

It must also be remembered that extreme cold and extreme heat are nowhere
perpetual. There is always some diversity of seasons, and there is no land
animal which passes its whole life where the temperature never rises above
the freezing point.


THE NECESSITY OF SOLAR LIGHT

Whether the higher animals and man could have been developed upon the earth
without solar light, even if all the other essential conditions were
present, is doubtful. That, however, is not the point I am at present
considering, but one that is much more fundamental. Without plant life land
animals at all events could never have come into existence, because they
have not the power of making protoplasm out of inorganic matter. The plant
alone can take the carbon out of the small proportion of carbonic acid in
the atmosphere, and with it, and the other necessary elements, as already
described, build up those wonderful carbon compounds which are the very
foundation of animal life. But it does this solely by the agency of solar
light, and even uses a special portion of that light. Not only, therefore,
is a sun needed to give light and heat, but it is quite possible that _any_
sun would not answer the purpose. A sun is required whose light possesses
those special rays which are effective for this operation, and as we know
that the stars differ greatly in their spectra, and therefore in the nature
of their light, all might not be able to effect this great transformation,
which is one of the very first steps in rendering animal life possible on
our earth, and therefore probably on all earths.


WATER A FIRST ESSENTIAL OF ORGANIC LIFE

It is hardly necessary to point out the absolute necessity of water, since
it actually constitutes a very large proportion of the material of every
living organism, and about three-fourths of our own bodies. Water,
therefore, must be present everywhere, in one form or another, on any globe
where life is possible. Neither animal nor plant can exist without it. It
must also be present in such quantity and so distributed as to be
constantly available on every part of a globe where life is to be
maintained; and it is equally necessary that it should have persisted in
equal profusion throughout those enormous geological epochs during which
life has been developing. We shall see later on how very special are the
conditions that have secured this continuous distribution of water on our
earth, and we shall also learn that this large amount of water, its wide
distribution, and its arrangement with regard to the land-surface, is an
essential factor in producing that limited range of temperature which, as
we have seen, is a primary condition for the development and maintenance of
life.


THE ATMOSPHERE MUST BE OF SUFFICIENT DENSITY AND COMPOSED OF SUITABLE GASES

The atmosphere of any planet on which life can be developed must have
several qualities which are unconnected with each other, and the
coincidence of which may be a rare phenomenon in the universe. The first of
these is a sufficient density, which is required for two purposes--as a
storer of heat, and in order to supply the oxygen, carbonic acid, and
aqueous vapour in sufficient quantities for the requirements of vegetable
and animal life.

As a reservoir of heat and a regulator of temperature, a rather dense
atmosphere is a first necessity, in co-operation with the large quantity
and wide distribution of water referred to in the last section. The very
different character of our south-west from our north-east winds is a good
illustration of its power of distributing heat and moisture. This it does
owing to the peculiar property it possesses of allowing the sun's rays to
pass freely through it to the earth which it warms, but acting like a
blanket in preventing the rapid escape of the non-luminous heat so
produced. But the heat stored up during the day is given out at night, and
thus secures a uniformity of temperature which would not otherwise exist.
This effect is strikingly seen at high altitudes, where the temperature
becomes lower and lower, till at a not very great elevation, even in the
tropics, snow lies on the ground all the year round. This is almost wholly
due to the rarity of the air, which, on that account, has not so much
capacity for heat. It also allows the heat it acquires to radiate more
freely than denser air, so that the nights are much colder. At about 18,000
feet high our atmosphere is exactly half its density at the sea-level. This
is considerably higher than the usual snow-line, even under the equator,
whence it follows that if our atmosphere was only half its present density
it would render the earth unsuitable for the higher forms of animal life.
It is not easy to say exactly what would be the result as regards climate;
but it seems likely that, except perhaps in limited areas in the tropics,
where conditions were very favourable, the whole land-surface would become
buried in snow and ice. This appears inevitable, because evaporation from
the oceans by direct sun-heat would be more rapid than now; but as the
vapour rose in the rare atmosphere it would rapidly become frozen, and snow
would fall almost perpetually, although it might not lie permanently on the
ground in the equatorial lowlands. It appears certain, therefore, that with
half our present bulk of atmosphere life would be hardly possible on the
earth on account of lowered temperature alone. And as there would certainly
be an added difficulty in the needful supply of oxygen to animals and
carbonic acid to plants, it seems highly probable that a reduction of
density of even one-fourth might be sufficient to render a large portion of
the globe a snow and ice-clad waste, and the remainder liable to such
extremes of climate that only low forms of life could have arisen and been
permanently maintained.


THE GASES OF THE ATMOSPHERE

Coming now to consider the constituent gases of the atmosphere, there is
reason to believe that they form a mixture as nicely balanced in regard to
animal and vegetable life as are the density and the temperature. At a
first view of the subject we might conclude that oxygen is the one great
essential for animal life, and that all else is of little importance. But
further consideration shows us that nitrogen, although merely a diluent of
the oxygen as regards the respiration of animals, is of the first
importance to plants, which obtain it from the ammonia formed in the
atmosphere and carried down into the soil by the rain. Although there is
only one part of ammonia to a million of air, yet upon this minute
proportion the very existence of the animal world depends, because neither
animals nor plants can assimilate the free nitrogen of the air into their
tissues.

Another fundamentally important gas in the atmosphere is carbonic acid,
which forms about four parts in ten thousand parts of air, and, as already
stated, is the source from which plants build up the great bulk of their
tissues, as well as those protoplasms and proteids so absolutely necessary
as food for animals. An important fact to notice here is, that carbonic
acid, so essential to plants, and to animals through plants, is yet a
poison to animals. When present in much more than the normal quantity, as
it often is in cities and in badly ventilated buildings, it becomes highly
prejudicial to health; but this is believed to be partly due to the various
corporeal emanations and other impurities associated with it. Pure carbonic
acid gas to the amount of even one per cent. in otherwise pure air can, it
is said, be breathed for a time without bad effects, but anything more than
this proportion will soon produce suffocation. It is probable, therefore,
that a very much smaller proportion than one per cent., if constantly
present, would be dangerous to life; though no doubt, if this had always
been the proportion, life might have been developed in adaptation to it.
Considering, however, that this poisonous gas is largely given out by the
higher animals as a product of respiration, it would evidently be dangerous
to the permanence of life if the quantity forming a constant constituent of
the atmosphere were much greater than it is.


AQUEOUS VAPOUR IN THE ATMOSPHERE

This water-gas, although it occurs in the atmosphere in largely varying
quantities, is yet, in two distinct ways, essential to organic life. It
prevents the too rapid loss of moisture from the leaves of plants when
exposed to the sun, and it is also absorbed by the upper surface of the
leaf and by the young shoots, which thus obtain both water and minute
quantities of ammonia when the supply by the roots is insufficient. But it
is of even more vital importance in supplying the hydrogen which, when
united with the nitrogen of the atmosphere by electrical discharges,
produces the ammonia, which is the main source of all the proteids of the
plant, which proteids are the very foundation of animal life.

From this brief statement of the purposes served by the various gases
forming our atmosphere, we see that they are to some extent antagonistic,
and that any considerable increase of one or the other would lead to
results that might be injurious either directly or in their ultimate
results. And as the elements which constitute the bulk of all living matter
possess properties which render them alone suitable for the purpose, we may
conclude that the proportions in which they exist in our atmosphere cannot
be very widely departed from wherever organic forms are developed.


THE ALTERNATION OF DAY AND NIGHT

Although it is difficult to decide positively whether alternations of light
and darkness at short intervals are absolutely essential for the
development of the various higher forms of life, or whether a world in
which light was constant might do as well, yet on the whole it seems
probable that day and night are really important factors. All nature is
full of rhythmic movements of infinitely varied kinds, degrees, and
durations. All the motions and functions of living things are periodic;
growth and repair, assimilation and waste, go on alternately. All our
organs are subject to fatigue and require rest. All kinds of stimulus must
be of short duration or injurious results follow. Hence the advantage of
darkness, when the stimuli of light and heat are partially removed, and we
welcome 'tired nature's sweet restorer, balmy sleep'--giving rest to all
the senses and faculties of body and mind, and endowing us with renewed
vigour for another period of activity and enjoyment of life.

Plants as well as animals are invigorated by this nightly repose; and all
alike benefit by these longer periods of greater and less amounts of work
caused by summer and winter, dry and wet seasons. It is a suggestive fact,
that where the influence of heat and light is greatest--within the
tropics--the days and nights are of equal length, giving equal periods of
activity and rest. But in cold and Arctic regions where, during the short
summer, light is nearly perpetual, and all the functions of life, in
vegetation especially, go on with extreme rapidity, this is followed by the
long rest of winter, with its short days and greatly lengthened periods of
darkness.

Of course, all this is rather suggestion than proof. It is possible that in
a world of perpetual day or in one of perpetual night, life _might_ have
been developed. But on the other hand, considering the great variety of
physical conditions which are seen to be necessary for the development and
preservation of life in its endless varieties, any prejudicial influences,
however slight, might turn the scale, and prevent that harmonious and
continuous evolution which we know _must_ have occurred.

So far I have only considered the question of day and night as regards the
presence or absence of light. But it is probably far more important in its
heat aspect; and here its period becomes of great, perhaps vital,
importance. With its present duration of twelve hours day and twelve night
on the average, there is not time, even between the tropics, for the earth
to become so excessively heated as to be inimical to life; while a
considerable portion of the heat, stored up in the soil, the water, and the
atmosphere, is given out at night, and thus prevents a too sudden and
injurious contrast of heat and cold. If the day and night were each very
much longer--say 50 or 100 hours--it is quite certain that, during a day of
that duration, the heat would become so great as to be inimical, perhaps
prohibitive, to most forms of life; while the absence of all sun-heat for
an equally long period would result in a temperature far below the
freezing point of water. It is doubtful whether any high forms of animal
life could have arisen under such great and continual contrasts of
temperature.

We will now proceed to point out the special features which, in our earth,
have combined to bring about and to maintain the various and complex
conditions we have seen to be essential for life as it exists around us.



CHAPTER XII

THE EARTH IN ITS RELATION TO THE DEVELOPMENT AND MAINTENANCE OF LIFE


The first circumstance to be considered in relation to the habitability of
a planet is its distance from the sun. We know that the heating power of
the sun upon our earth is ample for the development of life in an almost
infinite variety of forms; and we have a large amount of evidence to show
that, were it not for the equalising power of air and water, distributed as
they are with us, the heat received from the sun would be sometimes too
great and sometimes too little. In some parts of Africa, Australia, and
India, the sandy soil becomes so hot that an egg can be cooked by placing
it just below the surface. On the other hand, at an elevation of about
12,000 feet in lat. 40° it freezes every night, and throughout the day in
all places sheltered from the sun. Now, both these temperatures are adverse
to life, and if either of them persisted over a considerable portion of the
earth, the development of life would have been impossible. But the heat
derived from the sun is inversely as the square of the distance, so that at
half the distance we should have four times as much heat, and at twice the
distance only one-fourth of the heat. Even at two-thirds of the distance we
should receive more than twice as much heat; and, considering the facts as
to the extreme sensitiveness of protoplasm and the coagulation of albumen,
it seems certain that we are situated in what has been termed the temperate
zone of the solar system, and that we could not be removed far from our
present position without endangering a considerable portion of the life now
existing upon the earth, and in all probability rendering the actual
development of life, through all its phases and gradations, impossible.


THE OBLIQUITY OF THE ECLIPTIC

The effect of the obliquity of the earth's equator to its path round the
sun, upon which depend our varying seasons and the inequality of day and
night throughout all the temperate zones, is very generally known. But it
is not usually considered that this obliquity is of any great importance as
regards the suitability of the earth for the development and maintenance of
life; and it seems to have been passed over as an accident hardly worth
notice, because almost any other obliquity or none at all would have been
equally advantageous. But if we consider what the direction of the earth's
axis might possibly have been, we shall find that it is really a matter of
great importance from our present point of view.

Let us suppose, first, that the earth's axis was, like that of Uranus,
almost exactly in the plane of its orbit or directed towards the sun. There
can be little doubt that such a position would have rendered our world
unfitted for the development of life. For the result would be the most
tremendous contrasts of the seasons; at mid-winter, on one half the globe,
arctic night and more than arctic cold would prevail; while on the other
half there would be a midsummer of continuous day with a vertical sun and
such an amount of heat as nowhere exists with us. At the two equinoxes the
whole globe would enjoy equal day and night, all our present tropics and
part of the sub-tropical zone having the sun at noon so near to the zenith
as to have the essential of a tropical climate. But the change to about a
month of constant sunshine or a month of continuous night would be so
rapid, that it seems almost impossible that either vegetable or animal life
would ever have developed under such terrible conditions.

The other extreme direction of the earth's axis, exactly at right angles to
the plane of the orbit, would be much more favourable, but would still have
its disadvantages. The whole surface from equator to poles would enjoy
equal day and night, and every part would receive the same amount of
sun-heat all the year round, so that there would be no change of seasons;
but the heat received would vary with the latitude. In our latitude the
sun's altitude at noon all the year would be less than 40°, the same as now
occurs at the equinoxes, and we might therefore have a perpetual spring as
regards temperature. But the constancy of the heat in the equatorial and
tropical regions and of cold towards the poles would lead to a more
constant and more rapid circulation of air, and we should probably
experience such continuous north-westerly winds as to render our climate
always cold and probably very damp. Near the poles the sun would always be
on, or close to, the horizon, and would give so little heat that the sea
might be perpetually frozen and the land deeply snow-buried; and these
conditions would probably extend into the temperate zone, and possibly so
far south as to render life impossible in our latitudes, since whatever
results arose would be due to permanent causes, and we know how powerful
are snow and ice to extend their sway over adjacent areas if not
counteracted by summer heat or warm moist winds. On the whole, therefore,
it seems probable that this position of the earth's axis would result in a
much smaller portion of its surface being capable of supporting a luxuriant
and varied vegetable and animal life than is now the case; while the
extreme uniformity of conditions everywhere present might be so
antagonistic to the great law of rhythm that seems to pervade the universe,
and be in other ways so unfavourable, that life-development would probably
have taken quite a different course from that which it has taken.

It appears almost certain, therefore, that some intermediate position of
the axis would be the most favourable; and that which actually exists seems
to combine the advantage of change of seasons with good climatical
conditions over the largest possible area. We know that during the greater
part of the epoch of life-development this area was much greater than at
present, since a luxuriant vegetation of deciduous and evergreen trees and
shrubs extended up to and within the Arctic Circle, leading to the
formation of coal-beds both in palæozoic and tertiary times; the extremely
favourable conditions for organic life which then prevailed over so large a
portion of the globe's surface, and which persisted down to a
comparatively recent epoch, lead to the conclusion that no more favourable
degree of obliquity was possible than that which we actually possess. A
short account of the evidence on this interesting subject will now be
given.


PERSISTENCE OF MILD CLIMATES THROUGH GEOLOGIC TIME

The whole of the geological evidence goes to show that in remote ages the
climate of the earth was generally more uniform, though perhaps not warmer,
than it is now, and this can be best explained by a slightly different
distribution of sea and land, which allowed the warm waters of the tropical
oceans to penetrate into various parts of the continents (which were then
more broken up than they are now), and also to extend more freely into the
Arctic regions. So soon as we go back into the tertiary period, we find
indications of a warmer climate in the north temperate zone; and when we
reach the middle of that period, we find abundant indications, both in
plant and animal remains, of mild climates near to the Arctic Circle, or
actually within it.

On the west coast of Greenland, in 70° N. lat., there are found abundance
of fossil plants very beautifully preserved, among which are many different
species of oaks, beeches, poplars, plane-trees, vines, walnuts, plums,
chestnuts, sequoias, and numerous shrubs--137 species in all, indicating a
vegetation such as now grows in the north temperate parts of America and
Eastern Asia. And even further north, in Spitzbergen, in N. lat. 78° and
79°, a somewhat similar flora is found, not quite so varied, but with oaks,
poplars, birches, planes, limes, hazels, pines, and many aquatic plants
such as may now be found in West Norway and in Alaska, nearly twenty
degrees further south.

Still more remote, in the Cretaceous period, fossil plants have been found
in Greenland, consisting of ferns, cycads, conifers, and such trees and
shrubs as poplars, sassafras, andromedas, magnolias, myrtles, and many
others, similar in character and often identical in species with fossils of
the same period found in Central Europe and the United States, indicating a
widespread uniformity of climate, such as would be brought about by the
great ocean-currents carrying the warm waters of the tropics into the
Arctic seas.

Still further back, in the Jurassic period, we have proofs of a mild
climate in East Siberia and at Andö in Norway just within the Arctic
Circle, in numerous plant remains, and also remains of great reptiles
allied to those found in the same strata in all parts of the world. Similar
phenomena occur in the still earlier Triassic period; but we will pass on
to the much more remote Carboniferous period, during which most of the
great coal-beds of the world were formed from a luxuriant vegetation,
consisting mostly of ferns, giant horse-tails, and primitive conifers. The
luxuriance of these plants, which are often found beautifully preserved and
in immense quantities, is supposed to indicate an atmosphere in which
carbonic acid gas was much more abundant than now; and this is rendered
probable by the small number and low type of terrestrial animals,
consisting of a few insects and amphibia.

But the interesting point is, that true coal-beds, with similar fossils to
those of our own coal-measures, are found at Spitzbergen and at Bear Island
in East Siberia, both far within the Arctic Circle, again indicating a
great uniformity of climate, and probably a denser and more vapour-laden
atmosphere, which would act as a blanket over the earth and preserve the
heat brought to the Arctic seas by the ocean currents from the warmer
regions.

The still earlier Silurian rocks are also found abundantly in the Arctic
regions, but their fossils are entirely of marine animals. Yet they show
the same phenomena as regards climate, since the corals and cephalopodous
mollusca found in the Arctic beds closely resemble those of all other parts
of the earth.[16]

Many other facts indicate that throughout the enormous periods required for
the development of the varied forms of life upon the earth, the great
phenomena of nature were but little different from those that prevail in
our own times. The slow and gentle processes by which the various vegetable
and animal remains were preserved are shown by the perfect state in which
many of the fossils exist. Often trunks of trees, cycads, and tree-ferns
are found standing erect, with their roots still imbedded in the soil they
grew in. Large leaves of poplars, maples, oaks, and other trees are often
preserved in as perfect a state as if gathered by a botanist and dried
between paper for his herbarium, and the same is especially the case with
the beautiful ferns of the Permian and Carboniferous periods. Throughout
these and most other formations well-preserved ripple-marks are found in
the solidified mud or sand of old seashores, differing in no respect from
similar marks to be found on almost every coast to-day. Equally interesting
are the marks of rain-drops preserved in the rocks of almost all ages. Sir
Charles Lyell has given illustrations of recent impressions of rain-drops
on the extensive mud-flats of Nova Scotia, and also an illustration of
rain-drops on a slab of shale from the carboniferous formation of the same
country; and the two are as much alike as the prints of two different
showers a few days apart. The general size and form of the drops are almost
identical, and imply a great similarity in the general atmospheric
conditions.

We must not forget that this presence of rain throughout geological time
implies, as we have seen in our last chapter, a constant and universal
distribution of atmospheric dust. The two chief sources of this dust--the
total quantity of which in the atmosphere must be enormous--are volcanoes
and deserts, and we are therefore sure that these two great natural
phenomena have always been present. Of volcanoes we have ample independent
evidence in the presence of lavas and volcanic ashes, as well as actual
stumps or cores of old volcanoes, through all geological formations; and we
can have little doubt that deserts also were present, though perhaps not
always so extensive as they are now. It is a very suggestive fact that
these two phenomena, usually held to be blots on the fair face of nature,
and even to be opposed to belief in a beneficent Creator, should now be
proved to be really essential to the earth's habitability.

