Title: The Science of the Stars
Author: Maunder, E. Walter (Edward Walter)
Language: English
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  THE SCIENCE OF
  THE STARS


  BY E. WALTER MAUNDER, F.R.A.S.

  OF THE ROYAL OBSERVATORY, GREENWICH

  AUTHOR OF "ASTRONOMY WITHOUT A TELESCOPE"
  "THE ASTRONOMY OF THE BIBLE," ETC.



  LONDON: T. C. & E. C. JACK
  67 LONG ACRE, W.C., AND EDINBURGH
  NEW YORK: DODGE PUBLISHING CO.



{vii}

  CONTENTS

  CHAP.

  I.  ASTRONOMY BEFORE HISTORY
  II.  ASTRONOMY BEFORE THE TELESCOPE
  III.  THE LAW OF GRAVITATION
  IV.  ASTRONOMICAL MEASUREMENTS
  V.  THE MEMBERS OF THE SOLAR SYSTEM
  VI.  THE SYSTEM OF THE STARS
  INDEX



{9}

THE SCIENCE OF THE STARS



CHAPTER I

ASTRONOMY BEFORE HISTORY

The plan of the present series requires each volume to be complete in
about eighty small pages.  But no adequate account of the achievements
of astronomy can possibly be given within limits so narrow, for so
small a space would not suffice for a mere catalogue of the results
which have been obtained; and in most cases the result alone would be
almost meaningless unless some explanation were offered of the way in
which it had been reached.  All, therefore, that can be done in a work
of the present size is to take the student to the starting-point of
astronomy, show him the various roads of research which have opened out
from it, and give a brief indication of the character and general
direction of each.

That which distinguishes astronomy from all the other sciences is this:
it deals with objects that we cannot touch.  The heavenly bodies are
beyond our reach; we cannot tamper with them, or subject them to any
form of experiment; we cannot bring them into our laboratories to
analyse or dissect them.  We can only watch them and wait for such
indications as their {10} own movements may supply.  But we are
confined to this earth of ours, and they are so remote; we are so
short-lived, and they are so long-enduring; that the difficulty of
finding out much about them might well seem insuperable.

Yet these difficulties have been so far overcome that astronomy is the
most advanced of all the sciences, the one in which our knowledge is
the most definite and certain.  All science rests on sight and thought,
on ordered observation and reasoned deduction; but both sight and
thought were earlier trained to the service of astronomy than of the
other physical sciences.

It is here that the highest value of astronomy lies; in the discipline
that it has afforded to man's powers of observation and reflection; and
the real triumphs which it has achieved are not the bringing to light
of the beauties or the sensational dimensions and distances of the
heavenly bodies, but the vanquishing of difficulties which might well
have seemed superhuman.  The true spirit of the science can be far
better exemplified by the presentation of some of these difficulties,
and of the methods by which they have been overcome, than by many
volumes of picturesque description or of eloquent rhapsody.

There was a time when men knew nothing of astronomy; like every other
science it began from zero.  But it is not possible to suppose that
such a state of things lasted long, we know that there was a time when
men had noticed that there were two great lights in the sky--a greater
light that shone by day, a lesser light that shone by night--and there
were the stars also.  And this, the earliest observation of primitive
astronomy, is preserved for us, expressed in the simplest possible
language, in the first chapter of the first book {11} of the sacred
writings handed down to us by the Hebrews.

This observation, that there are bodies above us giving light, and that
they are not all equally bright, is so simple, so inevitable, that men
must have made it as soon as they possessed any mental power at all.
But, once made, a number of questions must have intruded themselves:
"What are these lights?  Where are they?  How far are they off?"

Many different answers were early given to these questions.  Some were
foolish; some, though intelligent, were mistaken; some, though wrong,
led eventually to the discovery of the truth.  Many myths, many
legends, some full of beauty and interest, were invented.  But in so
small a book as this it is only possible to glance at those lines of
thought which eventually led to the true solution.