Notwithstanding this prevalence of warm and uniform conditions, there is
also evidence of considerable changes of climate; and at two periods--in
the Eocene and in the remote Permian--there are even indications of
ice-action, so that some geologists believe that there were then actual
glacial epochs. But it seems more probable that they imply only local
glaciation, owing to there having been high land and other suitable
conditions for the production of glaciers in certain areas.

The whole bearing of the geological evidence indicates the wonderful
continuity of conditions favourable for life, and for the most part of
climatal conditions more favourable than those now prevailing, since a
larger extent of land towards the North Pole was available for an abundant
vegetation, and in all probability for an equally abundant animal life. We
know, too, that there was never any total break in life-development; no
epoch of such lowering or raising of temperature as to destroy all life; no
such general subsidence as to submerge the whole land-surface. Although the
geological record is in parts very imperfect, yet it is, on the whole,
wonderfully complete; and it presents to our view a continuous progress,
from simple to complex, from lower to higher. Type after type becomes
highly specialised in adaptation to local or climatal conditions, and then
dies out, giving room for some other type to arise and be specialised in
harmony with the changed conditions. The general character of the inorganic
change appears to have been from more insular to more continental
conditions, accompanied by a change from more uniform to less uniform
climates, from an almost sub-tropical warmth and moisture, extending up to
the Arctic Circle, to that diversity of tropical, temperate, and cold
areas, capable of supporting the greatest possible variety in the forms of
life, and which seems especially adapted to stimulate mankind to
civilisation and social development by means of the necessary struggle
against, and utilisation of, the various forces of nature.


WATER, ITS AMOUNT AND DISTRIBUTION ON THE EARTH

Although it is generally known that the oceans occupy more than two-thirds
of the whole surface of the globe, the enormous bulk of the water in
proportion to the land that rises above its surface is hardly ever
appreciated. But as this is a matter of the greatest importance, both as
regards the geological history of the globe and the special subject we are
here discussing, it will be necessary to enter into some details in regard
to it.

According to the best recent estimates, the land area of the globe is 0.28
of the whole surface, and the water area 0.72. But the mean height of the
land above the sea-level is found to be 2250 feet, while the mean depth of
the seas and oceans is 13,860 feet; so that though the water area is two
and a half times that of the land, the mean depth of the water is more
than six times the mean height of the land. This is, of course, due to the
fact that lowlands occupy most of the land-area, the plateaus and high
mountains a comparatively small portion of it; while, though the greatest
depths of the oceans about equal the greatest heights of the mountains, yet
over enormous areas the oceans are deep enough to submerge all the
mountains of Europe and temperate North America, except the extreme summits
of one or two of them. Hence it follows that the bulk of the oceans, even
omitting all the shallow seas, is more than thirteen times that of the land
above sea-level; and if all the land-surface and ocean-floors were reduced
to one level, that is, if the solid mass of the globe were a true oblate
spheroid, the whole would be covered with water about two miles deep. The
diagram here given will render this more intelligible and will serve to
illustrate what follows.

[Illustration: _Diagram of proportionate mean height of Land and depth of
Oceans_. _Land_ _Area. .28 of area of Globe._ _Ocean_ _Area .72 of area of
Globe._]

In this diagram the lengths of the sections representing land and ocean are
proportionate to their areas, while the thickness of each is proportionate
to their mean height and mean depth respectively. Hence the two sections
are in correct proportion to their cubic contents.

A mere inspection of this diagram is sufficient to disprove the old idea,
still held by a few geologists and by many biologists, that oceans and
continents have repeatedly changed places during geological times, or that
the great oceans have again and again been bridged over to facilitate the
distribution of beetles or birds, reptiles or mammals. We must remember
that although the diagram shows the continents and oceans as a whole, yet
it also shows, with quite sufficient accuracy, the proportions of each of
the great continents to the oceans which are adjacent to them. It must also
be borne in mind that there can be no elevation on a large scale without a
corresponding subsidence elsewhere; because if there were not a vast
unsupported hollow would be left beneath the rising land or in some part
adjacent to it.

Now, looking at the diagram and at a chart or globe, try to imagine the
ocean-bottom rising gradually, to form a continent joining Africa with
South America or with Australia (both of which are demanded by many
biologists): it is clear that, while such an elevation was going on, either
some continental land or some other part of the ocean-bed must sink to a
corresponding amount. We shall then see, that if such changes of elevation
on a continental scale have taken place again and again at different
periods, it would have been almost impossible, on every occasion, to avoid
a whole continent being submerged (or even all the continents) in order to
equalise subsidence with elevation while new continents were being raised
up from the abyssal depths of the ocean. We conclude, therefore, that with
the exception of a comparatively narrow belt around the continents, which
may be roughly indicated by the thousand fathom line of soundings, the
great ocean depths are permanent features of the earth's surface. It is
this stability of the general distribution of land and water that has
secured the continuity of life upon the earth. Had the great oceanic
basins, on the other hand, been unstable, changing places with the land at
various periods of geological time, they would, almost certainly, again and
again have swallowed up the land in their vast abysses, and have thus
destroyed all the organic life of the world.

There are many confirmatory proofs of this view (which is now widely
accepted by geologists and physicists), and a few of them may be briefly
stated.

1. None of the continents present us with marine deposits of any one
geological age and occupying a large part of the surface of each, as must
have been the case had they ever been sunk deep beneath the ocean and again
elevated; neither do any of them contain extensive formations corresponding
to the deep oceanic clays and oozes, which again they must have done had
they been at any time raised up from the ocean depths.

2. All the continents present an almost complete and continuous series of
rocks of _all_ geological ages, and in each of the great geological periods
there are found fresh water and estuarine deposits, and even old
land-surfaces, demonstrating continuity of continental or insular
conditions.

3. All the great oceans possess, scattered over them, a few or many islands
termed 'oceanic,' and characterised by a volcanic or coralline structure,
with no ancient stratified rocks in anyone of them; and in none of these
is there found a single indigenous land mammal or amphibian. It is
incredible that, if these oceans had ever contained extensive continents,
and if these oceanic islands are--as even now they are often alleged to
be--parts of these now submerged continents, not one fragment of any of the
old stratified rocks, which characterise all existing continents, should
remain to show their origin. In the Atlantic we find the Azores, Madeira,
and St. Helena; in the Indian Ocean, Mauritius, Bourbon, and Kerguelen
Island; in the Pacific, the Fiji, Samoan, Society, Sandwich, and Galapagos
Islands, all without exception telling us the same tale, that they have
been built up from the ocean depths by submarine volcanoes and coralline
growths, but have never formed part of continental areas.

4. The contours of the floors of all the great oceans, now fairly well
known through the soundings of exploring vessels and for submarine
telegraph lines, also give confirmatory evidence that they have never been
continental land. For if any part of them were a sunken continent, that
part must have retained some impress of its origin. Some of the numerous
mountain ranges which characterise _every_ continent would have remained.
We should find slopes of from 20° to 50° not uncommon, while valleys
bordered by rocky precipices, as in Lake Lucerne and a hundred others, or
isolated rock-walled mountains like Roraima, or ranges of precipices as in
the Ghâts of India or the Fiords of Norway, would frequently be met with.
But not a single feature of this kind has ever been found in the ocean
abysses. Instead of these we have vast plains, which, if the water were
removed, would appear almost exactly level, with no abrupt slopes
anywhere. When we consider that deposits from the land never reach these
remote ocean depths, and that there is no wave-action below a few hundred
feet, these continental features once submerged would be indestructible;
and their total absence is, therefore, itself a demonstration that none of
the great oceans are on the sites of submerged continents.


HOW OCEAN DEPTHS WERE PRODUCED

It is a very difficult problem to determine how the vast basins which are
filled by the great oceans, especially that of the Pacific, were first
produced. When the earth's surface was still in a molten state, it would
necessarily take the form of a true oblate spheroid, with a compression at
the poles due to its speed of rotation, which is supposed to have been very
great. The crust formed by the gradual cooling of such a globe would be of
the same general form, and, being thin, would easily be fractured or bent
so as to accommodate itself to any unequal stresses from the interior. As
the crust thickened and the whole mass slowly cooled and contracted,
fissures and crumpling would occur, the former serving as outlets for
volcanic activities whose results are found throughout all geological ages;
the latter producing mountain chains in which the rocks are almost always
curved, folded, or even thrust over each other, indicating the mighty
forces due to the adjustments of a solid crust upon a shrinking fluid or
semi-fluid interior.

But during this whole process there seem to be no forces at work that could
lead to the production of such a feature as the Pacific, a vast depression
covering nearly one-third of the whole surface of the globe. The Atlantic
Ocean, being smaller and nearly opposite to the Pacific, but approximately
of equal depth, may be looked upon as a complementary phenomenon which will
be probably explained as a result of the same causes as the vaster cavity.

So far as I am aware, there is only one suggested cause of the formation of
these great oceans that seems adequate; and as that cause is to some extent
supported by quite independent astronomical evidence, and also directly
bears upon the main subject of the present volume, it must be briefly
considered.

A few years ago, Professor George Darwin, of Cambridge, arrived at a
certain conclusion as to the origin of the moon, which is now comparatively
well known by Sir Robert Ball's popular account of it in his small volume,
_Time and Tide_. Briefly stated, it is as follows. The tides produce
friction on the earth and very slowly increase the length of our day, and
also cause the moon to recede further from us. The day is lengthened only
by a small fraction of a second in a thousand years, and the moon is
receding at an equally imperceptible rate. But as these forces are
constant, and have always acted on the earth and moon, as we go back and
back into the almost infinite past we come to a time when the rotation of
the earth was so rapid that gravity at the equator could hardly retain its
outer portion, which was spread out so that the form of the whole mass was
something like a cheese with rounded edges. And about the same epoch the
distance of the moon is found to have been so small that it was actually
touching the earth. All this is the result of mathematical calculation from
the known laws of gravitation and tidal effects; and as it is difficult to
see how so large a body as the moon could have originated in any other way,
it is supposed that at a still earlier period the moon and earth were one,
and that the moon separated from the parent mass owing to centrifugal force
generated by the earth's rapid rotation. Whether the earth was liquid or
solid at this epoch, and exactly how the separation occurred, is not
explained either by Professor Darwin or Sir Robert Ball; but it is a very
suggestive fact that, quite recently, it has been shown, by means of the
spectroscope, that double stars of short period _do_ originate in this way
from a single star, as already described in our sixth chapter; but in these
cases it seems probable that the parent star is in a gaseous state.

These investigations of Professor G. Darwin have been made use of by the
Rev. Osmond Fisher (in his very interesting and important work, _Physics of
the Earth's Crust_) to account for the basins of the great oceans, the
Pacific being the chasm left when the larger portion of the mass of the
moon parted from the earth.

Adopting, as I do, the theory of the origin of the earth by meteoric
accretion of solid matter, we must consider our planet as having been
produced from one of those vast rings of meteorites which in great numbers
still circulate round the sun, but which at the much earlier period now
contemplated were both more numerous and much more extensive. Owing to
irregularities of distribution in such a ring and through disturbance by
other bodies, aggregations of various size would inevitably occur, and the
largest of these would in time draw in to itself all the rest, and thus
form a planet. During the early stages of this process the particles would
be so small and would come together so gradually, that little heat would be
produced, and there would result merely a loose aggregation of cold matter.
But as the process went on and the mass of the incipient planet became
considerable--perhaps half that of the earth--the rest of the ring would
fall in with greater and greater velocity; and this, added to the
gravitative compression of the growing mass might, when nearly its present
size, have produced sufficient heat to liquefy the outer layers, while the
central portion remained solid and to some extent incoherent, with probably
large quantities of heavy gases in the interstices. When the amount of the
meteoric accretions became so reduced as to be insufficient to keep up the
heat to the melting-point, a crust would form, and might have reached about
half or three-fourths of its present thickness when the moon became
separated.

Let us now try to picture to ourselves what happened. We should have a
globe somewhat larger than our earth is now, both because it then contained
the material of the moon and also because it was hotter, revolving so
rapidly as to be very greatly flattened at the poles; while the equatorial
belt bulged out enormously, and would probably have separated in the form
of a ring with a very slight increase of the time of rotation, which is
supposed to have been about four hours. This globe would have a
comparatively thin crust, beneath which there was molten rock to an
unknown depth, perhaps a few hundreds, perhaps more than a thousand miles.
At this time the attraction of the sun acting on the molten interior
produced tides in it, causing the thin crust to rise and fall every two
hours, but to so small an extent--only about a foot or so--as not
necessarily to fracture it; but it is calculated that this slight rhythmic
undulation coincided with the normal period of undulation due to such a
large mass of heavy liquid, and so tended to increase the instability due
to rapid rotation.

The bulk of the moon is about one-fiftieth part that of the earth, and an
easy calculation shows us that, taking the area of the Pacific, Atlantic,
and Indian Oceans combined as about two-thirds that of the globe, it would
require a thickness (or depth) of about forty miles to furnish the material
for the moon. We must, of course, assume that there were some inequalities
in the thickness of the crust and in its comparative rigidity, so that when
the critical moment came and the earth could no longer retain its
equatorial protuberance against the centrifugal force due to rotation
combined with the tidal undulations caused by the sun, instead of a
continuous ring slowly detaching itself, the crust gave way in two or more
great masses where it was weakest, and as the tidal wave passed under it
and a quantity of the liquid substratum rose with it, the whole would break
up and collect into a sub-globular mass a short distance from the earth,
and continue revolving with it for some time at about the same rate as the
surface had rotated. But as tidal action is always equal on opposite sides
of a globe, there would be a similar disruption there, forming, it may be
supposed, the Atlantic basin, which, as may be seen on a small globe, is
almost exactly opposite a part of the Central Pacific. So soon as these two
great masses had separated from the earth, the latter would gradually
settle down into a state of equilibrium, and the molten matter of the
interior, which would now fill the great oceanic basins up to a level of a
few mile below the general surface would soon cool enough to form a thin
crust. The larger portion of the nascent moon would gradually attract to
itself the one or more smaller portions and form our satellite; and from
that time tidal friction by both moon and sun would begin to operate and
would gradually lengthen our day and, more rapidly, our month in the way
explained in Sir Robert Ball's volume.

A very interesting point may now be referred to, because it seems
confirmatory of this origin of the great ocean basins. In Mr. Osmond
Fisher's work it is explained how the variations in the force of gravity,
at numerous points all over the world, have been determined by observations
with the pendulum, and also how these variations afford a measure of the
thickness of the solid crust, which is of less specific gravity than the
molten interior on which it rests. By this means a very interesting result
was obtained. The observations on numerous oceanic islands proved that the
sub-oceanic crust was considerably more dense than the crust under the
continents, but also thinner, the result being to bring the average mass of
the sub-oceanic crust and oceans to an equality with that of the
continental crust, and this causes the whirling earth to be in a state of
balance, or equilibrium. Now, both the thinness and the increased density
of the crust seem to be well explained by this theory of the origin of the
oceanic basins. The new crust would necessarily for a long time be thinner
than the older portion, because formed so much later, but it would very
soon become cool enough to allow the aqueous vapour of the atmosphere and
that given off through fissures from the molten interior to collect in the
ocean basins, which would thenceforth be cooled more rapidly and kept at a
uniform temperature and also under a uniform pressure, and these conditions
would lead to the steady and continuous increase of thickness, with a
greater compactness of structure than in the continental areas. It is no
doubt to this uniformity of conditions, with a lowering of the bottom
temperature throughout the greater part of geological time, till it has
become only a few degrees above the freezing-point, that we owe the
remarkable persistence of the vast and deep ocean basins on which, as we
have seen, the continuity of life on the earth has largely depended.

There is one other fact which lends some support to this theory of the
origin of the ocean basins--their almost complete symmetry with regard to
the equator. Both the Atlantic and Pacific basins extend to an equal
distance north and south of the equator, an equality which could hardly
have been produced by any cause not directly connected with the earth's
rotation. The polar seas which are coterminous with the two great oceans
are very much shallower, and cannot, therefore, be considered as forming
part of the true oceanic basins.


WATER AS AN EQUALISER OF TEMPERATURE

The importance of water in regulating the temperature of the earth is so
great that, even if we had enough water on the land for all the wants of
plants and animals, but had no great oceans, it is almost certain that the
earth could not have produced and sustained the various forms of life which
it now possesses.

The effect of the oceans is twofold. Owing to the great specific heat of
water, that is, its property of absorbing heat slowly but to a large
amount, and giving it out with equal slowness, the surface-waters of the
oceans and seas are heated by the sun so that by the evening of a bright
day they have become quite warm to a depth of several feet. But air has
much less specific heat than water, a pound of water in cooling one degree
being capable of warming four pounds of air one degree; but as air is 770
times as light as water, it follows that the heat from one cubic foot of
water will warm more than 3000 cubic feet of air as much as it cools
itself. Hence the enormous surface of the seas and oceans, the larger part
of which is within the tropics, warms the whole of the lower and denser
portions of the air, especially during the night, and this warmth is
carried to all parts of the earth by the winds, and thus ameliorates the
climate. Another quite distinct effect is due to the great ocean currents,
like the Gulf Stream and the Japan Current, which carry the warm water of
the tropics to temperate and arctic regions, and thus render many countries
habitable which would otherwise suffer the rigour of an almost arctic
winter. These currents are, however, directly due to the winds, and
properly belong to the section on the atmosphere.

The other equalising action, due primarily to the great area of the seas
and oceans, is a result of the vast evaporating surface from which the land
derives almost all its water in the form of rain and rivers; and it is
quite evident that if there were not sufficient water-surface to produce an
ample supply of vapour for this purpose, arid districts would occupy more
and more of the earth's surface. How much water-surface is necessary for
life we do not know; but if the proportions of water and land-surfaces were
reversed, it seems probable that the larger proportion of the earth might
be uninhabitable. The vapour thus produced has also a very great effect in
equalising temperature; but this also is a point which will come better
under our next chapter on the atmosphere.

       *       *       *       *       *

There are, however, some matters connected with the water-supply of the
earth, and its relation to the development of life, that call for a few
remarks here. What has determined the total quantity of water on the earth
or on other planets does not appear to be known; but presumably it would
depend, partially or wholly, on the mass of the planet being sufficient to
enable it to retain by its gravitative force the oxygen and hydrogen of
which water is composed. As the two gases are so easily combined to form
water, but can only be separated under special conditions, its quantity
would be dependent on the supply of hydrogen, which is but rarely found on
the earth in a free state. The important fact, however, is, that we do
possess so great a quantity of water, that if the whole surface of the
globe was as regularly contoured as are the continents, and merely wrinkled
with mountain chains, then the existing water would cover the whole globe
nearly two miles deep, leaving only the tops of high mountains above its
surface as rows of small islands, with a few larger islands formed by what
are now the high plateaus of Tibet and the Southern Andes.

Now there seems no reason why this distribution of the water should not
have occurred--in fact it seems probable that it would have occurred, had
it not been for the fortunate coincidence of the formation of enormously
deep ocean basins. So far as I am aware, no sufficient explanation of the
formation of these basins has been given but that of Mr. Osmond Fisher, as
here described, and that depends upon three unique circumstances: (1) the
formation of a satellite at a very late period of the planet's development
when there was already a rather thick crust; (2) the satellite being far
larger in proportion to its primary than any other in the solar system; and
(3) its having been produced by fission from its primary on account of
extremely rapid rotation, combined with solar tides in its molten interior,
and a rate of oscillation of that molten interior coinciding with the tidal
period.[17]

Whether this very remarkable theory of the origin of our moon is the true
one, and if so, whether the explanation it seems to afford of the great
oceanic basins is correct, I am not mathematician enough to judge. The
tidal theory of the origin of the moon, as worked out mathematically by
Professor G.H. Darwin, has been supported by Sir Robert Ball and accepted
by many other astronomers; while the researches of the Rev. Osmond Fisher
into the _Physics of the Earth's Crust_, together with his mathematical
abilities and his practical work as a geologist, entitle his opinion on the
question of the mode of origin of the ocean basins to the highest respect.
And, as we have seen, the existence of these vast and deep ocean basins,
produced by the agency of a series of events so remarkable as to be quite
unique in the solar system, played an important part in rendering the earth
fit for the development of the higher forms of animal life, while without
them it seems not improbable that the conditions would have been such as to
render any varied forms of terrestrial life hardly possible.

FOOTNOTES:

[16] For a fuller account of this Arctic fauna and flora see the works of
Sir C. Lyell, Sir A. Geikie, and other geologists. A full summary of it is
also given in the author's _Island Life_.

[17] Professor G.H. Darwin states that it is nearly certain that no other
satellite nor any of the planets originated in the same way as the moon.



CHAPTER XIII

THE EARTH IN RELATION TO LIFE: ATMOSPHERIC CONDITIONS


We have seen in our tenth chapter that the physical basis of
life--protoplasm--consists of the four elements, oxygen, nitrogen,
hydrogen, and carbon, and that both plants and animals depend largely upon
the free oxygen in the air to carry on their vital processes; while the
carbonic acid and ammonia in the atmosphere seem to be absolutely essential
to plants. Whether life could have arisen and have been highly developed
with an atmosphere composed of different elements from ours it is, of
course, impossible to say; but there are certain physical conditions which
seem absolutely essential whatever may be the elements which compose it.

The first of these essentials is an atmosphere which shall be of such
density at the surface of the planet, and of so great a bulk, as to be not
too rare to fulfil its various functions at all altitudes where there is a
considerable area of land. What determines the total quantity of gaseous
matter on the surface of a planet will be, mainly, its mass, together with
the average temperature of its surface.

The molecules of gases are in a state of rapid motion in all directions,
and the lighter gases have the most rapid motions. The average speed of
the motion of the molecules has been roughly determined under varying
conditions of pressure and temperature, and also the probable maximum and
minimum rates, and from these data, and certain known facts as to planetary
atmospheres, Mr. G. Johnstone Stoney, F.R.S., has calculated what gases
will escape from the atmospheres of the earth and the other planets. He
finds that all the gases which are constituents of air have such
comparatively low molecular rates of motion that the force of gravity at
the upper limits of the earth's atmosphere is amply sufficient to retain
them; hence the stability in its composition. But there are two other
gases, hydrogen and helium, which are both known to enter the atmosphere,
but never accumulate so as to form any measurable portion of it, and these
are found to have sufficient molecular motion to escape from it. With
regard to hydrogen, if the earth were much larger and more massive than it
is, so as to retain the hydrogen, disastrous consequences might ensue,
because, whenever a sufficient quantity of this gas accumulated, it would
form an explosive mixture with the oxygen of the atmosphere, and a flash of
lightning or even the smallest flame would lead to explosions so violent
and destructive as perhaps to render such a planet unsuited for the
development of life. We appear, therefore, to be just at the major limit of
mass to secure habitability, except in such planets as may have no
continuous supply of free hydrogen.

       *       *       *       *       *

Perhaps the most important mechanical functions of the atmosphere dependent
on its density are: (1) the production of winds, which in many ways bring
about an equalisation of temperature, and which also produce
surface-currents on the ocean; and (2) the distribution of moisture over
the earth by means of clouds which also have other important functions.

Winds depend primarily on the local distribution of heat in the air,
especially on the great amount of heat constantly present in the equatorial
zone, due to the sun being always nearly vertical at noon, and to its being
similarly vertical at each tropic once a year, with a longer day, leading
to even higher temperatures than at the equator, and producing also that
continuous belt of arid lands or deserts which almost encircle the globe in
the region of the tropics. Heated air being lighter, the colder air from
the temperate zones continually flows towards it, lifting it up and causing
it to flow over, as it were, to the north and south. But as the inflow
comes from an area of less rapid to one of more rapid rotation, the course
of the air is diverted, and produces the north-east and south-east trades;
while the overflow from the equator going to an area of less rapid
rotation, turns westward and produces the south-west winds so prevalent
over the north Atlantic and the north temperate zone generally, and the
north-west in the southern hemisphere.

It is outside the zone of the equable trade-winds, and in a region a few
degrees on each side of the tropics, that destructive hurricanes and
typhoons prevail. These are really enormous whirlwinds due to the intensely
heated atmosphere over the arid regions already mentioned, causing an
inrush of cool air from various directions, thus setting up a rotatory
motion which increases in rapidity till equilibrium is restored. The
hurricanes of the West Indies and Mauritius, and the typhoons of the
Eastern seas, are thus caused. Some of these storms are so violent that no
human structures can resist them, while the largest and most vigorous trees
are torn to pieces or overturned by them. But if our atmosphere were much
denser than it is, its increased weight would give it still greater
destructive force; and if to this were added a somewhat greater amount of
sun-heat--which might be due either to our greater proximity to the sun or
to the sun's greater size or greater heat-intensity, these tempests might
be so increased in frequency and violence as to render considerable
portions of the earth uninhabitable.

The constant and equable trade-winds have a very important function in
initiating those far-reaching ocean-currents which are of the greatest
importance in equalising temperature. The well known Gulf Stream is to us
the most important of these currents, because it plays the chief part in
giving us the mild climate we enjoy in common with the whole of Western
Europe, a mildness which is felt to a considerable distance within the
Arctic Circle; and, in conjunction with the Japan current, which does the
same for the whole of the temperate regions of the North Pacific, renders a
large portion of the globe better adapted for life than it would be without
these beneficial influences.

These equalising currents, however, are almost entirely due to the form and
position of the continents, and especially to the fact that they are so
situated as to leave vast expanses of ocean along the equatorial zone, and
extending north and south to the arctic and antarctic regions. If with the
same amount of land the continents had been so grouped as to occupy a
considerable portion of the equatorial oceans--such as would have been the
case had Africa been turned so as to join South America, and Asia been
brought to the south-east so as to take the place of part of the equatorial
Pacific, then the great ocean-currents could have been but feeble or have
hardly existed. Without these currents much of the north and south
temperate lands would have been buried in ice, while the largest portion of
the continents would have been so intensely heated as perhaps to be
unsuited for the development of the higher forms of animal life, since we
have shown (in chapters X. and XI.) how delicate is the balance and how
narrow the limits of temperature which are required.

There seems to be no reason whatever why some such distribution of the sea
and land should not have existed, had it not been for the admittedly
exceptional conditions which led to the production of our satellite, thus
necessarily forming vast chasms along the region of the equator where
centrifugal force as well as the internal solar tides were most powerful,
and where the thin crust was thus compelled to give way. And as the highest
authorities declare that there are no indications of such an origin of
satellites in the case of any other planet, the whole series of conditions
favourable to life on the earth become all the more remarkable.


CLOUDS, THEIR IMPORTANCE AND THEIR CAUSES

Few persons have any adequate conception of the real nature of clouds and
of the important part they take in rendering our world a habitable and an
enjoyable one.

On the average, the rainfall over the oceans is much less than over the
land, the whole region of the trade-winds having usually a cloudless sky
and very little rain; but in the intervening belt of calms, near to the
equator, a cloudy sky and heavy rains are frequent. This arises from the
fact that the warm, moist air over the ocean is raised upwards, by the cold
and heavy air from north and south, into a cooler region where it cannot
hold so much aqueous vapour, which is there condensed and falls as rain.
Generally, wherever the winds blow over extensive areas of water on to the
land, especially if there are mountains or elevated plateaus which cause
the moisture-laden air to rise to heights where the temperature is lower,
clouds are formed and more or less rain falls. But if the land is of an
arid nature and much heated by the sun, the air becomes capable of holding
still more aqueous vapour, and even dense rain-clouds disperse without
producing any rainfall. From these simple causes, with the large area of
sea as compared with the land upon our earth, by far the larger portion of
the surface is well supplied with rain, which, falling most abundantly in
the elevated and therefore cooler regions, percolates the soil, and gives
rise to those innumerable springs and rivulets which moisten and beautify
the earth, and which, uniting together, form streams and rivers, which
return to the seas and oceans whence they were originally derived.


CLOUDS AND RAIN DEPEND UPON ATMOSPHERIC DUST

The beautiful system of aqueous circulation by means of the atmosphere as
sketched above was long thought to explain the whole process, and to
require no further elucidation; but about a quarter of a century back a
curious experiment was made which indicated that there was another factor
in the process which had been entirely overlooked. If a small jet of steam
is sent into two large glass receivers, one filled with ordinary air, the
other with air which has been filtered by passing through a thick layer of
cotton wool so as to keep back all particles of solid matter, the first
vessel will be instantly filled with condensed cloudy-looking vapour, while
in the other vessel the air and vapour will remain perfectly transparent
and invisible. Another experiment was then made to imitate more nearly what
occurs in nature. The two vessels were prepared as before, but a small
quantity of water was placed in each vessel and allowed to evaporate till
the air was nearly saturated with vapour, which remained invisible in both.
Both vessels were then slightly cooled, when instantly a dense cloud was
formed in that filled with unfiltered air, while the other remained quite
clear. These experiments proved that the mere cooling of air below the dew
point will not cause the aqueous vapour in it to condense into drops so as
to form mist, fog, or cloud, unless small particles of solid or liquid
matter are present to act as nuclei upon which condensation begins. The
density of a cloud will therefore depend not only on the quantity of vapour
in the air, but on the presence of an abundance of minute dust-particles on
which condensation can begin.

That such dust exists everywhere in the air, even up to great heights, is
not a supposition but a proved fact. By exposing glass plates covered with
glycerine in different places and at different altitudes the number of
these particles in each cubic foot of air has been determined; and it is
found that not only are they present everywhere at low levels, but that
there are a considerable number even at the tops of the highest mountains.
These solid particles also act in another way. By radiation in the higher
atmosphere they become very cold, and thus condense the vapour by contact,
just as the points of grass-blades condense it to form dew.

When steam is escaping from an engine we see a mass of dense white vapour,
a miniature cloud; and if we are near it in cold, damp weather, we feel
little drops of rain produced from it. But on a fine, warm day it rises
quickly and soon melts away, and entirely disappears. Exactly the same
thing happens on a larger scale in nature. In fine weather we may have
abundant clouds continually passing high overhead, but they never produce
rain, because as the minute globules of water slowly fall towards the
earth, the warm dry air again turns them into invisible vapour. Again, in
fine weather, we often see a small cloud on a mountain top which remains
there a considerable time, even though a brisk wind is blowing. The
mountain top is colder than the surrounding air, and the invisible vapour
becomes condensed into cloud by passing over it, but the moment these cloud
particles are carried past the summit into the warmer and drier air they
are again evaporated and disappear. On Table Mountain, near Cape Town, this
phenomenon occurs on a large scale, and is termed the table-cloth, the mass
of white fleecy cloud seeming to hang over the flat mountain top to some
distance down where it remains for several months, while all around there
is bright sunshine.

Another phenomenon that indicates the universal presence of dust to
enormous heights in the atmosphere is the blue colour of the sky. This is
caused by the presence of such excessively minute particles of dust through
an enormous thickness of the higher atmosphere--probably up to a height of
twenty or thirty miles, or more--that they reflect only the light of short
wave-length from the blue end of the spectrum. This also has been proved by
experiment. If a glass cylinder, several feet long, is filled with pure air
from which all solid particles have been removed by filtering and passing
over red-hot platinum wires, and a ray of electric light is passed through
it, the interior, when viewed laterally, appears quite dark, the light
passing through in a straight line and not illuminating the air. But if a
little more air is passed through the filter so rapidly as to allow only
the minutest particles of dust to enter with it, the vessel becomes
gradually filled with a blue haze, which gradually deepens into a beautiful
blue, comparable with that of the sky. If now some of the unfiltered air
is admitted, the blue fades away into the ordinary tint of daylight.

Since it has been known that liquid oxygen is blue, many people have
concluded that this explains the blue colour of the sky. But it has really
nothing to do with the point at issue. The blue of the liquid oxygen
becomes so excessively faint in the gas, further attenuated as it is by the
colourless nitrogen, that it would have no perceptible colour in the whole
thickness of our atmosphere. Again, if it had a perceptible blue tint we
could not see it against the blackness of space behind it; but white
objects seen through it, such as the moon and clouds, should all appear
blue, which they do not do. The blue we see is from the whole sky, and is
therefore reflected light; and as pure air is quite transparent, there must
be solid or liquid particles so minute as to reflect blue light only. In
the lower atmosphere the rain-producing particles are larger, and reflect
all the rays, thus diluting the blue colour near the horizon, and, by
refraction and reflection combined, producing the various beautiful hues of
sunrise and sunset.

This production of exquisite colours by the dust in the atmosphere, though
adding greatly to the enjoyment of life, cannot be considered essential to
it; but there is another circumstance connected with atmospheric dust
which, though little appreciated, might have effects which can hardly be
calculated. If there were no dust in the atmosphere, the sky would appear
black even at noon, except in the actual direction of the sun; and the
stars would be visible in the day as well as at night. This would follow
because air does not reflect light, and is not visible. We should therefore
receive no light from the sky itself as we do now, and the north side of
every hill, house, and other solid objects, would be totally dark, unless
there were any surfaces in that direction to reflect the light. The surface
of the ground at a little distance would be in sunshine, and this would be
the only source of light wherever direct sunlight was cut off. To get a
good amount of pleasant light in houses it would be necessary to have them
built on nearly level ground, or on ground rising to the north, and with
walls of glass all round and down to the floor line, to receive as much as
possible of the reflected light from the ground. What effect this kind of
light would have on vegetation it is difficult to say, but trees and shrubs
would probably grow laterally towards the south, east, and west, so as to
get as much direct sunshine as possible.

A more important result would be that, as sunshine would be perpetual
during the day, so much evaporation would take place that the soil would
become arid and almost bare in places that are now covered with vegetation,
and plants like the cactuses of Arizona and the euphorbias of South Africa
would occupy a large portion of the surface.

Returning now from this collateral subject of light and colour to the more
important aspect of the question--the absence of cloud and rain--we have to
consider what would happen, and in what way the enormous quantity of water
which would be evaporated under continual sunshine would be returned to the
earth.

The first and most obvious means would be by abnormally abundant dews,
which would be deposited almost every night on every form of leafy
vegetation. Not only would all grass and herbage, but all the outer leaves
of shrubs and trees, condense so much moisture as to take the place of rain
so far as the needs of such vegetation were concerned. But without
arrangements for irrigation cultivation would be almost impossible, because
the bare soil would become intensely heated during the day, and would
retain so much of its heat through the night so as to prevent any dew
forming upon it.

Some more effective mode, therefore, of returning the aqueous vapour of the
atmosphere to the earth and ocean, would be required, and this, I believe,
would be done by means of hills and mountains of sufficient height to
become decidedly colder than the lowlands. The air from over the oceans
would be constantly loaded with moisture, and whenever the winds blew on to
the land the air would be carried up the slopes of the hills into the
colder regions, and there be rapidly condensed upon the vegetation, and
also on the bare earth and rocks of northern slopes, and wherever they
cooled sufficiently during the afternoon or night to be below the
temperature of the air. The quantity of vapour thus condensed would reduce
the atmospheric pressure, which would lead to an inrush of air from below,
bringing with it more vapour, and this might give rise to perpetual
torrents, especially on northern and eastern slopes. But as the evaporation
would be much greater than at the present time, owing to perpetual
sunshine, so the water returned to the earth would be greater, and as it
would not be so uniformly distributed over the land as it is now, the
result would perhaps be that extensive mountain sides would become
devastated by violent torrents, rendering permanent vegetation almost
impossible; while other and more extensive areas, in the absence of rain,
would become arid wastes that would support only the few peculiar types of
vegetation that are characteristic of such regions.

Whether such conditions as here supposed would prevent the development of
the higher forms of life it is impossible to say, but it is certain that
they would be very unfavourable, and might have much more disastrous
consequences than any we have here suggested. We can hardly suppose that,
with winds and rock-formations at all like what they are now, any world
could be wholly free from atmospheric dust. If, however, the atmosphere
itself were much less dense than it is, say one-half, which might very
easily have been the case, then the winds would have less carrying power,
and at the elevations at which clouds are usually formed there would not be
enough dust-particles to assist in their formation. Hence fogs close to the
earth's surface would largely take the place of clouds floating far above
it, and these would certainly be less favourable to human life and to that
of many of the higher animals than existing conditions.

The world-wide distribution of atmospheric dust is a remarkable phenomenon.
As the blue colour of the sky is universal, the whole of the higher
atmosphere must be pervaded by myriads of ultra-microscopical particles,
which, by reflecting the blue rays only, give us not only the azure vault
of heaven, but in combination with the coarser dust of lower altitudes,
diffused daylight, the grand forms and motions of the fleecy clouds, and
the 'gentle rain from heaven' to refresh the parched earth and make it
beautiful with foliage and flowers. Over every part of the vast Pacific
Ocean, whose islands must produce a minimum of dust, the sky is always
blue, and its thousand isles do not suffer for want of rain. Over the great
forest-plain of the Amazon valley, where the production of dust must be
very small, there is yet abundance of rain-clouds and of rain. This is due
primarily to the two great natural sources of dust--the active volcanoes,
together with the deserts and more arid regions of the world; and, in the
second place, to the density and wonderful mobility of the atmosphere,
which not only carries the finest dust-particles to an enormous height, but
distributes them through its whole extent with such wonderful uniformity.

Every dust-particle is of course much heavier than air, and in a
comparatively short time, if the atmosphere were still, would fall to the
ground. Tyndall found that the air of a cellar under the Royal Institution
in Albemarle Street, which had not been opened for several months, was so
pure that the path of a beam of electric light sent through it was quite
invisible. But careful experiments show that not only is the air in
continual motion, but the motion is excessively irregular, being hardly
ever quite horizontal, but upwards and downwards and in every intermediate
direction, as well as in countless whirls and eddies; and this complexity
of motion must extend to a vast height, probably to fifty miles or more,
in order to provide a sufficient thickness of those minutest particles
which produce the blue of the sky.

All this complexity of motion is due to the action of the sun in heating
the surface of the earth, and the extreme irregularity of that surface both
as regards contour and its capacity for heat-absorption. In one area we
have sand or rock or bare clay, which, when exposed to bright sunshine,
become scorching hot; in another area we have dense vegetation, which,
owing to evaporation caused by the sunshine, remains comparatively cool,
and also the still cooler surfaces of rivers and Alpine lakes. But if the
air were much less dense than it is, these movements would be less
energetic, while all the dust that was raised to any considerable height
would, by its own weight, fall back again to the earth much more rapidly
than it does now. There would thus be much less dust permanently in the
atmosphere, and this would inevitably lead to diminished rainfall and,
partially, to the other injurious effects already described.