As the greater light, the lesser light, and the stars were carefully
watched, it was seen not only that they shone, but that they appeared
to move; slowly, steadily, and without ceasing.  The stars all moved
together like a column of soldiers on the march, not altering their
positions relative to each other.  The lesser light, the Moon, moved
with the stars, and yet at the same time among them.  The greater
light, the Sun, was not seen with the stars; the brightness of his
presence made the day, his absence brought the night, and it was only
during his absence that the stars were seen; they faded out of the sky
before he came up in the morning, and did not reappear again until
after he passed out of sight in the evening.  But there came a time
when it was realised that there were stars shining in the sky all day
long as well as at night, and this discovery was one of the greatest
and most important ever made, {12} because it was the earliest
discovery of something quite unseen.  Men laid hold of this fact, not
from the direct and immediate evidence of their senses, but from
reflection and reasoning.  We do not know who made this discovery, nor
how long ago it was made, but from that time onward the eyes with which
men looked upon nature were not only the eyes of the body, but also the
eyes of the mind.

It followed from this that the Sun, like the Moon, not only moved with
the general host of the stars, but also among them.  If an observer
looks out from any fixed station and watches the rising of some bright
star, night after night, he will notice that it always appears to rise
in the same place; so too with its setting.  From any given observing
station the direction in which any particular star is observed to rise
or set is invariable.

Not so with the Sun.  We are accustomed to say that the Sun rises in
the east and sets in the west.  But the direction in which the Sun
rises in midwinter lies far to the south of the east point; the
direction in which he rises in midsummer lies as far to the north.  The
Sun is therefore not only moving with the stars, but among them.  This
gradual change in the position of the Sun in the sky was noticed in
many ancient nations at an early time.  It is referred to in Job
xxxviii. 12: "Hast thou commanded the morning since thy days; and
caused the dayspring to know his place?"

And the apparent path of the Sun on one day is always parallel to its
path on the days preceding and following.  When, therefore, the Sun
rises far to the south of east, he sets correspondingly far to the
south of west, and at noon he is low down in the south.  His course
during the day is a short one, and the daylight {13} is much shorter
than the night, and the Sun at noon, being low down in the sky, has not
his full power.  The cold and darkness of winter, therefore, follows
directly upon this position of the Sun.  These conditions are reversed
when the Sun rises in the north-east.  The night is short, the daylight
prolonged, and the Sun, being high in the heavens at noon, his heat is
felt to the full.

Thus the movements of the Sun are directly connected with the changes
of season upon the Earth.  But the stars also are connected with those
seasons; for if we look out immediately after it has become dark after
sunset, we shall notice that the stars seen in the night of winter are
only in part those seen in the nights of summer.

In the northern part of the sky there are a number of stars which are
always visible whenever we look out, no matter at what time of the
night nor what part of the year.  If we watch throughout the whole
night, we see that the whole heavens appear to be slowly
turning--turning, as if all were in a single piece--and the pivot about
which it is turning is high up in the northern sky.  The stars,
therefore, are divided into two classes.  Those near this invisible
pivot--the "Pole" of the Heavens, as we term it--move round it in
complete circles; they never pass out of sight, but even when lowest
they clear the horizon.  The other stars move round the same pivot in
curved paths, which are evidently parts of circles, but circles of
which we do not see the whole.  These stars rise on the eastern side of
the heavens and set on the western, and for a greater or less space of
time are lost to sight below the horizon.  And some of these stars are
visible at one time of the year, others at another; some being seen
during the {14} whole of the long nights of winter, others throughout
the short nights of summer.  This distinction again, and its connection
with the change of the seasons on the earth, was observed many ages
ago.  It is alluded to in Job xxxviii. 32: "Canst thou lead forth the
Signs of the Zodiac in their season, or canst thou guide the Bear with
her train?" (R.V., Margin).  The Signs of the Zodiac are taken as
representing the stars which rise and set, and therefore have each
their season for being "led forth," while the northern stars, which are
always visible, appearing to be "guided" in their continual movement
round the Pole of the sky in perfect circles, are represented by "the
Bear with her train."

The changes in position of the Sun, the greater light, must have
attracted attention in the very earliest ages, because these changes
are so closely connected with the changes of the seasons upon the
Earth, which affect men directly.  The Moon, the lesser light, goes
through changes of position like the Sun, but these are not of the same
direct consequence to men, and probably much less notice was taken of
them.  But there were changes of the Moon which men could not help
noticing--her changes of shape and brightness.  One evening she may be
seen soon after the Sun has set, as a thin arch of light, low down in
the sunset sky.  On the following evenings she is seen higher and
higher in the sky, and the bow of light increases, until by the
fourteenth day it is a perfect round.  Then the Moon begins to diminish
and to disappear, until, on the twenty-ninth or thirtieth day after the
first observation, she is again seen in the west after sunset as a
narrow crescent.  This succession of changes gave men an important
measure of time, and, in an age when artificial means of light were
difficult to procure, moonlight was of the greatest {15} value, and the
return of the moonlit portion of the month was eagerly looked for.