ATMOSPHERIC ELECTRICITY

We have already seen that vegetable organisms obtain the chief part of the
nitrogen in their tissues from ammonia produced in the atmosphere and
carried into the earth by rain. This substance can only be thus produced by
the agency of electrical discharges, or lightning, which cause the
combination of the hydrogen in the aqueous vapour with the free nitrogen of
the air. But clouds are important agents in the accumulation of
electricity in sufficient amount to produce the violent discharges we know
as lightning, and it is doubtful whether without them there would be any
discharges through the atmosphere capable of decomposing the aqueous vapour
in it. Not only are clouds beneficial in the production of rain, and also
in moderating the intensity of continuous sun-heat, but they are also
requisite for the formation of chemical compounds in vegetables which are
of the highest importance to the whole animal kingdom. So far as we know,
animal life could not exist on the earth's surface without this source of
nitrogen, and therefore without clouds and lightning; and these, we have
just seen, depend primarily on a due proportion of dust in the atmosphere.

But this due proportion of dust is mainly supplied by volcanoes and
deserts, and its distribution and constant presence in the air depends upon
the density of the atmosphere. This again depends on two other factors: the
force of gravity due to the mass of the planet, and the absolute quantity
of the free gases constituting the atmosphere.

We thus find that the vast, invisible ocean of air in which we live, and
which is so important to us that deprivation of it for a few minutes is
destructive of life, produces also many other beneficial effects of which
we usually take little account, except at times when storm or tempest, or
excessive heat or cold, remind us how delicate is the balance of conditions
on which our comfort, and even our lives, depend.

But the sketch I have here attempted to give of its varied functions shows
us that it is really a most complex structure, a wonderful piece of
machinery, as it were, which in its various component gases, its actions
and reactions upon the water and the land, its production of electrical
discharges, and its furnishing the elements from which the whole fabric of
organic life is composed and perpetually renewed, may be truly considered
to be the very source and foundation of life itself. This is seen, not only
in the fact of our absolute dependence upon it every minute of our lives,
but in the terrible effects produced by even a slight degree of impurity in
this vital element. Yet it is among those nations that claim to be the most
civilised, those that profess to be guided by a knowledge of the laws of
nature, those that most glory in the advance of science, that we find the
greatest apathy, the greatest recklessness, in continually rendering impure
this all-important necessary of life, to such a degree that the health of
the larger portion of their populations is injured and their vitality
lowered, by conditions which compel them to breathe more or less foul and
impure air for the greater part of their lives. The huge and
ever-increasing cities, the vast manufacturing towns belching forth smoke
and poisonous gases, with the crowded dwellings, where millions are forced
to live under the most terrible insanitary conditions, are the witnesses to
this criminal apathy, this incredible recklessness and inhumanity.

For the last fifty years and more the inevitable results of such conditions
have been fully known; yet to this day nothing of importance _has_ been
done, nothing is being done. In this beautiful land there is ample space
and a superabundance of pure air for every individual. Yet our wealthy and
our learned classes, our rulers and law-makers, our religious teachers and
our men of science, all alike devote their lives and energies to anything
or everything but this. Yet _this_ is the one great and primary essential
of a people's health and well-being, to which _everything_ should, for the
time, be subordinate. Till this is done, and done thoroughly and
completely, our civilisation is naught, our science is naught, our religion
is naught, and our politics are less than naught--are utterly despicable;
are below contempt.

It has been the consideration of our wonderful atmosphere in its various
relations to human life, and to all life, which has compelled me to this
cry for the children and for outraged humanity. Will no body of humane men
and women band themselves together, and take no rest till this crying evil
is abolished, and with it nine-tenths of all the other evils that now
afflict us? Let _everything_ give way to this. As in a war of conquest or
aggression nothing is allowed to stand in the way of victory, and all
private rights are subordinated to the alleged public weal, so, in this war
against filth, disease, and misery let nothing stand in the way--neither
private interests nor vested rights--and we shall certainly conquer. This
is the gospel that should be preached, in season and out of season, till
the nation listens and is convinced. Let this be our claim: Pure air and
pure water for every inhabitant of the British Isles. Vote for no one who
says 'It can't be done.' Vote only for those who declare 'It shall be
done.' It may take five or ten or twenty years, but all petty
ameliorations, all piecemeal reforms, must wait till this fundamental
reform is effected. Then, when we have enabled our people to breathe pure
air, and drink pure water, and live upon simple food, and work and play and
rest under healthy conditions, they will be in a position to decide (for
the first time) what other reforms are really needed.

Remember! We claim to be a people of high civilisation, of advanced
science, of great humanity, of enormous wealth! For very shame do not let
us say 'We _cannot_ arrange matters so that our people may all breathe
unpolluted, unpoisoned air!'



CHAPTER XIV

THE EARTH IS THE ONLY HABITABLE PLANET IN THE SOLAR SYSTEM


Having shown in the last three chapters how numerous and how complex are
the conditions which alone render life possible on our earth, how nicely
balanced are opposing forces, and how curious and delicate are the means by
which the essential combinations of the elements are brought about, it will
be a comparatively easy task to show how totally unfitted are all the other
planets either to develop or to preserve the higher forms of life, and, in
most cases, any forms above the lowest and most rudimentary. In order to
make this clear we will take the most important of the conditions in order,
and see how the various planets fulfil them.


MASS OF A PLANET AND ITS ATMOSPHERE

The height and density of the atmosphere of a planet is important as
regards life in several ways. On its density depends its power of carrying
moisture; of holding a sufficient supply of dust-particles for the
formation of clouds; of carrying ultra-microscopic particles to such a
height and in such quantity as to diffuse the light of the sun by
reflection from the whole sky; of raising waves in the ocean and thus
aerating its waters, and of producing the ocean currents which so greatly
equalise temperature. Now this density depends on two factors: the mass of
the planet and the quantity of the atmospheric gases. But there is good
reason to think that the latter depends directly upon the former, because
it is only when a certain mass is attained that any of the lighter
permanent gases can be held on the surface of a planet. Thus, according to
Dr. G. Johnstone Stoney, who has specially studied this subject, the moon
cannot retain even such a heavy gas as carbonic acid, or the still heavier
carbon disulphide; while no particle of oxygen, nitrogen, or water-vapour
can possibly remain on it, owing to the fact of its mass being only about
one-eightieth that of the earth. It is believed that there are considerable
quantities of gases in the stellar spaces, and probably also within the
solar system, but perhaps in the liquid or solid form. In that state they
might be attracted by any small mass such as the moon, but the heat of its
surface when exposed to the solar rays would quickly restore them to the
gaseous condition, when they would at once escape.

It is only when a planet attains a mass at least a quarter that of the
earth that it is capable of retaining water-vapour, one of the most
essential of the gases; but with so small a mass as this, its whole
atmosphere would probably be so limited in amount and so rare at the
planet's surface that it would be quite unable to fulfil the various
purposes for which an atmosphere is required in order to support life. For
their adequate fulfilment the mass of a planet cannot be much less than
that of the earth. Here we come to one of those nice adjustments of which
so many have been already pointed out. Dr. Johnstone Stoney arrives at the
conclusion that hydrogen escapes from the earth. It is continually produced
in small quantities by submarine volcanoes, by fissures in volcanic
regions, from decaying vegetation, and from some other sources; yet, though
sometimes found in minute quantities, it forms no regular constituent of
our atmosphere.[18]

The quantity of hydrogen combined with oxygen to form the mass of water in
our vast and deep oceans is enormous. Yet if it had been only one-tenth
more than it actually is, the present land-surface would have been almost
all submerged. How the adjustments occurred so that there was exactly
enough hydrogen to fill the vast ocean basins with water to such a depth as
to leave enough land-surface for the ample development of vegetable and
animal life, and yet not so much as to be injurious to climate, it is
difficult to imagine. Yet the adjustment stares us in the face. First, we
have a satellite unique in size as compared with its primary, and
apparently in lateness of origin; then we have a mode of origin for that
satellite said to be certainly unique in the solar system; as a consequence
of this origin, it is believed, we have enormously deep ocean basins
symmetrically placed with regard to the equator--an arrangement which is
very important for ocean circulation; then we must have had the right
quantity of hydrogen, obtained in some unknown way, which formed water
enough to fill these chasms, so as to leave an ample area of dry land, but
which one-tenth more water would have ingulfed; and, lastly, we have oxygen
enough left to form an atmosphere of sufficient density for all the
requirements of life. It could not be that the surplus hydrogen escaped
when the water had been produced, because it escapes very slowly, and it
combines so easily with free oxygen by means of even a spark, as to make it
certain that _all_ the available hydrogen was used up in the oceanic
waters, and that the supply from the earth's interior has been since
comparatively small in amount.

There is yet one more adjustment to be noticed. All the facts now referred
to show that the earth's mass is sufficient to bring about the conditions
favourable for life. But if our globe had been a little larger, and
proportionately denser, in all probability no life would have been
possible. Between a planet of 8000 and one of 9500 miles diameter is not a
large difference, when compared with the enormous range of size of the
other planets. Yet this slight increase in diameter would give two-thirds
increase in bulk, and, with a corresponding increase of density due to the
greater gravitative force, the mass would be about double what it is. But
with double the mass the quantity of gases of all sorts attracted and
retained by gravity would probably have been double; and in that case there
would have been double the quantity of water produced, as no hydrogen could
then escape. But the _surface_ of the globe would only be one half greater
than at present, in which case the water would have sufficed to cover the
whole surface several miles deep.


HABITABILITY OF OTHER PLANETS

When we look to the other planets of our system we see everywhere
illustrations of the relation of size and mass to habitability. The smaller
planets, Mercury and Mars, have not sufficient mass to retain water-vapour,
and, without it, they cannot be habitable. All the larger planets can have
very little solid matter, as indicated by their very low density
notwithstanding their enormous mass. There is, therefore, very good reason
for the belief that the adaptability of a planet for a full development of
life is _primarily_ dependent, within very narrow limits, on its size and,
more directly, on its mass. But if the earth owes its specially constituted
atmosphere and its nicely adjusted quantity of water to such general causes
as here indicated, and the same causes apply to the other planets of the
solar system, then the only planet on which life can be possible is Venus.
As, however, it may be urged that exceptional causes may have given other
planets an equal advantage in the matter of air and water, we will briefly
consider some of the other conditions which we have found to be essential
in the case of the earth, but which it is almost impossible to conceive as
existing, to the required extent, on any of the other planets of the solar
system.


A SMALL AND DEFINITE RANGE OF TEMPERATURE

We have already seen within what narrow limits the temperature on a
planet's surface must be maintained in order to develop and support life.
We have also seen how numerous and how delicate are the conditions, such as
density of atmosphere, extent and permanence of oceans, and distribution of
sea and land, which are requisite, even with us, in order to render
possible the continuous preservation of a sufficiently uniform temperature.
Slight alterations one way or another might render the earth almost
uninhabitable, through its being liable to alternations of too great heat
or excessive cold. How then can we suppose that any other of the planets,
which have either very much more or very much less sun-heat than we
receive, could, by any possible modification of conditions, be rendered
capable of producing and supporting a full and varied life-development?

Mars receives less than half the amount of sun-heat per unit of surface
that we do. And as it is almost certain that it contains no water (its
polar snows being caused by carbonic acid or some other heavy gas) it
follows that, although it may produce vegetable life of some low kinds, it
must be quite unsuited for that of the higher animals. Its small size and
mass, the latter only one-ninth that of the earth, may probably allow it to
possess a very rare atmosphere of oxygen and nitrogen, if those gases exist
there, and this lack of density would render it unable to retain during the
night the very moderate amount of heat it might absorb during the day. This
conclusion is supported by its low reflecting power, showing that it has
hardly any clouds in its scanty atmosphere. During the greater part of the
twenty-four hours, therefore, its surface-temperature would probably be
much below the freezing point of water; and this, taken in conjunction with
the total absence of aqueous vapour or liquid water, would add still
further to its unsuitability for animal life.

In Venus the conditions are equally adverse in the other direction. It
receives from the sun almost double the amount of heat that we receive, and
this alone would render necessary some extraordinary combination of
modifying agencies in order to reduce and render uniform the excessively
high temperature. But it is now known that Venus has one peculiarity which
is in itself almost prohibitive of animal life, and probably of even the
lowest forms of vegetable life. This peculiarity is, that through tidal
action caused by the sun, its day has been made to coincide with its year,
or, more properly, that it rotates on its axis in the same time that it
revolves round the sun. Hence it always presents the same face to the sun;
and while one half has a perpetual day, the other half has perpetual night,
with perpetual twilight through refraction in a narrow belt adjoining the
illuminated half. But the side that never receives the direct rays of the
sun must be intensely cold, approximating, in the central portions, to the
zero of temperature, while the half exposed to perpetual sunshine of double
intensity to ours must almost certainly rise to a temperature far too great
for the existence of protoplasm, and probably, therefore, of any form of
animal life.

Venus appears to have a dense atmosphere, and its brilliancy suggests that
we see the upper surface of a cloud-canopy, and this would no doubt greatly
reduce the excessive solar heat. Its mass, being a little more than
three-fourths that of the earth, would enable it to retain the same gases
as we possess. But under the extraordinary conditions that prevail on the
surface of this planet, it is hardly possible that the temperature of the
illuminated side can be preserved in a sufficient state of uniformity for
the development of life in any of its higher forms.

Mercury possesses the same peculiarity of keeping one face always towards
the sun, and as it is so much smaller and so much nearer the sun its
contrasts of heat and cold must be still more excessive, and we need hardly
discuss the possibility of this planet being habitable. Its mass being only
one-thirtieth that of the earth, water-vapour will certainly escape from
it, and, most probably, nitrogen and oxygen also, so that it can possess
very little atmosphere; and this is indicated by its low reflecting power,
no less than 83 per cent. of the sun's light being absorbed, and only 17
per cent. reflected, whereas clouds reflect 72 per cent. This planet is
therefore intensely heated on one side and frozen on the other; it has no
water and hardly any atmosphere, and is therefore, from every point of
view, totally unfitted for supporting living organisms.

Even if it is supposed that, in the case of Venus, its perpetual
cloud-canopy may keep down the surface temperature within the limits
necessary for animal life, the extraordinary turmoil in its atmosphere
caused by the excessively contrasted temperatures of its dark and light
hemispheres must be extremely inimical to life, if not absolutely
prohibitive of it. For on the greater part of the hemisphere that never
receives a ray of light or heat from the sun all the water and aqueous
vapour must be turned into ice or snow, and it seems almost impossible that
the air itself can escape congelation. It could only do so by a very rapid
circulation of the whole atmosphere, and this would certainly be produced
by the enormous and permanent difference of temperature between the two
hemispheres. Indications of refraction by a dense atmosphere are visible
during the planet's transit over the sun's disc, and also when it is in
conjunction with the sun, and the refraction is so great that Venus is
believed to have an atmosphere much higher than ours. But during the rapid
circulation of such an atmosphere, heated on one half the planet and cooled
on the other, most of the aqueous vapour must be taken out of it on the
dark side as fast as it is produced on the heated side, though sufficient
may remain to produce a canopy of very lofty clouds analogous to our cirri.
The occasional visibility of the dark side of Venus may be caused by an
electrical glow due to the friction of the perpetually overflowing and
inflowing atmosphere, this being increased by reflection from a vast
surface of perpetual snow. If we consider all the exceptional features of
this planet, it appears certain that the conditions as regards climate
cannot now be such as to maintain a temperature within the narrow limits
essential for life, while there is little probability that at any earlier
period it can have possessed and maintained the necessary stability during
the long epochs which are requisite for its development.

Before considering the condition of the larger planets, it will be well to
refer to an argument which has been supposed to minimise the difficulties
already stated as to those planets which approach nearest to the earth in
size and distance from the sun.


THE ARGUMENT FROM EXTREME CONDITIONS ON THE EARTH

In reply to the evidence showing how nice are the adaptations required for
life-development, it is often objected that life does _now_ exist under
very extreme conditions--under tropic heat and arctic snows; in the
burnt-up desert as well as in the moist tropical forest; in the air as well
as in the water; on lofty mountains as well as on the level lowlands. This
is no doubt true, but it does not prove that life could have been developed
in a world where any of these extremes of climate characterised the whole
surface. The deserts are inhabited because there are oases where water is
attainable, as well as in the surrounding fertile areas. The arctic regions
are inhabited because there is a summer, and during that summer there is
vegetation. If the surface of the ground were always frozen, there would be
no vegetation and no animal life.

The late Mr. R.A. Proctor put this argument of the diversity of conditions
under which life actually does exist on the earth as well probably as it
can be put. He says: 'When we consider the various conditions under which
life is found to prevail, that no difference of climatic relations, or of
elevation, of land, or of air, or of water, of soil in land, of freshness
or saltness in water, of density in air, appears (so far as our researches
have extended) to render life impossible, we are compelled to infer that
the power of supporting life is a quality which has an exceedingly wide
range in nature.'

This is true, but with certain reservations. The only species of animal
which does really exist under the most varied conditions of climate is man,
and he does so because his intellect renders him to some extent the ruler
of nature. None of the lower animals have such a wide range, and the
diversity of conditions is not really so great as it appears to be. The
strict limits are nowhere permanently overpassed, and there is always the
change from winter to summer, and the possibility of migration to less
inhospitable areas.


THE GREAT PLANETS ALL UNINHABITABLE

Having already shown that the condition of Mars, both as regards water,
atmosphere, and temperature, is quite unfitted to maintain life, a view in
which both general principles and telescopic examination perfectly agree,
we may pass on to the outer planets, which, however, have long been given
up as adapted for life even by the most ardent advocates for 'life in other
worlds.' Their remoteness from the sun--even Jupiter being five times as
far as the earth, and therefore receiving only one twenty-fifth of the
light and heat that we receive per unit of surface--renders it almost
impossible, even if other conditions were favourable, that they should
possess surface-temperatures adequate to the necessities of organic life.
But their very low densities, combined with very large size, renders it
certain that they none of them have a solidified surface, or even the
elements from which such a surface could be formed.

It is supposed that Jupiter and Saturn, as well as Uranus and Neptune,
retain a considerable amount of internal heat, but certainly not sufficient
to keep the metallic and other elements of which the sun and earth consist
in a state of vapour, for if so they would be planetary stars and would
shine by their own light. And if any considerable portion of their bulk
consisted of these elements, whether in a solid or a liquid state, their
densities would necessarily be much greater than that of the earth instead
of very much less--Jupiter is under one-fourth the density of the earth,
Saturn under an eighth, while Uranus and Neptune are of intermediate
densities, though much less in bulk even than Saturn.

It thus appears that the solar system consists of two groups of planets
which differ widely from each other. The outer group of four very large
planets are almost wholly gaseous, and probably consist of the permanent
gases--those which can only be liquefied or solidified at a very low
temperature. In no other way can their small density combined with enormous
bulk be accounted for.

The inner group also of four planets are totally unlike the preceding. They
are all of small size, the earth being the largest. They are all of a
density roughly proportionate to their bulk. The earth is both the largest
and the densest of the group; not only is it situated at that distance from
the sun which, through solar heat alone, allows water to remain in the
liquid state over almost the whole of its surface, but it possesses
numerous characteristics which secure a very equable temperature, and which
have secured to it very nearly the same temperature during those enormous
geological periods in which terrestrial life has existed. We have already
shown that no other planet possesses these characteristics now, and it is
almost equally certain that they never have possessed them in the past, and
never will possess them in the future.


A LAST ARGUMENT FOR HABITABILITY OF THE PLANETS

Although it has been admitted by the late Mr. Proctor and some other
astronomers that most of the planets are not _now_ habitable, yet, it is
often urged, they may have been so in the past or may become so in the
future. Some are now too hot, others are now too cold; some have now no
water, others have too much; but all go through their appointed series of
stages, and during some of these stages life may be or may have been
possible. This argument, although vague, will appeal to some readers, and
it may, therefore, be necessary to reply to it. This is the more necessary
as it is still made use of by astronomers. In a criticism of my article in
_The Fortnightly Review_, M. Camille Flammarion, of the Paris Observatory,
dramatically remarks: 'Yes, life is universal, and eternal, for time is one
of its factors. Yesterday the moon, to-day the earth, to-morrow Jupiter. In
space there are both cradles and tombs.'[19]

It is thus suggested that the moon was once inhabited and that Jupiter will
be inhabited in some remote future; but no attempt is made to deal with the
essential physical conditions of these very diverse objects, rendering them
not only _now_, but always, unfitted to develop and to maintain terrestrial
or aerial life. This vague supposition--it can hardly be termed an
argument--as regards past or future adaptability for life, of all the
planets and some of the satellites in the solar system, is, however,
rendered invalid by an equally general objection to which its upholders
appear never to have given a moment's consideration; and as it is an
objection which still further enforces the view as to the unique position
of the earth in the solar system, it will be well to submit it to the
judgment of our readers.


LIMITATION OF THE SUN'S HEAT

It is well known that there is, and has been for nearly half a century, a
profound difference of opinion between geologists and physicists as to the
actual or possible duration in years of life upon the earth. The
geologists, being greatly impressed with the vast results produced by the
slow processes of the wearing away of the rocks and the deposit of the
material in seas or lakes, to be again upheaved to form dry land, and to be
again carved out by rain and wind, by heat and cold, by snow and ice, into
hills and valleys and grand mountain ranges; and further, by the fact that
the highest mountains in every part of the globe very often exhibit on
their loftiest summits stratified rocks which contain marine organisms, and
were therefore originally laid down beneath the sea; and, yet again, by
the fact that the loftiest mountains are often the most recent, and that
these grand features of the earth's surface are but the latest examples of
the action of forces that have been at work throughout all geological
time--studying all their lives the detailed evidences of all these changes,
have come to the conclusion that they imply enormous periods only to be
measured by scores or hundreds of millions of years.

And the collateral study of fossil remains in the long series of
rock-formations enforces this view. In the whole epoch of human history,
and far back into prehistoric times during which man existed on the earth,
although several animals have become extinct, yet there is no proof that
any new one has been developed. But this human era, so far as yet known,
going back certainly to the glacial epoch and almost certainly to
pre-glacial times, cannot be estimated at less than a million, some think
even several million years; and as there have certainly been some
considerable alterations of level, excavation of valleys, deposits of great
beds of gravel, and other superficial changes during this period, some kind
of a scale of measurement of geological time has been obtained, by
comparison with the very minute changes that have occurred during the
historical period. This scale is admittedly a very imperfect one, but it is
better than none at all; and it is by comparing these small changes with
the far greater ones which have occurred during every successive step
backward in geological history that these estimates of geological time have
been arrived at. They are also supported by the palæontologists, to whom
the vast panorama of successive forms of life is an ever-present reality.
Directly they pass into the latest stage of the Tertiary period--the
Pliocene of Sir Charles Lyell--all over the world new forms of life appear
which are evidently the forerunners of many of our still existing species;
and as they go a little further back, into the Miocene, there are
indications of a warmer climate in Europe, and large numbers of mammals
resembling those which now inhabit the tropics, but of quite distinct
species and often of distinct genera and families. And here, though we have
only reached to about the middle of the Tertiary period, the changes in the
forms of life, in the climate, and in the land-surfaces are so great when
compared with the very minute changes during the human epoch, as to require
us to multiply the time elapsed many times over. Yet the whole of the
Tertiary period, during which _all_ the great groups of the higher animals
were developed from a comparatively few generalised ancestral forms, is yet
the shortest by far of the three great geological periods--the Mesozoic or
Secondary, having been much longer, with still vaster changes both in the
earth's crust and in the forms of life; while the Palæozoic or Primary,
which carries us back to the earliest forms of life as represented by
fossilised remains, is always estimated by geologists to be at least as
long as the other two combined, and probably very much longer.

From these various considerations most geologists who have made any
estimates of geological time from the period of the earliest fossiliferous
rocks, have arrived at the conclusion that about 200 millions of years are
required. But from the variety of the forms of life at this early period
it is concluded that a very much greater duration is needed for the whole
epoch of life. Speaking of the varied marine fauna of the Cambrian period,
the late Professor Ramsay says:--'In this earliest known varied life we
find no evidence of its having lived near the beginning of the zoological
series. In a broad sense, compared with what must have gone before, both
biologically and physically, all the phenomena connected with this old
period seem, to my mind, to be of quite a recent description; and the
climates of seas and lands were of the very same kind as those the world
enjoys at the present day.' And Professor Huxley held very similar views
when he declared: 'If the very small differences which are observable
between the crocodiles of the older Secondary formations and those of the
present day furnish any sort of an approximation towards an estimate of the
average rate of change among reptiles, it is almost appalling to reflect
how far back in Palæozoic times we must go before we can hope to arrive at
that common stock from which the crocodiles, lizards, _Ornithoscelida_, and
_Plesiosauria_, which had attained so great a development in the Triassic
epoch, must have been derived.'

Now, in opposition to these demands of the geologists, in which they are
almost unanimous, the most celebrated physicists, after full consideration
of all possible sources of the heat of the sun, and knowing the rate at
which it is now expending heat, declare, with complete conviction, that our
sun cannot have existed as a heat-giving body for so long a period, and
they would therefore reduce the time during which life can possibly have
existed on the earth to about one-fourth of that demanded by geologists.
In one of his latest articles, Lord Kelvin says:--'Now we have irrefragable
dynamics proving that the whole life of our sun as a luminary is a very
moderate number of million years, probably less than 50 million, possibly
between 50 and 100' (_Phil. Mag._, vol. ii., Sixth Ser., p. 175, Aug.
1901). In my _Island Life_ (chap. X.) I have myself given reasons for
thinking that both the stratigraphical and biological changes may have gone
on more quickly than has been supposed, and that geological time (meaning
thereby the time during which the development of life upon the earth has
been going on) may be reduced so as possibly to be brought within the
maximum period allowed by physicists; but there will certainly be no time
to spare, and any planets dependent on our sun whose period of habitability
is either past or to come, cannot possibly have, or have had, sufficient
time for the necessarily slow evolution of the higher life-forms. Again,
all physicists hold that the sun is now cooling, and that its future life
will be much less than its past. In a lecture at the Royal Institution
(published in _Nature Series_, in 1889), Lord Kelvin says:--'It would, I
think, be exceedingly rash to assume as probable anything more than twenty
million years of the sun's light in the past history of the earth, or to
reckon more than five or six million years of sunlight for time to come.'

These extracts serve to show that, unless either geologists or physicists
are very far from any approach to accuracy in their estimates of past or
future age of the sun, there is very great difficulty in bringing them into
harmony or in accounting for the actual facts of the geological history of
the earth and of the whole course of life-development upon it. We are,
therefore, again brought to the conclusion that there has been, and is, no
time to spare; that the _whole_ of the available past life-period of the
sun has been utilised for life-development on the earth, and that the
future will be not much more than may be needed for the completion of the
grand drama of human history, and the development of the full possibilities
of the mental and moral nature of man.

We have here, then, a very powerful argument, from a different point of
view than any previously considered, for the conclusion that man's place in
the solar system is altogether unique, and that no other planet either has
developed or can develop such a full and complete life-series as that which
the earth has actually developed. Even if the conditions had been more
favourable than they are seen to be on other planets, Mercury, Venus, and
Mars could not possibly have preserved equability of conditions long enough
for life-development, since for unknown ages they must have been passing
slowly towards their present wholly _unsuitable_ conditions; while Jupiter
and the planets beyond him, whose epoch of life-development is supposed to
be in the remote future when they shall have slowly cooled down to
habitability, will then be still more faintly illuminated and scantily
warmed by a rapidly cooling sun, and may thus become, at the best, globes
of solid ice. This is the teaching of science--of the best science of the
twentieth century. Yet we find even astronomers who, more than any other
exponents of science, should give heed to the teachings of the sister
sciences to which they owe so much, indulging in such rhapsodies as the
following:--'In our solar system, this little earth has not obtained any
special privileges from Nature, and it is strange to wish to confine life
within the circle of terrestrial chemistry.' And again: 'Infinity
encompasses us on all sides, life asserts itself, universal and eternal,
our existence is but a fleeting moment, the vibration of an atom in a ray
of the sun, and our planet is but an island floating in the celestial
archipelago, to which no thought will ever place any bounds.'[20]

In place of such 'wild and whirling words,' I have endeavoured to state the
sober conclusions of the best workers and thinkers as to the nature and
origin of the world in which we live, and of the universe which on all
sides surrounds us. I leave it to my readers to decide which is the more
trustworthy guide.


FOOTNOTES:

[18] _Transactions of Royal Dublin Society_, vol. vi. (ser. ii.), part
xiii. 'Of Atmospheres upon Planets and Satellites.' By G. Johnstone Stoney,
F.R.S., etc. etc.

[19] _Knowledge_, June 1903.

[20] M. Camille Flammarion, in _Knowledge_, June 1903.



CHAPTER XV

THE STARS--HAVE THEY PLANETARY SYSTEMS? ARE THEY BENEFICIAL TO US?


Most of the writers on the Plurality of Worlds, from Fontenelle to Proctor,
taking into consideration the enormous number of the stars and their
apparent uselessness to our world, have assumed that many of them _must_
have systems of planets circling round them, and that some of these
planets, at all events, _must_ possess inhabitants, some, perhaps, lower,
but others no doubt higher than ourselves. One of our well-known modern
astronomers, writing only ten years ago, adopts the same view. He says:
'The suns which we call stars were clearly not created for our benefit.
They are of very little practical use to the earth's inhabitants. They give
us very little light; an additional small satellite--one considerably
smaller than the moon--would have been much more useful in this respect
than the millions of stars revealed by the telescope. They must therefore
have been formed for some other purpose.... We may therefore conclude, with
a high degree of probability, that the stars--at least those with spectra
of the solar type--form centres of planetary systems somewhat similar to
our own.'[21] The author then discusses the conditions necessary for life
analogous to that of our earth, as regards temperature, rotation, mass,
atmosphere, water, etc., and he is the only writer I have met with who has
considered these conditions; but he touches on them very briefly, and he
arrives at the conclusion that, in the case of the stars of solar type, it
is probable that _one_ planet, situated at a proper distance, would be
fitted to support life. He estimates roughly that there are about ten
million stars of this type, that is, closely resembling our sun, and that
if only one in ten of these has a planet at the proper distance and
properly constituted in other respects, there will be one million worlds
fitted for the support of animal life. He therefore concludes that there
are probably many stars having life-bearing planets revolving round them.

There are, however, many considerations not taken account of by this writer
which tend to reduce very considerably the above estimate. It is now known
that immense numbers of the stars of smaller magnitudes are nearer to us
than are the majority of the stars of the first and second magnitudes, so
that it is probable that these, as well as a considerable proportion of the
very faint telescopic stars, are really of small dimensions. We have
evidence that many of the brightest stars are much larger than our sun, but
there are probably ten times as many that are much smaller. We have seen
that the whole of the past light and heat-giving duration of our sun has,
according to the best authorities, been only just sufficient for the
development of life upon the earth. But the duration of a sun's heat-giving
power will depend mainly upon its mass, together with its constituent
elements. Suns which are much smaller than ours are, therefore, from that
cause alone, unsuited to give adequate light and heat for a sufficient
time, and with sufficient uniformity, for life-development on planets, even
if they possess any at the right distance, and with the extensive series of
nicely adjusted conditions which I have shown to be necessary.

Again, we must, probably, rule out as unfitted for life-development the
whole region of the Milky Way, on account of the excessive forces there in
action, as shown by the immense size of many of the stars, their enormous
heat-giving power, the crowding of stars and nebulous matter, the great
number of star-clusters, and, especially, because it is the region of 'new
stars,' which imply collisions of masses of matter sufficiently large to
become visible from the immense distance we are from them, but yet
excessively small as compared with suns the duration of whose light is to
be measured by millions of years. Hence the Milky Way is the theatre of
extreme activity and motion; it is comparatively crowded with matter
undergoing continual change, and is therefore not sufficiently stable for
long periods to be at all likely to possess habitable worlds.

We must, therefore, limit our possible planetary systems suitable for
life-development, to stars situated inside the circle of the Milky Way and
far removed from it--that is, to those composing the solar cluster. These
have been variously estimated to consist of a few hundred or many thousand
stars--at all events to a very small number as compared with the 'hundreds
of millions' in the whole stellar universe. But even here we find that
only a portion are probably suitable. Professor Newcomb arrives at the
conclusion--as have some other astronomers--that the stars in general have
a much smaller mass in proportion to the light they give than our sun has;
and, after an elaborate discussion, he finally concludes that the brighter
stars are, on the average, much less dense than our sun. In all
probability, therefore, they cannot give light and heat for so long a
period, and as this period in the case of our sun has only been just
sufficient, the number of suns of the solar type and of a sufficient mass
may be very limited. Yet further, even among stars having a similar
physical constitution to our sun and of an equal or greater mass, only a
portion of their period of luminosity would be suitable for the support of
planetary life. While they are in process of formation by accretions of
solid or gaseous masses, they would be subject to such fluctuations of
temperature, and to such catastrophic outbursts when any larger mass than
usual was drawn towards them, that the whole of this period--perhaps by far
the longest portion of their existence--must be left out of the account of
planet-producing suns. Yet all these are to us stars of various degrees of
brilliancy. It is almost certain that it is only when the growth of a sun
is nearly completed, and its heat has attained a maximum, that the epoch of
life-development is likely to begin upon any planets it may possess at the
most suitable distance, and upon which all the requisite conditions should
be present.

It may be said that there are great numbers of stars beyond our solar
cluster and yet within the circle of the Milky Way, as well as others
towards the poles of the Milky Way, which I have not here referred to. But
of these regions very little is known, because it is impossible to tell
whether stars in these directions are situated in the outer portion of the
solar cluster or in the regions beyond it. Some astronomers appear to think
that these regions may be nearly empty of stars, and I have endeavoured to
represent what seems to be the general view on this very difficult subject
in the two diagrams of the stellar universe at pp. 300, 301. The regions
beyond our cluster and above or below the plane of the Milky Way are those
where the small irresolvable nebulæ abound, and these may indicate that
sun-formation is not yet active in those regions. The two charts of Nebulæ
and Clusters at the end of the volume illustrate, and perhaps tend to
support this view.


DOUBLE AND MULTIPLE STAR SYSTEMS

We have already seen, in our sixth chapter, how rapid and extraordinary has
been the discovery of what are termed spectroscopic binaries--pairs of
stars so close together as to appear like a single star in the most
powerful telescopes. The systematic search for such stars has only been
carried on for a few years, yet so many have been already found, and their
numbers are increasing so rapidly, as to quite startle astronomers. One of
the chief workers in this field, Professor Campbell of the Lick
Observatory, has stated his opinion that, as accuracy of measurement
increases, these discoveries will go on till--'the star that is not a
spectroscopic binary will prove to be the rare exception,'--and other
astronomers of eminence have expressed similar views. But these close
revolving star-systems are generally admitted to be out of the category of
life-producing suns. The tidal disturbances mutually produced must be
enormous, and this must be inimical to the development of planets, unless
they were very close to each sun, and thus in the most unfavourable
position for life.

We thus see that the result of the most recent researches among the stars
is entirely opposed to the old idea that the countless myriads of stars
_all_ had planets circulating round them, and that the ultimate purpose of
their existence was, that they should be supporters of life, as our sun is
the supporter of life upon the earth. So far is this from being the case,
that vast numbers of stars have to be put aside as wholly unfitted for such
a purpose; and when by successive eliminations of this nature we have
reduced the numbers which may possibly be available to a few millions, or
even to a few thousands, there comes the last startling discovery, that the
entire host of stars is found to contain binary systems in such rapidly
increasing numbers, as to lead some of the very first astronomers of the
day to the conclusion that single stars may someday be found to be the rare
exception! But this tremendous generalisation would, at one stroke, sweep
away a large proportion of the stars which other successive
disqualifications had spared, and thus leave our sun, which is certainly
single, and perhaps two or three companion orbs, alone among the starry
host as possible supporters of life on some one of the planets which
circulate around them.

But we do not really _know_ that any such suns exist. If they exist we do
not _know_ that they possess planets. If any do possess planets these may
not be at the proper distance, or be of the proper mass, to render life
possible. If these primary conditions should be fulfilled, and if there
should possibly be not only one or two, but a dozen or more that so far
fulfil the first few conditions which are essential, what probability is
there that all the other conditions, all the other nice adaptations, all
the delicate balance of opposing forces that we have found to prevail upon
the earth, and whose combination here is due to exceptional conditions
which exist in the case of no other known planet--should _all_ be again
combined in some of the possible planets of these possibly existing suns?

I submit that the probability is _now_ all the other way. So long as we
could assume that all the stars might be, in all essentials, like our sun,
it seemed almost ludicrous to suppose that our sun alone should be in a
position to support life. But when we find that enormous classes like the
gaseous stars of small density, the solar stars while increasing in size
and temperature, the stars which are much smaller than our sun, the
nebulous stars, probably all the stars of the Milky Way, and lastly that
enormous class of spectroscopic doubles--veritable Aaron's rods which
threaten to swallow up all the rest--that all these are for various reasons
unlikely to have attendant planets adapted to develop life, then the
probabilities seem to be enormously against there being any considerable
number of suns possessing attendant habitable earths. Just as the
habitability of all the planets and larger satellites, once assumed as so
extremely probable as to amount almost to a certainty, is now generally
given up, so that in speculating on life in stellar systems Mr. Gore
assumes that only _one_ planet to each sun can be habitable; in like manner
it may, and I believe will, turn out, that of all the myriad stars, the
more we learn about them, the smaller and smaller will become the scanty
residue which, with any probability, we can suppose to illuminate and
vivify habitable earths. And when with this scanty probability we combine
the still scantier probability that any such planet will possess
simultaneously, and for a sufficiently long period, _all_ the highly
complex and delicately balanced conditions known to be essential for a full
life-development, the conception that on this earth alone has such
development been completed will not seem so wildly improbable a conjecture
as it has hitherto been held to be.


ARE THE STARS BENEFICIAL TO US?

When I suggested in my first publication on this subject that some
emanations from the stars _might_ be beneficial or injurious, and that a
central position _might_ be essential in order to render these emanations
equable, one of my astronomical critics laughed the idea to scorn, and
declared that 'we might wander into outer space without losing anything
more serious than we lose when the night is cloudy and we cannot see the
stars.'[22] How my critic knows that this is so he does not tell us. He
states it positively, with no qualification, as if it were an established
fact. It may be as well to inquire, therefore, if there is any evidence
bearing upon the point at issue.

Astronomers are so fully occupied with the vast number and variety of the
phenomena presented by the stellar universe and the various difficult
problems arising therefrom, that many lesser but still interesting
inquiries have necessarily received little attention. Such a minor problem
is the determination of how much heat or other active radiation we receive
from the stars; yet a few observations have been made with results that are
of considerable interest.