These early astronomical observations were simple and obvious, and of
great practical value.  The day, month, and year were convenient
measures of time, and the power of determining, from the observation of
the Sun and of the stars, how far the year had progressed was most
important to farmers, as an indication when they should plough and sow
their land.  Such observations had probably been made independently by
many men and in many nations, but in one place a greater advance had
been made.  The Sun and Moon are both unmistakable, but one star is
very like another, and, for the most part, individual stars can only be
recognised by their positions relative to others.  The stars were
therefore grouped together into +Constellations+ and associated with
certain fancied designs, and twelve of these designs were arranged in a
belt round the sky to mark the apparent path of the Sun in the course
of the year, these twelve being known as the "+Signs of the
Zodiac+"--the Ram, Bull, Twins, Crab, Lion, Virgin, Balance, Scorpion,
Archer, Goat, Water-pourer, and Fishes.  In the rest of the sky some
thirty to thirty-six other groups, or constellations, were formed, the
Bear being the largest and brightest of the constellations of the
northern heavens.

But these ancient constellations do not cover the entire heavens; a
large area in the south is untouched by them.  And this fact affords an
indication both of the time when and the place where the old stellar
groups were designed, for the region left untouched was the region
below the horizon of 40° North latitude, about 4600 years ago.  It is
probable, therefore, that the ancient astronomers who carried out this
great work {16} lived about 2700 B.C., and in North latitude 37° or
38°.  The indication is only rough, but the amount of uncertainty is
not very large; the constellations must be at least 4000 years old,
they cannot be more than 5000.

All this was done by prehistoric astronomers; though no record of the
actual carrying out of the work and no names of the men who did it have
come down to us.  But it is clear from the fact that the Signs of the
Zodiac are arranged so as to mark out the annual path of the Sun, and
that they are twelve in number--there being twelve months in the
year--that those who designed the constellations already knew that
there are stars shining near the Sun in full daylight, and that they
had worked out some means for determining what stars the Sun is near at
any given time.

Another great discovery of which the date and the maker are equally
unknown is referred to in only one of the ancient records available to
us.  It was seen that all along the eastern horizon, from north to
south, stars rise, and all along the western horizon, from north to
south, stars set.  That is what was seen; it was the fact observed.
There is no hindrance anywhere to the movement of the stars--they have
a free passage under the Earth; the Earth is unsupported in space.
That is what was _thought_; it was the inference drawn.  Or, as it is
written in Job xxvi. 7, "He (God) stretcheth out the north over empty
space, and hangeth the earth upon nothing."

The Earth therefore floats unsupported in the centre of an immense
star-spangled sphere.  And what is the shape of the Earth?  The natural
and correct inference is that it is spherical, and we find in some of
the early Greek writers the arguments which establish this inference as
clearly set forth as they would be to-day.  {17} The same inference
followed, moreover, from the observation of a simple fact, namely, that
the stars as observed from any particular place all make the same angle
with the horizon as they rise in the east, and all set at the same
angle with it in the west; but if we go northward, we find that angle
steadily decreasing; if we go southward, we find it increasing.  But if
the Earth is round like a globe, then it must have a definite size, and
that size can be measured.  The discoveries noted above were made by
men whose names have been lost, but the name of the first person whom
we know to have measured the size of the Earth was ERATOSTHENES.  He
found that the Sun was directly overhead at noon at midsummer at Syene
(the modern Assouan), in Egypt, but was 7° south of the "zenith"--the
point overhead--at Alexandria, and from this he computed the Earth to
be 250,000 stadia (a stadium = 606 feet) in circumference.

Another consequence of the careful watch upon the stars was the
discovery that five of them were planets; "wandering" stars; they did
not move all in one piece with the rest of the celestial host.  In this
they resemble the Sun and Moon, and they further resemble the Moon in
that, though too small for any change of shape to be detected, they
change in brightness from time to time.  But their movements are more
complicated than those of the other heavenly bodies.  The Sun moves a
little slower than the stars, and so seems to travel amongst them from
west to east; the Moon moves much slower than the stars, so her motion
from west to east is more pronounced than that of the Sun.  But the
five planets sometimes move slower than the stars, sometimes quicker,
and sometimes at the same rate.  Two of the five, which we now know as
Mercury {18} and Venus, never move far from the Sun, sometimes being
seen in the east before he rises in the morning, and sometimes in the
west after he has set in the evening.  Mercury is the closer to the
Sun, and moves more quickly; Venus goes through much the greater
changes of brightness.  Jupiter and Saturn move nearly at the same
average rate as the stars, Saturn taking about thirteen days more than
a year to come again to the point of the sky opposite to the Sun, and
Jupiter about thirty-four days.  Mars, the fifth planet, takes two
years and fifty days to accomplish the same journey.

These planetary movements were not, like those of the Sun and Moon and
stars, of great and obvious consequence to men.  It was important to
men to know when they would have moonlight nights, to know when the
successive seasons of the year would return.  But it was no help to men
to know when Venus was at her brightest more than when she was
invisible.  She gave them no useful light, and she and her companion
planets returned at no definite seasons.  Nevertheless, men began to
make ordered observations of the planets--observations that required
much more patience and perseverance than those of the other celestial
lights.  And they set themselves with the greatest ingenuity to unravel
the secret of their complicated and seemingly capricious movements.

This was a yet higher development than anything that had gone before,
for men were devoting time, trouble, and patient thought, for long
series of years, to an inquiry which did not promise to bring them any
profit or advantage.  Yet the profit which it actually did bring was of
the highest order.  It developed men's mental powers; it led to the
devising of {19} instruments of precision for the observations; it led
to the foundation of mathematics, and thus lay at the root of all our
modern mechanical progress.  It brought out, in a higher degree,
ordered observation and ordered thought.



{20}

CHAPTER II

ASTRONOMY BEFORE THE TELESCOPE

There was thus a real science of astronomy before we have any history
of it.  Some important discoveries had been made, and the first step
had been taken towards cataloguing the fixed stars.  It was certainly
known to some of the students of the heavens, though perhaps only to a
few, that the Earth was a sphere, freely suspended in space, and
surrounded on all sides by the starry heavens, amongst which moved the
Sun, Moon, and the five planets.  The general character of the Sun's
movement was also known; namely, that he not only moved day by day from
east to west, as the stars do, but also had a second motion inclined at
an angle to the first, and in the opposite direction, which he
accomplished in the course of a year.

To this sum of knowledge, no doubt, several nations had contributed.
We do not know to what race we owe the constellations, but there are
evidences of an elementary acquaintance with astronomy on the part of
the Chinese, the Babylonians, the Egyptians, and the Jews.  But in the
second stage of the development of the science the entire credit for
the progress made belongs to the Greeks.

The Greeks, as a race, appear to have been very little apt at
originating ideas, but they possessed, beyond all other races, the
power of developing and perfecting crude ideas which they had obtained
from other sources, {21} and when once their attention was drawn to the
movements of the heavenly bodies, they devoted themselves with striking
ingenuity and success to devising theories to account for the
appearances presented, to working out methods of computation, and,
last, to devising instruments for observing the places of the
luminaries in which they were interested.

In the brief space available it is only possible to refer to two or
three of the men whose commanding intellects did so much to help on the
development of the science.  EUDOXUS of Knidus, in Asia Minor (408-355
B.C.), was, so far as we know, the first to attempt to represent the
movements of the heavenly bodies by a simple mathematical process.  His
root idea was something like this.  The Earth was in the centre of the
universe, and it was surrounded, at a great distance from us, by a
number of invisible transparent shells, or spheres.  Each of these
spheres rotated with perfect uniformity, though the speed of rotation
differed for different spheres.  One sphere carried the stars, and
rotated from east to west in about 23 h. 56 m.  The Sun was carried by
another sphere, which rotated from west to east in a year, but the
pivots, or poles, of this sphere were carried by a second, rotating
exactly like the sphere of the stars.  This explained how it is that
the ecliptic--that is to say, the apparent path of the Sun amongst the
stars--is inclined 23-½° to the equator of the sky, so that the Sun is
23-½° north of the equator at midsummer and 23-½° south of the equator
at midwinter, for the poles of the sphere peculiar to the Sun were
supposed to be 23-½° from the poles of the sphere peculiar to the
stars.  Then the Moon had three spheres; that which actually carried
the Moon having its poles 5° from the poles of the sphere peculiar to
the {22} Sun.  These poles were carried by a sphere placed like the
sphere of the Sun, but rotating in 27 days; and this, again, had its
poles in the sphere of the stars.  The sphere carrying the Moon
afforded the explanation of the wavy motion of the Moon to and fro
across the ecliptic in the course of a month, for at one time in the
month the Moon is 5° north of the ecliptic, at another time 5° south.
The motions of the planets were more difficult to represent, because
they not only have a general daily motion from east to west, like the
stars, and a general motion from west to east along the ecliptic, like
the Sun and Moon, but from time to time they turn back on their course
in the ecliptic, and "retrograde."  But the introduction of a third and
fourth sphere enabled the motions of most of the planets to be fairly
represented.  There were thus twenty-seven spheres in all--four for
each of the five planets, three for the Moon, three for the Sun
(including one not mentioned in the foregoing summary), and one for the
stars.  These spheres were not, however, supposed to be solid
structures really existing; the theory was simply a means for
representing the observed motions of the heavenly bodies by
computations based upon a series of uniform movements in concentric
circles.