In the years 1900 and 1901 Mr. E.F. Nichols of the Yerkes Observatory made
a series of experiments with a radiometer of special construction, to
determine the heat emitted by certain stars. The result arrived at was,
that Vega gave about 1/200000000 of the heat of a candle at one metre
distance, and Arcturus about 2.2 times as much.

In 1895 and 1896 Mr. G.M. Minchin made a series of experiments on the
_Electrical Measurement of Starlight_, by means of a photo-electric cell of
peculiar construction which is sensitive to the whole of the rays in the
spectrum, and also to some of the ultra-red and ultra-violet rays. Combined
with this was a very delicate electrometer. The telescope employed to
concentrate the light was a reflector of two feet aperture. Mr. Minchin was
assisted in the experiments by the late Professor G.F. Fitzgerald, F.R.S.,
of Trinity College, Dublin, which may be considered guarantee of the
accuracy of the observations. The following are the chief results
obtained:--

    -----------------------------------------------------------------------
    ---------------------------------+-------------+-----------+-----------
                                     | Deflection  |  Light    |
          Source of Light.           |      in     |    in     | E. M. F.
                                     | Millimetres | Candles.  |  Volts.
    ---------------------------------+-------------+-----------+-----------
    1896  Candle at 10 feet distance |    18.70    |           |
          Betelgeuse (0.9 mag.)      |    12.80    |   0.685   |   0.026
          Aldebaran (1.1 mag.)       |     5.21    |   0.279   |   0.012
          Procyon (0.5 mag)          |     4.89    |   0.261   |   0.011
          Alpha Cygni (1.3 mag.)     |     4.90    |   0.262   |   0.011
          Polaris (2.1 mag.)         |     3.10    |   0.166   |   0.007
    ---------------------------------+-------------+-----------+-----------
                   1 volt.           |   432.00    |           |
                                     |             |           |
    1895  Arcturus (0.3 mag.)        |     8.2     |   1.01    |   0.019
          Vega (0.1 mag)             |    11.5     |   1.42    |   0.026
          Candle at 10 feet          |     8.1     |           |
    ---------------------------------+-------------+-----------+-----------

     N.B.--The standard candle shone directly on the cell, whereas
     the star's light was concentrated by a 2-foot mirror.

The sensitive surface on which the light of the stars was concentrated was
1/20 inch in diameter. We must therefore diminish the amount of candle
light in this table in the proportion of the square of the diameter of the
mirror (in 1/20 of an inch) to one, equal to 1/230400. If we make the
necessary reduction in the case of Vega, and also equalise the distance at
which the candle was placed, we find the following result:--

    Observer.        Star.               Candle power at 10 ft.

    Minchin.         Vega                1/162250
    Nichols.          "                  1/22000000

This enormous difference in the result is no doubt largely due to the fact
that Mr. Nichols's apparatus measured heat alone, whereas Mr. Minchin's
cell measured almost all the rays. And this is further shown by the fact
that, whereas Mr. Nichols found Arcturus a red star, hotter than Vega a
white one, Mr. Minchin, measuring also the light-giving and some of the
chemical rays, found Vega considerably more energetic than Arcturus. These
comparisons also suggest that other modes of measurement might give yet
higher results, but it will no doubt be urged that such minute effects must
necessarily be quite inoperative upon the organic world.

There are, however, some considerations which tend the other way. Mr.
Minchin remarks on the unexpected fact that Betelgeuse produces more than
double the electrical energy of Procyon, a much brighter star. This
indicates that many of the stars of smaller visual magnitudes may give out
a large amount of energy, and it is this energy, which we now know can take
many strange and varied forms, that would be likely to influence organic
life. And as to the quantity being too minute to have any effect, we know
that the excessively minute amount of light from the very smallest
telescopic stars produces such chemical changes on a photographic plate as
to form distinct images, with comparatively small lenses or reflectors and
with an exposure of two or three hours. And if it were not that the
diffused light of the surrounding sky also acts upon the plate and blurs
the faint images, much smaller stars could be photographed.

We know that not all the rays, but a portion only, are capable of producing
these effects; we know also that there are many kinds of radiation from the
stars, and probably some yet undiscovered comparable with the X rays and
other new forms of radiation. We must also remember the endless variety
and the extreme instability of the protoplasmic products in the living
organism, many of which are perhaps as sensitive to special rays as is the
photographic plate.

And we are not here limited to action for a few minutes or a few hours, but
throughout the whole night and day, and continued whenever the sky is clear
for months or years. Thus the cumulative effect of these very weak
radiations may become important. It is probable that their action would be
most influential on plants, and here we find all the conditions requisite
for its accumulation and utilisation in the large amount of leaf-surface
exposed to it. A large tree must present some hundreds of superficial feet
of receptive surface, while even shrubs and herbs often have a leaf-area of
greater superficial extent than the object-glasses of our largest
telescopes. Some of the highly complex chemical processes that go on in
plants may be helped by these radiations, and their action would be
increased by the fact that, coming from every direction over the whole
surface of the heavens, the rays from the stars would be able to reach and
act upon every leaf of the densest masses of foliage. The large amount of
growth that takes place at night may be in part due to this agency.

Of course all this is highly speculative; but I submit, in view of the fact
that the light of the very faintest stars _does_ produce distinct chemical
changes, that even the very minute heat-effects are measureable, as well as
the electro-motive forces caused by them; and further, that when we
consider the millions, perhaps hundreds of millions of stars, all acting
simultaneously on any organism which may be sensitive to them, the
supposition that they do produce some effect, and possibly a very important
effect, is not one to be summarily rejected as altogether absurd and not
worth inquiring into.

It is not, however, these possible direct actions of the stars upon living
organisms to which I attach much weight as regards our central position in
the stellar universe. Further consideration of the subject has convinced me
that the fundamental importance of that position is a physical one, as has
already been suggested by Sir Norman Lockyer and some other astronomers.
Briefly, the central position appears to be the only one where suns can be
sufficiently stable and long-lived to be capable of maintaining the long
process of life-development in any of the planets they may possess. This
point will be further developed in the next (and concluding) chapter.

FOOTNOTES:

[21] _The Worlds of Space_, by J.E. Gore, chapter iii.

[22] _The Fortnightly Review_, April 1903, p. 60.



CHAPTER XVI

STABILITY OF THE STAR-SYSTEM: IMPORTANCE OF OUR CENTRAL POSITION: SUMMARY
AND CONCLUSION


One of the greatest difficulties with regard to the vast system of stars
around us is the question of its permanence and stability, if not
absolutely and indefinitely, yet for periods sufficiently long to allow for
the many millions of years that have certainly been required for our
terrestrial life-development. This period, in the case of the earth, as I
have sufficiently shown, has been characterised throughout by extreme
uniformity, while a continuance of that uniformity for a few millions of
years in the future is almost equally certain.

But our mathematical astronomers can find no indications of such stability
of the stellar universe as a whole, if subject to the law of gravitation
alone. In reply to some questions on this point, my friend Professor George
Darwin writes as follows:--'A symmetrical annular system of bodies might
revolve in a circle with or without a central body. Such a system would be
unstable. If the bodies are of unequal masses and not symmetrically
disposed, the break-up of the system would probably be more rapid than in
the ideal case of symmetry.'

This would imply that the great annular system of the Milky Way is
unstable. But if so, its existence at all is a greater mystery than ever.
Although in detail its structure is very irregular, as a whole it is
wonderfully symmetrical; and it seems quite impossible that its generally
circular ring-like form can be the result of the chance aggregation of
matter from any pre-existing different form. Star-clusters are equally
unstable, or, rather, nothing is known or can be predicated about their
stability or instability, according to Professors Newcomb and Darwin.

Mr. E.T. Whittaker (Secretary to the Royal Astronomical Society), to whom
Professor G. Darwin sent my questions, writes:--'I doubt whether the
principal phenomena of the stellar universe are consequences of the law of
gravitation at all. I have been working myself at spiral nebulæ, and have
got a first approximation to an explanation--but it is electro-dynamical
and not gravitational. In fact, it may be questioned whether, for bodies of
such tremendous extent as the Milky Way or nebulæ, the effect which we call
gravitation is given by Newton's law; just as the ordinary formulæ of
electrostatic attraction break down when we consider charges moving with
very great velocities.'

Accepting these statements and opinions of two mathematicians who have
given special attention to similar problems, we need not limit ourselves to
the laws of gravitation as having determined the present form of the
stellar universe; and this is the more important because we may thus escape
from a conclusion which many astronomers seem to think inevitable, viz.
that the observed proper motions of the stars cannot be explained by the
gravitative forces of the system itself. In chapter VIII. of this work I
have quoted Professor Newcomb's calculation as to the effect of gravitation
in a universe of 100 million stars, each five times the mass of our sun,
and spread over a sphere which it would take light 30,000 years to cross;
then, a body falling from its outer limits to the centre could at the
utmost acquire a velocity of twenty-five miles a second; and therefore, any
body in any part of such a universe having a greater velocity would pass
away into infinite space. Now, as several stars have, it is believed, much
more than this velocity, it follows not only that they will inevitably
escape from our universe, but that they do not belong to it, as their great
velocity must have been acquired elsewhere. This seems to have been the
idea of the astronomer who stated that, even at the very moderate speed of
our sun, we should in five million years be deep in the actual stream of
the Milky Way. To this I have already sufficiently replied; but I now wish
to bring before my readers an excellent illustration of the importance of
the late Professor Huxley's remark, that the results you got out of the
'mathematical mill' depend entirely on what you put into it.

In the _Philosophical Magazine_ (January 1902) is a remarkable article by
Lord Kelvin, in which he discusses the very same problem as that which
Professor Newcomb had discussed at a much earlier date, but, starting from
different assumptions, equally based on ascertained facts and probabilities
deduced from them, brings out a very different result.

Lord Kelvin postulates a sphere of such a radius that a star at its
confines would have a parallax of one-thousandth part of a second (0".001),
equivalent to 3215 light-years. Uniformly distributed through this sphere
there is matter equal in mass to 1000 million suns like ours. If this
matter becomes subject to gravitation, it all begins to move at first with
almost infinite slowness, especially near its centre; but nevertheless, in
twenty-five million years many of these suns would have acquired velocities
of from twelve to twenty miles a second, while some would have less and
some probably more than seventy miles a second. Now such velocities as
these agree generally with the measured velocities of the stars, hence Lord
Kelvin thinks there may be as much matter as 1000 million suns within the
above-named distance. He then states that if we suppose there to be 10,000
million suns within the same sphere, velocities would be produced very much
greater than the known star-velocities; hence it is probable that there is
very much less matter than 10,000 million times the sun's mass. He also
states that if the matter were not uniformly distributed within the sphere,
then, whatever was the irregularity, the acquired motions would be greater;
again indicating that the 1000 million suns would be ample to produce the
observed effects of stellar motion. He then calculates the average distance
apart of each of the 1000 million stars, which he finds to be about 300
millions of millions of miles. Now the nearest star to our sun is about
twenty-six million million of miles distant, and, as the evidence shows, is
situated in the denser part of the solar cluster. This gives ample
allowance for the comparative emptiness of the space between our cluster
and the Milky Way, as well as of the whole region towards the poles of the
Milky Way (as shown by the diagrams in chapter IV.), while the comparative
density of extensive portions of the Galaxy itself may serve to make up the
average.

Now, previous writers have come to a different conclusion from the same
general line of argument, because they have started with different
assumptions. Professor Newcomb, whose statement made some years back is
usually followed, assumed 100 million stars each five times as large as our
sun, equal to 500 million suns in all, and he distributed them equally
throughout a sphere 30,000 light-years in diameter. Thus he has half the
amount of matter assumed by Lord Kelvin, but nearly five times the extent,
the result being that gravity could only produce a maximum speed of
twenty-five miles a second; whereas on Lord Kelvin's assumption a maximum
speed of seventy miles a second would be produced, or even more. By this
latter calculation we find no insuperable difficulty in the speed of any of
the stars being beyond the power of gravitation to produce, because the
rates here given are the direct results of gravitation acting on bodies
almost uniformly distributed through space. Irregular distribution, such as
we see everywhere in the universe, might lead to both greater and less
velocities; and if we further take account of collisions and near
approaches of large masses resulting in explosive disruptions, we might
have almost any amount of motion as the result, but as this motion would
be produced by gravitation within the system, it could equally well be
controlled by gravitation.

[Illustration: DIAGRAM OF STELLAR UNIVERSE (Plan). 1. Central part of Solar
Cluster. 2. Sun's Orbit (Black spot). 3. Outer limit of Solar Cluster. 4.
Milky Way.]

In order that my readers may better understand the calculations of Lord
Kelvin, and also the general conclusions of astronomers as to the form and
dimensions of the stellar universe, I have drawn two diagrams, one showing
a plan on the central plane of the Milky Way, the other a section through
its poles. Both are on the same scale, and they show the total diameter
across the Milky Way as being 3600 light-years, or about half that
postulated by Lord Kelvin for his hypothetical universe. I do this because
the dimensions given by him are those which are sufficient to lead to
motions near the centre such as the stars now possess in a minimum period
of twenty-five million years after the initial arrangement he supposes, at
which later epoch which we are now supposed to have reached, the whole
system would of course be greatly reduced in extent by aggregations
towards and near the centre. These dimensions also seem to accord
sufficiently with the actual distances of stars as yet measured. The
smallest parallax which has been determined with any certainty, according
to Professor Newcomb's list, is that of Gamma Cassiopeiæ, which is
one-hundredth of a second (0".01), while Lord Kelvin gives none smaller
than 0".02, and these will all be included within the solar cluster as I
have shown it.

[Illustration: DIAGRAM OF STELLAR UNIVERSE (Section). Section through Poles
of Milky Way.]

It must be clearly understood that these two illustrations are merely
diagrams to show the main features of the stellar universe according to the
best information available, with the proportionate dimensions of these
features, so far as the facts of the distribution of the stars and the
views of those astronomers who have paid most attention to the subject can
be harmonised. Of course it is not suggested that the whole arrangement is
so regular as here shown, but an attempt has been made by means of the
dotted shading to represent the comparative densities of the different
portions of space around us, and a few remarks on this point may be needed.

The solar cluster is shown very dense at the central portion, occupying
one-tenth of its diameter, and it is near the outside of this dense centre
that our sun is supposed to be situated. Beyond this there seems to be
almost a vacuity, beyond which again is the outer portion of the cluster
consisting of comparatively thinly scattered stars, thus forming a kind of
ring-cluster, resembling in shape the beautiful ring-nebula in Lyra, as has
been suggested by several astronomers. There is some direct evidence for
this ring-form. Professor Newcomb in his recent book on _The Stars_ gives
a list of all stars of which the parallax is fairly well known. These are
sixty-nine in number; and on arranging them in the order of the amount of
their parallax, I find that no less than thirty-five of them have
parallaxes between 0".1 and 0".4 of a second, thus showing that they
constitute part of the dense central mass; while three others, from 0".4 to
0".75, indicate those which are our closest companions at the present time,
but still at an enormous distance. Those which have parallaxes of less than
the tenth and down to one-hundredth of a second are only thirty-one in all;
but as they are spread over a sphere ten times the diameter, and therefore
a thousand times the cubic content of the sphere containing those above
one-tenth of a second, they ought to be immensely more numerous even if
very much more thinly scattered. The interesting point, however, is, that
till we get down to a parallax of 0".06, there are only three stars as yet
measured, whereas those between 0".02 and 0".06, an equal range of
parallax, are twenty-six in number, and as these are scattered in all
directions they indicate an almost vacant space followed by a moderately
dense outer ring.

In the enormous space between our cluster and the Milky Way, and also above
and below its plane to the poles of the Galaxy, stars appear to be very
thinly scattered, perhaps more densely in the plane of the Milky Way than
above and below it where the irresolvable nebulæ are so numerous; and there
may not improbably be an almost vacant space beyond our cluster for a
considerable distance, as has been supposed, but this cannot be known till
some means are discovered of measuring parallaxes of from one-hundredth to
one five-hundredth of a second.

These diagrams also serve to indicate another point of considerable
importance to the view here advocated. By placing the solar system towards
the outer margin of the dense central portion of the solar cluster (which
may very possibly include a large proportion of dark stars and thus be much
more dense towards the centre than it appears to us), it may very well be
supposed to revolve, with the other stars composing it, around the centre
of gravity of the cluster, as the force of gravity towards that centre
might be perhaps twenty or a hundred times greater than towards the very
much less dense and more remote outer portions of the cluster. The sun, as
indicated on the diagrams, is about thirty light-years from that centre,
corresponding to a parallax of a little more than one-tenth of a second,
and an actual distance of 190 millions of millions of miles, equal to about
70,000 times the distance of the sun from Neptune. Yet we see that this
position is so little removed from the exact centre of the whole stellar
universe, that if any beneficial influences are due to that central
position in regard to the Galaxy, it will receive them perhaps to as full
an extent as if situated at the actual centre. But if it is situated as
here shown, there is no further difficulty as to its proper motion carrying
it from one side to the other of the Milky Way in less time than has been
required for the development of life upon the earth. And if the solar
cluster is really sub-globular, and sufficiently condensed to serve as a
centre of gravity for the whole of the stars of the cluster to revolve
around, all the component stars which are not situated in the plane of its
equator (and that of the Milky Way) must revolve obliquely at various
angles up to an angle of 90°. These numerous diverging motions, together
with the motions of the nearer stars outside the cluster, some of which may
revolve round other centres of gravity made up largely of dark bodies,
would perhaps sufficiently account for the apparent random motions of so
many of the stars.


UNIFORM HEAT-SUPPLY DUE TO CENTRAL POSITION

We now come to a point of the greatest interest as regards the problem we
are investigating. We have seen how great is the difference in the
estimates of geologists and those of physicists as to the time that has
elapsed during the whole development of life. But the position we have now
found for the sun, in the outer portion of the central star-cluster, may
afford a clue to this problem. What we require is, some mode of keeping up
the sun's heat during the enormous geological periods in which we have
evidence of a wonderful uniformity in the earth's temperature, and
therefore in the sun's heat-emission. The great central ring-cluster with
its condensed central mass, which presumably has been forming for a much
longer period than our sun has been giving heat to the earth, must during
all this time have been exerting a powerful attraction on the diffused
matter in the spaces around it, now apparently almost void as compared with
what they may have been. Some scanty remnants of that matter we see in the
numerous meteoric swarms which have been drawn into our system. A position
towards the outside of this central aggregation of suns would evidently be
very favourable for the growth by accretion of any considerable mass. The
enormous distance apart of the outer components (the outer ring) of the
cluster would allow a large amount of the inflowing meteoritic matter to
escape them, and the larger suns situated near the surface of the inner
dense cluster would draw to themselves the greater part of this matter.[23]
The various planets of our system were no doubt built up from a portion of
the matter that flowed in near the plane of the ecliptic, but much of that
which came from all other directions would be drawn towards the sun itself
or to its neighbouring suns. Some of this would fall directly into it;
other masses coming from different directions and colliding with each other
would have their motion checked, and thus again fall into the sun; and so
long as the matter falling in were not in too large masses, the slow
additions to the sun's bulk and increase of its heat would be sufficiently
gradual to be in no way prejudicial to a planet at the earth's distance.

The main point I wish to suggest here is, that by far the greater portion
of the matter of the whole stellar universe has, either through gravitation
or in combination with electrical forces, as suggested by Mr. Whittaker,
become drawn together into the vast ring-formed system of the Milky Way,
which is, presumably, slowly revolving, and has thus been checked in its
original inflow toward the centre of mass of the stellar universe. It has
also probably drawn towards itself the adjacent portions of the scattered
material in the spaces around it in all directions.

Had the vast mass of matter postulated by Lord Kelvin acquired no motion of
revolution, but have fallen continuously towards the centre of mass, the
motions developed when the more distant bodies approached that centre would
have been extremely rapid; while, as they must have fallen in from every
direction, they would have become more and more densely aggregated, and
collisions of the most catastrophic nature would frequently have occurred,
and this would have rendered the central portion of the universe the
_least_ stable and the _least_ fitted to develop life.

But, under the conditions that actually prevail, the very reverse is the
case. The quantity of matter remaining between our cluster and the Milky
Way being comparatively small, the aggregation into suns has gone on more
regularly and more slowly. The motions acquired by our sun and its
neighbours have been rendered moderate by two causes: (1) their nearness to
the centre of the very slowly aggregating cluster where the motion due to
gravitation is least in amount; and (2) the slight differential attraction
away from the centre by the Milky Way on the side nearest to us. Again,
this protective action of the Milky Way has been repeated, on a smaller
scale, by the formation of the outer ring of the solar cluster, which has
thus preserved the inner central cluster itself from a too abundant direct
inflow of large masses of matter.

But although the matter composing the outer portion of the original
universe has been to a large extent aggregated into the vast system of the
Milky Way, it seems probable, perhaps even certain, that some portion would
escape its attractive forces and would pass through its numerous open
spaces--indicated by the dark rifts, channels, and patches, as already
described--and thus flow on unchecked towards the centre of mass of the
whole system. The quantity of matter thus reaching the central cluster from
the enormously remote spaces beyond the Milky Way might be very small in
comparison with what was retained to build up that wonderful star-system;
but it might yet be so large in total amount as to play an important part
in the formation of the central group of suns. It would probably flow
inwards almost continuously, and when it ultimately reached the solar
cluster, it would have attained a very high velocity. If, therefore, it
were widely diffused, and consisted of masses of small or moderate size as
compared with planets or stars, it would furnish the energy requisite for
bringing these slowly aggregating stars to the required intensity of heat
for forming luminous suns.

Here, then, I think, we have found an adequate explanation of the very
long-continued light and heat-emitting capacity of our sun, and probably of
many others in about the same position in the solar cluster. These would at
first gradually aggregate into considerable masses from the slowly moving
diffused matter of the central portions of the original universe; but at a
later period they would be reinforced by a constant and steady inrush of
matter from its very outer regions, and therefore possessing such high
velocities as to materially aid in producing and maintaining the requisite
temperature of a sun such as ours, during the long periods demanded for
continuous life-development. The enormous extension and mass of the
original universe of diffused matter (as postulated by Lord Kelvin) is thus
seen to be of the greatest importance as regards this ultimate product of
evolution, because, without it, the comparatively slow-moving and cool
central regions might not have been able to produce and maintain the
requisite energy in the form of heat; while the aggregation of by far the
larger portion of its matter in the great revolving ring of the galaxy was
equally important, in order to prevent the too great and too rapid inflow
of matter to these favoured regions.

It appears, then, that if we admit as probable some such process of
development as I have here indicated, we can dimly see the bearing of all
the great features of the stellar universe upon the successful development
of life. These are, its vast dimensions; the form it has acquired in the
mighty ring of the Milky Way; and our position near to, but not exactly in,
its centre. We know that the star-system _has_ acquired these forms,
presumably from some simple and more diffused condition. We know that we
_are_ situated near the centre of this vast system. We know that our sun
_has_ emitted light and heat, almost uniformly, for periods incompatible
with rapid aggregation and the equally rapid cooling which physicists
consider inevitable. I have here suggested a mode of development which
would lead to a very slow but continuous growth of the more central suns;
to an excessively long period of nearly stationary heat-giving power; and
lastly, an equally long period of very gradual cooling--a period the
commencement of which our sun may have just entered upon.

Descending now to terrestrial physics, I have shown that, owing to the
highly complex nature of the adjustments required to render a world
habitable and to retain its habitability during the æons of time requisite
for life-development, it is in the highest degree improbable that the
required conditions and adaptations should have occurred in any other
planets of any other suns, which _might_ occupy an equally favourable
position with our own, and which were of the requisite size and heat-giving
power.

Lastly, I submit that the whole of the evidence I have here brought
together leads to the conclusion that our earth is almost certainly the
only inhabited planet in our solar system; and, further, that there is no
inconceivability--no improbability even--in the conception that, in order
to produce a world that should be precisely adapted in every detail for the
orderly development of organic life culminating in man, such a vast and
complex universe as that which we know exists around us, may have been
absolutely required.


SUMMARY OF ARGUMENT

As the last ten chapters of this volume embody a connected argument leading
to the conclusion above stated, it may be useful to my readers to summarise
rather fully the successive steps of this argument, the facts on which it
rests, and the various subsidiary conclusions arrived at.

(1) One of the most important results of modern astronomy is to have
established the unity of the vast stellar universe which we see around us.
This rests upon a great mass of observations, which demonstrate the
wonderful complexity in detail of the arrangement and distribution of stars
and nebulæ, combined with a no less remarkable general symmetry, indicating
throughout a single inter-dependent system, not a number of totally
distinct systems so far apart as to have no physical relations with each
other, as was once supposed.

(2) This view is supported by numerous converging lines of evidence, all
tending to show that the stars are not infinite in number, as was once
generally believed, and which view is even now advocated by some
astronomers. The very remarkable calculations of Lord Kelvin, referred to
in the early part of this chapter, give a further support to this view,
since they show that if the stars extended much beyond those we see or can
obtain direct knowledge of, and with no very great change in their average
distance apart, then the force of gravitation towards the centre would have
produced on the average more rapid motions than the stars generally
possess.

(3) An overwhelming consensus of opinion among the best astronomers
establishes the fact of our nearly central position in the stellar
universe. They all agree that the Milky Way is nearly circular in form.
They all agree that our sun is situated almost exactly in its medial plane.
They all agree that our sun, although not situated at the exact centre of
the galactic circle, is yet not very far from it, because there are no
unmistakable signs of our being nearer to it at any one point and farther
away from the opposite point. Thus the nearly central position of our sun
in the great star-system is almost universally admitted.

On the question of the solar-cluster there is more difference of opinion;
though here, again, all are agreed that there is such a cluster. Its size,
form, density, and exact position are somewhat uncertain, but I have, as
far as possible, been guided by the best available evidence. If we adopt
Lord Kelvin's general idea of the gradual condensation of an enormous
diffused mass of matter towards its common centre of gravity, that centre
would be approximately the centre of this cluster. Also, as gravitational
force at and near this centre would be comparatively small, the motions
produced there would be slow, and collisions, being due only to
differential motions, when they did occur would be very gentle. We might
therefore expect many dark aggregations of matter here, which may explain
why we do not find any special crowding of visible stars in the direction
of this centre; while, as no star has a sensible disc, the dark stars if at
great distances would hardly ever be seen to occult the bright ones. Thus,
it seems to me, the controlling force may be explained which has retained
our sun in approximately the same orbit around the centre of gravity of
this central cluster during the whole period of its existence as a sun and
our existence as a planet; and has thus saved us from the
possibility--perhaps even the certainty--of disastrous collisions or
disruptive approaches to which suns, in or near the Milky Way, and to a
less extent elsewhere, are or have been exposed. It seems quite probable
that in that region of more rapid and less controlled motions and more
crowded masses of matter, no star can remain in a nearly stable condition
as regards temperature for sufficiently long periods to allow of a complete
system of life-development on any planet it may possess.

(4) The various proofs are next stated that assure us of the almost
complete uniformity of matter, and of material physical and chemical laws,
throughout our universe. This I believe no one seriously disputes; and it
is a point of the greatest importance when we come to consider the
conditions required for the development and maintenance of life, since it
assures us that very similar, if not identical, conditions must prevail
wherever organic life is or can be developed.

(5) This leads us on to the consideration of the essential characteristics
of the living organism, consisting as it does of some of the most abundant
and most widely distributed of these material elements, and being always
subject to the general laws of matter. The best authorities in physiology
are quoted, as to the extreme complexity of the chemical compounds which
constitute the physical basis for the manifestation of life; as to their
great instability; their wonderful mobility combined with permanence of
form and structure; and the altogether marvellous powers they possess of
bringing about unique chemical transformations and of building up the most
complicated structures from simple elements.

I have endeavoured to put the broad phenomena of vegetable and animal life
in a way that will enable my readers to form some faint conception of the
intricacy, the delicacy, and the mystery of the myriad living forms they
see everywhere around them. Such a conception will enable them to realise
how supremely grand is organic life, and to appreciate better, perhaps, the
absolute necessity for the numerous, complex and delicate adaptations of
inorganic nature, without which it is impossible for life either to exist
now, or to have been developed during the immeasurable past.

(6) The general conditions which are absolutely essential for life thus
manifested on our planet are then discussed, such as, solar light and heat;
water universally distributed on the planet's surface and in the
atmosphere; an atmosphere of sufficient density, and composed of the
several gases from which alone protoplasm can be formed; some alternations
of light and darkness, and a few others.

(7) Having treated these conditions broadly, and explained why they are
important and even indispensable for life, we next proceed to show how they
are fulfilled upon the earth, and how numerous, how complex, and often how
exact are the adjustments needed to bring them about, and maintain them
almost unchanged throughout the vast æons of time occupied in the
development of life. Two chapters are devoted to this subject; and it is
believed that they contain facts that will be new to many of my readers.
The combinations of causes which lead to this result are so varied, and in
several cases dependent on such exceptional peculiarities of physical
constitution, that it seems in the highest degree improbable that they can
_all_ be found again combined either in the solar system or even in the
stellar universe. It will be well here just to enumerate these conditions,
which are all essential within more or less narrow limits:--

     Distance of planet from the sun.

     Mass of planet.

     Obliquity of its ecliptic.

     Amount of water as compared with land.

     Surface distribution of land and water.

     Permanence of this distribution, dependent probably on the
     unique origin of our moon.

     An atmosphere of sufficient density, and of suitable component
     gases.

     An adequate amount of dust in the atmosphere.

     Atmospheric electricity.

Many of these act and react on each other, and lead to results of great
complexity.

(8) Passing on to other planets of the solar system, it is shown that none
of them combine all the complex conditions which are found to work
harmoniously together on the earth; while in most cases there is some one
defect which alone removes them from the category of possible
life-producing and life-supporting planets. Among these are the small size
and mass of Mars, being such that it cannot retain aqueous vapour; and the
fact that Venus rotates on its axis in the same time as it takes to revolve
round the sun. Neither of these facts was known when Proctor wrote upon the
question of the habitability of the planets. All the other planets are now
given up--and were given up by Proctor himself--as possible life-bearers in
their present stage; but he and others have held that, if not suitable now,
some of them may have been the scene of life-development in the past,
while others will be so in the future.

In order to show the futility of this supposition, the problem of the
duration of the sun as a stable heat-giver is discussed; and it is shown
that it is only by reducing the periods claimed by geologists and
biologists for life-development upon the earth, and by extending the time
allowed by physicists to its utmost limits, that the two claims can be
harmonised. It follows that the whole period of the sun's duration as a
light and heat-giver has been required for the development of life upon the
earth; and that it is only upon planets whose phases of development
synchronise with that of the earth that the evolution of life is possible.
For those whose material evolution has gone on quicker or slower, there has
not been, or will not be, time enough for the development of life.

(9) The problem of the stars as possibly having life-supporting planets is
next dealt with, and reasons are given why in only a minute portion of the
whole is this possible. Even in that minute portion, reduced probably to a
few of the component suns of the solar-cluster, a large proportion seems
likely to be ruled out by being close binary systems, and another large
portion by being in process of aggregation. In those remaining, whether
they may be reckoned by tens or by hundreds we cannot say, the chances
against the same complex combination of conditions as those which we find
on the earth occurring on any planet of any other sun are enormously great.

(10) I then refer, briefly, to some recent measurements of star-radiation,
and suggest that they may thus possibly have important effects on the
development of vegetable and animal life; and, finally, I discuss the
problem of the stability of the stellar universe and the special advantage
we derive from our central position, suggested by some of the latest
researches of our great mathematician and physicist--Lord Kelvin.


CONCLUSIONS

Having thus brought together the whole of the available evidence bearing
upon the questions treated in this volume, I claim that certain definite
conclusions have been reached and proved, and that certain other
conclusions have enormous probabilities in their favour.

The conclusions reached by modern astronomers are: (1) That the stellar
universe forms one connected whole; and, though of enormous extent, is yet
finite, and its extent determinable.

(2) That the solar system is situated in the plane of the Milky Way, and
not far removed from the centre of that plane. The earth is therefore
nearly in the centre of the stellar universe.

(3) That this universe consists throughout of the same kinds of matter, and
is subjected to the same physical and chemical laws.

The conclusions which I claim to have shown to have enormous probabilities
in their favour are--

(4) That no other planet in the solar system than our earth is inhabited or
habitable.

(5) That the probabilities are almost as great against any other sun
possessing inhabited planets.

(6) That the nearly central position of our sun is probably a permanent
one, and has been specially favourable, perhaps absolutely essential, to
life-development on the earth.

       *       *       *       *       *

These latter conclusions depend upon the combination of a large number of
special conditions, each of which must be in definite relation to many of
the others, and must all have persisted simultaneously during enormous
periods of time. The weight to be given to this kind of reasoning depends
upon a full and fair consideration of the _whole_ evidence as I have
endeavoured to present it in the last seven chapters of this book. To this
evidence I appeal.

       *       *       *       *       *

This completes my work as a connected argument, founded wholly on the facts
and principles accumulated by modern science; and it leads, if my facts are
substantially correct and my reasoning sound, to one great and definite
conclusion--that man, the culmination of conscious organic life, has been
developed here only in the whole vast material universe we see around us. I
claim that this is the logical outcome of the evidence, if we consider and
weigh this evidence without any prepossessions whatever. I maintain that it
is a question as to which we have no right to form _a priori_ opinions not
founded upon evidence. And evidence opposed to this conclusion, or even as
to its improbability, we have absolutely none whatever.

But, if we admit the conclusion, nothing that need alarm either the
scientific or the religious mind necessarily follows, because it can be
explained or accounted for in either of two distinct ways. One
considerable body, including probably the majority of men of science, will
admit that the evidence does apparently lead to this conclusion, but will
explain it as due to a fortunate coincidence. There might have been a
hundred or a thousand life-bearing planets, had the course of evolution of
the universe been a little different, or there might have been none at all.
They would probably add, that, as life and man _have_ been produced, that
shows that their production was possible; and therefore, if not now then at
some other time, if not here then in some other planet of some other sun,
we should be sure to have come into existence; or if not precisely the same
as we are, then something a little better or a little worse.

The other body, and probably much the largest, would be represented by
those who, holding that mind is essentially superior to matter and distinct
from it, cannot believe that life, consciousness, mind, are products of
matter. They hold that the marvellous complexity of forces which appear to
control matter, if not actually to constitute it, are and must be
mind-products; and when they see life and mind apparently rising out of
matter and giving to its myriad forms an added complexity and unfathomable
mystery, they see in this development an additional proof of the supremacy
of mind. Such persons would be inclined to the belief of the great
eighteenth century scholar, Dr. Bentley, that the soul of one virtuous man
is of greater worth and excellency than the sun and all his planets and all
the stars in the heavens; and when they are shown that there are strong
reasons for thinking that man _is_ the unique and supreme product of this
vast universe, they will see no difficulty in going a little further, and
believing that the universe was actually brought into existence for this
very purpose.

With infinite space around us and infinite time before and behind us, there
is no incongruity in this conception. A universe as large as ours for the
purpose of bringing into existence many myriads of living, intellectual,
moral, and spiritual beings, with unlimited possibilities of life and
happiness, is surely not _more_ out of proportion than is the complex
machinery, the life-long labour, the ingenuity and invention which we have
bestowed upon the production of the humble, the trivial, _pin_. Neither is
the apparent waste of energy so great in such a universe, comparatively, as
the millions of acorns, produced during its life by an oak, every one of
which might grow to be a tree, but of which only _one_ does actually, after
several hundred years, produce the _one_ tree which is to replace the
parent. And if it is said that the acorns are food for bird and beast, yet
the spores of ferns and the seeds of orchids are not so, and countless
millions of these go to waste for every one which reproduces the parent
form. And all through the animal world, especially among the lower types,
the same thing is seen. For the great majority of these entities _we_ can
see no use whatever, either of the enormous variety of the species, or the
vast hordes of individuals. Of beetles alone there are at least a hundred
thousand distinct species now living, while in some parts of sub-arctic
America mosquitoes are sometimes so excessively abundant that they obscure
the sun. And when we think of the myriads that have existed through the
vast ages of geological time, the mind reels under the immensity of, to
us, apparently useless life.

All nature tells us the same strange, mysterious story, of the exuberance
of life, of endless variety, of unimaginable quantity. All this life upon
our earth has led up to and culminated in that of man. It has been, I
believe, a common and not unpopular idea that during the whole process of
the rise and growth and extinction of past forms, the earth has been
preparing for the ultimate--Man. Much of the wealth and luxuriance of
living things, the infinite variety of form and structure, the exquisite
grace and beauty in bird and insect, in foliage and flower, may have been
mere by-products of the grand mechanism we call nature--the one and only
method of developing humanity.

And is it not in perfect harmony with this grandeur of design (if it be
design), this vastness of scale, this marvellous process of development
through all the ages, that the material universe needed to produce this
cradle of organic life, and of a being destined to a higher and a permanent
existence, should be on a corresponding scale of vastness, of complexity,
of beauty? Even if there were no such evidence as I have here adduced for
the unique position and the exceptional characteristics which distinguish
the earth, the old idea that all the planets were inhabited, and that all
the stars existed for the sake of other planets, which planets existed to
develop life, would, in the light of our present knowledge, seem utterly
improbable and incredible. It would introduce monotony into a universe
whose grand character and teaching is endless diversity. It would imply
that to produce the living soul in the marvellous and glorious body of
man--man with his faculties, his aspirations, his powers for good and
evil--that this was an easy matter which could be brought about anywhere,
in any world. It would imply that man is an animal and nothing more, is of
no importance in the universe, needed no great preparations for his advent,
only, perhaps, a second-rate demon, and a third or fourth-rate earth.
Looking at the long and slow and complex growth of nature that preceded his
appearance, the immensity of the stellar universe with its thousand million
suns, and the vast æons of time during which it has been developing--all
these seem only the appropriate and harmonious surroundings, the necessary
supply of material, the sufficiently spacious workshop for the production
of that planet which was to produce first, the organic world, and then,
Man.

In one of his finest passages our great world-poet gives us _his_
conception of the grandeur of human nature--'What a piece of work is man!
How noble in reason! How infinite in faculty! In form and moving, how
express and admirable! In action how like an angel! In apprehension how
like a god!' And for the development of such a being what is a universe
such as ours? However vast it may seem to our faculties, it is as a mere
nothing in the ocean of the infinite. In infinite space there may be
infinite universes, but I hardly think they would be all universes of
matter. That would indeed be a low conception of infinite power! Here, on
earth, we see millions of distinct species of animals, millions of
different species of plants, and each and every species consisting often
of many millions of individuals, no two individuals exactly alike; and when
we turn to the heavens, no two planets, no two satellites alike; and
outside our system we see the same law prevailing--no two stars, no two
clusters, no two nebulæ alike. Why then should there be other universes of
the _same_ matter and subject to the _same_ laws--as is implied by the
conception that the stars are infinite in number, and extend through
infinite space?

Of course there may be, and probably are, other universes, perhaps of other
kinds of matter and subject to other laws, perhaps more like our
conceptions of the ether, perhaps wholly non-material, and what we can only
conceive of as spiritual. But, unless these universes, even though each of
them were a million times vaster than our stellar universe, were also
infinite in number, they could not fill infinite space, which would extend
on all sides beyond them, so that even a million million such universes
would shrink to imperceptibility when compared with the vast beyond!

Of infinity in any of its aspects we can really know nothing, but that it
exists and is inconceivable. It is a thought that oppresses and overwhelms.
Yet many speak of it glibly as if they _knew_ what it contains, and even
use that assumed knowledge as an argument against views that are
unacceptable to themselves. To me its existence is absolute but
unthinkable--that way madness lies.

    'O night! O stars, too rudely jars
    The finite with the infinite!'

I will conclude with one of the finest passages relating to the infinite
that I am acquainted with, from the pen of the late R.A. Proctor:

'Inconceivable, doubtless, are these infinities of time and space, of
matter, of motion, and of life. Inconceivable that the whole universe can
be for all time the scene of the operation of infinite power, omnipresent,
all-knowing. Utterly incomprehensible how Infinite Purpose can be
associated with endless material evolution. But it is no new thought, no
modern discovery, that we are thus utterly powerless to conceive or
comprehend the idea of an Infinite Being, Almighty, All-knowing,
Omnipresent, and Eternal, of whose inscrutable purpose the material
universe is the unexplained manifestation. Science is in presence of the
old, old mystery; the old, old questions are asked of her--"Canst thou by
searching find out God? Canst thou find out the Almighty unto perfection?
It is as high as heaven; what canst thou do? deeper than hell; what canst
thou know?" And science answers these questions as they were answered of
old--"As touching the Almighty we cannot find Him out."'

       *       *       *       *       *

The following beautiful lines--among the latest products of Tennyson's
genius--so completely harmonise with the subject-matter of the present
volume, that no apology is needed for quoting them here:--

(_The Question_)

    Will my tiny spark of being
      Wholly vanish in your deeps and heights?
    Must my day be dark by reason,
      O ye Heavens, of your boundless nights,
    Rush of Suns and roll of systems,
      And your fiery clash of meteorites?

(_The Answer_)

    'Spirit, nearing yon dark portal
      At the limit of thy human state,
    Fear not thou the hidden purpose
      Of that Power which alone is great,
    Nor the myriad world, His shadow,
      Nor the silent Opener of the Gate.'

FOOTNOTE:

[23] Since writing this chapter I have seen a paper by Luigi d'Auria
dealing mathematically with 'Stellar Motion, etc.,' and am pleased to see
that, from quite different considerations, he has found it necessary to
place the solar system at a distance from the centre not very much more
remote than the position I have given it. He says: 'We have good reasons to
suppose that the solar system is rather near the centre of the Milky Way,
and as this centre would, according to our hypothesis, coincide with the
centre of the Universe, the distance of 159 light years assumed is not too
great, nor can it be very much smaller.'--_Journal of the Franklin
Institute_, March 1903.



INDEX


    Adrianus Tollius on stone axes, 203.

    Air criminally poisoned by us, 260.

    Albedo explained, 162.

    Algol and its companion, 39;
      change of colour of, 41.

    Allen, Prof. F.J., on living matter, 193;
      on importance of nitrogen, 195;
      on physical conditions essential for life, 196.

    Alpha Centauri, nearest star, 74.

    Ammonia, importance of, to life, 195.

    Anaximander's cosmic theory, 2.

    Angles of a minute and second, 80.

    Arcturus, rapid motion of, 172.

    Argument of book, summary of, 310.

    Astronomers, the first, 2.

    Astronomy, the new, 24.

    Astrophysics, a new science, 32.

    Atmosphere, qualities requisite for life, 210;
      requisite composition of, 212;
      aqueous vapour in, 214;
      and life, 243;
      effects of density of, 245;
      a complex structure, 259;
      its vital importance to us, 260.


    Ball, Sir R., on dark stars, 143;
      _Time and Tide_, 233.

    Barnham, S.W., on double stars, 123.

    Blue of sky due to dust, 251.

    Boeddicker's map of Milky Way, 164.

    Brewster, Sir D., against Whewell, 15.


    Campbell, Prof., on spectroscopic binaries, 125;
      on uncertainty of sun's motions, 179;
      on number of binary systems, 286.

    Carbon compounds, vast numbers of, 194.

    Carbonic acid gas essential for life, 196.

    Central position of sun, importance of, 305.

    Chaldeans the first astronomers, 2.

    Chalmers Dr., on plurality of worlds, 13.

    Chamberlin, T.C., origin of nebulæ, 120;
      on stellar disruption, 186.

    Chromosphere, the sun's, 107.

    Clerke, Miss A.M., on limits of star system, 138;
      on Milky Way, 158, 160;
      on solar cluster, 165;
      on uncertainty of the sun's motion, 177.

    Climate, persistence of mild, 222.

    Clouds, importance of, to life, 248.

    Clusters in relation to Galaxy, 67.

    Comte, on impossibility of real knowledge of the stars, 25.

    Conclusions of the book, 317;
      bearing of, on science and on religion, 319.

    Corona of sun, 108.

    Criticisms of article in _Fortnightly Review_, 168, 180.


    Darwin, Prof. G., on meteoritic hypothesis, 133;
      on origin of moon, 233;
      on instability of annular systems, 295.

    Day and night, uses of, 215.

    Diagrams of star-distribution, 62, 66.

    Diffraction-gratings, 30.

    Disruption of stellar bodies, 187.

    Doppler principle, the, 37.

    Double stars, evolution of, 123;
      not fitted for life, 286.

    Dust, importance of, 249.

    Dust-free air, results of, 254.


    Earth, first measured, 5;
      in relation to life, 218;
      the only habitable planet, 262;
      cannot retain hydrogen, 264;
      supposed extreme conditions of, 271.

    Earth's mass, how related to life, 265.

    Ecliptic, obliquity of, in relation to life, 219.

    Electricity, effects of atmospheric, 257;
      atmospheric, how caused, 258.

    Elements, change in spectra of, 129;
      in the sun, 184;
      in meteorites, 185;
      in organic structures, 201.

    Empedocles an early astronomer, 3.

    Eudoxus on motions of planets, 3.

    Evolution of the stars, 128.

    Explanations of life-processes, 202.


    Faculæ of sun, 105.

    Fisher, Rev. O., on oceanic basins, 234;
      on thin sub-oceanic crust, 237.

    Fizeau measures speed of light, 79.

    Flammarion, C., on universality of life, 274, 281.

    Fontenelle on plurality of worlds, 9.


    Galileo on star measurement, 74.

    Geological climates, 222.

    Geologists on duration of sun's heat, 275.

    Germinal vesicle, M'Kendrick on, 202.

    Gill, Sir D., on systematic star-motions, 178.

    Globular clusters, stability of, 126;
      and variables, 127.

    Gore, Mr. J.E., on stars in Galaxy, 60;
      on mass of binary stars, 97;
      on remoteness of bright stars, 140;
      on limits of star system, 145;
      on limited number of stars, 151;
      on life on planets of other suns, 282, 289.

    Gould on solar cluster, 165.

    Gould's map of Milky Way, 164.

    Gravitation, motions produced by, on Lord Kelvin's hypothesis, 298.


    Haliburton, Professor W.D., on proteids, 200.

    Heat and cold on earth's surface, 207.

    Heat-supply, our long-continued, accounted for, 305.

    Herschel, Sir J., on Milky Way, 50;
      on limits of the star-system, 147.

    Heliometer, description of, 89.

    Huggins, Sir W., on spectra of stars, 32;
      measures radial motion, 37.

    Huxley, Prof., on protoplasm, 198;
      on duration of life, 278.

    Hydrogen, why not in atmosphere, 240;
      escapes from earth, 264.


    Infinity, unknowable, 323;
      Proctor on, 324.


    Jupiter's satellites show speed of light, 79.


    Kapteyn on solar cluster, 166.

    Kelvin, Lord, on the sun's age, 279;
      on a suggested primitive form of star-system, 298.

    Kirchhoff, discovers spectrum-analysis, 28.


    Laws of matter uniform throughout universe, 187.

    Leaves, importance of, 197.

    Lee, Dr., on origin of double stars, 123.

    Lewis, on remote bright stars, 141.

    Life, unity of organic, 189;
      definitions of, 191;
      conditions essential for, 206;
      water essential for, 210;
      atmosphere for, 210;
      dependent on temperature, 218;
      now improbable in stars, 288;
      conditions essential for, summarised, 314.

    Life-processes, explanations of, 202.

    Light, velocity of measured, 79;
      necessity of solar, 209;
      from sky due to dust, 252.

    Light-journey explained, 75.

    Light-ratio shows stars to be limited, 152.

    Living bodies, essential points in, 192.

    Lockyer, Sir. N., on inorganic evolution, 117;
      on evolution of stars, 130;
      on Milky Way, 159;
      on position of solar system, 161.

    Luigi d'Auria on stellar motion, 306.


    M'Kendrick, Prof., on germinal vesicle, 202.

    Magnetism and sun-spots, 106.

    Man, Shakespeare on, 322.

    Mars, has no water, 266;
      excessive temperatures on, 267.

    Matter of universe uniform, 183.

    Maunder on dark stars, 143.

    Maxwell Hall, Mr., on star-motions, 178.

    Measurement of star-distances, 85;
      difficulty of, 86.

    Mercury not habitable, 266.

    Meteorites, elements in, 185;
      not primitive bodies, 186.

    Meteoritic hypothesis, 113;
      Proctor on, 114;
      explains nebulæ, 116;
      Dr. Roberts on, 119.

    Milky Way, the, 48;
      form of, 51, 159;
      description of, 52;
      telescopic view of, 57;
      stars in relation to, 59;
      Mr. Gore on, 60;
      density of stars in, 61;
      clusters and nebulæ in relation to, 67;
      probable distance of, 96;
      forms a great circle, 157, 162;
      Prof. Newcomb on, 158;
      probably no life in, 284;
      diagrams of, 300;
      revolution of, important to us, 307.

    Million, how to appreciate a, 82.

    Minchin, G.M., on radiation from stars, 290.

    Monck, Mr. W.H.S., on non-infinity of stars, 144;
      on uncertainty of sun's motion, 177.

    Moon, why no atmosphere, 263.

    Moon's supposed origin, 233.

    Motion, in line of sight, 35.

    Motions, imperceptible, 39.


    Nebulæ, with gaseous spectra, 43;
      in relation to Galaxy, 66;
      distribution of, 69;
      many forms of, 70;
      gaseous, 71;
      meteoritic theory of, 116;
      planetary and annular, 175;
      Dr. Roberts on spiral, 117, 174;
      Chamberlin on origin of, 120.

    Nebular hypothesis, 98, 111;
      objection to, 112.

    Newcomb, Prof. S., on star distribution, 61;
      on parallax of stars, 94;
      on stability of star clusters, 126;
      on scarcity of single stars, 128;
      on limits of star system, 138;
      on Milky Way, 158, 160;
      on solar cluster, 167;
      on star velocities, 171;
      on average small mass of stars, 285;
      on star-motions, 297.

    Newton, Sir Isaac, on sun's habitability, 9.

    Nichols, E.F., on heat of stars, 290.

    Nitrogen, its importance to life, 195.

    Non-habitability of great planets, 272.


    Ocean and land, diagram of, 228.

    ---- basins, permanence of, 229.

    ---- ---- symmetry of, 238.

    ---- depths, how produced, 232.

    Oceans, effect of, on temperature, 239;
      curious relations of, 264.

    Organic products, diversity of, 195.


    Photographic astronomy, 43;
      measures of star-distances, 89.

    Photosphere, the, 105.

    Physicists on sun's duration, 278.

    Pickering's measurements of Algol, 40.

    Planets, supposed habitability of, 266, 269;
      the great, uninhabitable, 272;
      internal heat of great, 273;
      a last argument for habitability of, 274;
      have probably no life, 315.

    Planets' motions first explained, 3;
      mass and atmosphere, 262.

    Pleiades, number of stars in, 67;
      a drifting cluster, 177.

    Plurality of worlds, early writers on, 9;
      Proctor on, 18.

    Posidonius measures the earth, 5.

    Pritchard's photographic measures of star-distance, 89.

    Proctor, R.A., on other worlds, 18;
      on form of Galaxy, 51;
      on Herschel's views, 101;
      on stellar universe, 103;
      on meteoritic theory, 114;
      on infinities, 136;
      on star-drift, 176;
      on life under varied conditions, 271;
      on infinity, 324.

    Proctor's _Old and New Astronomy_, 46;
      chart of stars, 60.

    Prominences of sun, 107.

    Proteids, formation of, 199;
      Prof. Haliburton on, 200.


    Protoplasm, complexity of, 194;
      a mechanism, 198;
      sensibility of, to heat, 208.

    Ptolemaic system of the heavens, 4.


    Radial motion, 35.

    Radiation from stars, 290.

    Rain in the Carboniferous age, 225;
      dependent on dust, 249.

    Ramsay, Prof., on geological climates, 278.

    Ranyard, on star-discs, 98;
      on infinite universe, 137;
      on mass of Orion nebula, 173.

    Religious bearing of my conclusions, 319.

    Reproduction, marvel of, 201.

    Reversing layer of sun, 107.

    Roberts, A.W., on birth of double stars, 123.

    ---- Dr. I., on limits of star-system, 148;
      on spiral nebulæ, 117;
      on meteoritic theory, 119;
      photographs of nebulæ, 45, 174.

    Roche limit explained, 120, 187.


    Sanderson, Prof. Burdon, on living matter, 192.

    Scientific and agnostic opinion on my conclusions, 318.

    Secchi's classification of stars, 33.

    Single stars perhaps rare, 128.

    Solar apex, position of, 176.

    Solar cluster, the, 165;
      diagram showing, 300;
      evidence for, 302;
      importance to us, 306-7, 312.

    Solar system, position of, 304.

    Sorby on constitution of meteorites, 186.

    Spectra, varieties of, 34;
      of elements, changes in, 129.

    Spectroscopic binaries, abundance of, 125;
      great numbers of, 286.

    Spectrum analysis, discovery of, 26.

    Spencer, H., on status of nebulæ, 102.

    Spiral nebulæ, origin of, 120.

    Stars, proved to be suns, 32;
      invisible, 39;
      classification of, 33;
      spectroscopic double, 42;
      distribution of the, 47;
      number of visible, 48;
      description of Milky Way, 52;
      in relation to Milky Way, 59;
      distances of, 74;
      measurement of distance of, 85;
      mass of binary, 97;
      evolution of double, 122;
      spectroscopic double, 123;
      clusters of, 125;
      evolution of the, 128;
      classification of, 130;
      the hottest, 131;
      when cooling give more heat, 132;
      cycle of evolution and decay, 133;
      supposed infinite number of, 135;
      not infinite, 138;
      law of diminishing numbers of, 149;
      systematic motions of, 178;
      in relation to life, 282, 287;
      possible use of their emanations, 289.

    Star-clusters and variables, 127.

    Star-density, diagram of, 66.

    Star-drift, Proctor on, 176.

    Starlight, electrical measure of, 290;
      possible uses of, 292.

    Star-motions, Prof. Newcomb on, 297.

    Star-system, limited, 145;
      stability of, 295;
      supposed primitive form of, 297.

    Stellar motion, Luigi d'Auria on, 306.

    ---- universe, shape of, 49;
      unity of, 100;
      evolution of, 103;
      diagrams of, 300.

    Stoney, Dr., on atmospheres and gravity, 263.

    Sun, a typical star, 104;
      brightness of, 104;
      heat of, 104;
      surface of, 105;
      surroundings of, 106-110;
      corona of, 108;
      colour of, 111;
      elements in, 184.

    Sun's distance, measure of, 76.

    ---- heat, supposed limits of, 275.

    ---- life, all required to develop earth-life, 280.

    ---- motion through space, 91, 169.

    ---- ---- uncertain, 177.

    Sun-spots, nature of, 105.

    Symmetry of oceans, cause of, 238.


    Temperature, essential for life, 206;
      equalised by water, 239;
      as regards life on planets, 267.

    Tennyson on man and the universe, 325.


    Uniformity of matter, 183.

    Unity of stellar universe, 100.

    Universe of stars, how its form has affected our sun and earth, 308.

    Universe not disproportionate if man is its sole product, 320.


    VENUS, radial motions of, 38;
      diagram of transit of, 77;
      life barely possible on, 266;
      adverse climatic conditions of, 268.


    WATER, an essential for life, 210;
      its amount and distribution, 227;
      an equaliser of temperature, 239.

    Wave-lengths, how measured, 31.

    Whewell, on plurality of worlds, 8, 15;
      on man as the highest product of the universe, 14.

    Whittaker, Mr. E.T., on gravitative and electro-dynamical forces, 296.

    Winds, importance of, to life, 246.


    ZODIACAL light, 109.

    Printed by T. and A. CONSTABLE, Printers to His Majesty,
    at the Edinburgh University Press



[Illustration: THE NEBULÆ AND CLUSTERS OF THE NORTHERN HEAVENS. Drawn upon
an equal surface projection from Dr. DREYER'S catalogue of 1888 The Milky
Way from a Dr. BOEDDICKER'S Drawing by SIDNEY WATERS]

[Illustration: THE NEBULÆ AND CLUSTERS OF THE SOUTHERN HEAVENS. Drawn upon
an equal surface projection from Dr. DREYER'S catalogue of 1888 The Milky
Way from URANOMETRIA ARGENTINA by SIDNEY WATERS]



  _WORKS BY THE SAME AUTHOR._


  TRAVELS ON THE AMAZON AND RIO-NEGRO.

  PALM-TREES OF THE AMAZON, AND THEIR USES (_out of print_).

  THE MALAY ARCHIPELAGO.

  NATURAL SELECTION AND TROPICAL NATURE.

  THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS. _2 vols._

  ISLAND LIFE, OR INSULAR FAUNAS AND FLORAS.

  DARWINISM.

  AUSTRALASIA. _2 vols._

  BAD TIMES--CAUSES OF DEPRESSION OF TRADE (_out of print_).

  LAND NATIONALISATION, ITS NECESSITY AND ITS AIMS.

  VACCINATION A DELUSION.

  MIRACLES AND MODERN SPIRITUALISM.

  THE WONDERFUL CENTURY (_New and Illustrated Edition_).

  THE WONDERFUL CENTURY READER.

  STUDIES SCIENTIFIC AND SOCIAL.

       *       *       *       *       *


Transcriber's Notes:


Obvious punctuation and spelling errors repaired.

Inconsistent hyphenation has been repaired.

The oe and ae ligatures in the text has been left as it appears in the
original book.

Italic text is denoted by _underscores_ and bold text by =equal signs=.

In ambiguous cases, the text has been left as it appears in the original
book. In particular, many mismatched quotation marks have not been changed.

The remaining corrections made:

  Page v, "Fort-nightly" changed to "Fortnightly".
  Page 106, "terrestial" changed to "terrestrial".





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