But this assumption that each heavenly body moves in its path at a
uniform rate was soon seen to be contrary to fact.  A reference to the
almanac will show at once that the Sun's movement is not uniform.  Thus
for the year 1910-11 the solstices and equinoxes fell as given on the
next page:

{23}

  _Epoch                           Time                    Interval_

  Winter Solstice   1910 Dec.  22 d. 5 h. 12 m. P.M.   89 d.  0 h. 42 m.
  Spring Equinox    1911 Mar.  21 "  5 "  54 "  P.M.   92 "  19 "  41 "
  Summer Solstice   191l June  22 "  1 "  35 "  P.M.   93 "  14 "  43 "
  Autumn Equinox    1911 Sept. 24 "  4 "  18 "  A.M.   89 "  18 "  36 "
  Winter Solstice   1911 Dec.  22 " 10 "  54 "  P.M.

so that the winter half of the year is shorter than the summer half;
the Sun moves more quickly over the half of its orbit which is south of
the equator than over the half which is north of it.

The motion of the Moon is more irregular still, as we can see by taking
out from the almanac the times of new and full moon:

             _New Moon                 Interval to Full Moon_

  Dec. 1910  1 d. 9 h. 10.7 m. P.M.    14 d. 13 h. 54.4 m.
   "    "   31 "  4 "  21.2 "  P.M.    14 "   6 "   4.8 "
  Jan. 1911 30 "  9 "  44.7 "  A.M.    14 "   0 "  52.8 "
  March "    1 "  0 "  31.1 "  A.M.    13 "  23 "  27.4 "
    "   "   30 "  0 "  37.8 "  P.M.    14 "   1 "  58.8 "
  April "   28 " 10 "  25.0 "  P.M.    14 "   7 "  44.7 "
  May   "   28 "  6 "  24.4 "  A.M.    14 "  15 "  26.3 "
  June  "   26 "  1 "  19.7 "  P.M.    14 "  23 "  33.7 "
  July  "   25 "  8 "  12.0 "  P.M.    15 "   6 "  42.7 "
  Aug.  "   24 "  4 "  14.3 "  A.M.    15 "  11 "  42.4 "
  Sept. "   22 "  2 "  37.4 "  P.M.    15 "  13 "  33.7 "
  Oct.  "   22 "  4 "   9.3 "  A.M.    15 "  11 "  38.8 "
  Nov.  "   20 "  8 "  49.4 "  P.M.    15 "   6 "   2.5 "
  Dec.  "   20 "  3 "  40.3 "  P.M.    14 "  21 "  49.4 "

{24}

           _Full Moon                  Interval to New Moon_

  Dec. 1910 16 d  11 h.  5.1 m. A.M.   15 d.  5 h. 16.1 m.
  Jan. 1911 14 "  10 "  26.0 "  P.M.   15 "  11 "  18.7 "
  Feb.  "   13 "  10 "  37.5 "  A.M.   15 "  13 "  53.6 "
  March "   14 "  11 "  58.5 "  P.M.   15 "  12 "  39.3 "
  April "   13 "   2 "  36.6 "  P.M.   15 "   7 "  48.4 "
  May   "   13 "   6 "   9.7 "  A.M.   15 "   0 "  14.7 "
  June  "   11 "   9 "  50.7 "  P.M.   14 "  15 "  29.0 "
  July  "   11 "   0 "  53.4 "  P.M.   14 "   7 "  18.6 "
  Aug.  "   10 "   2 "  54.7 "  A.M.   14 "   1 "  19.6 "
  Sept. "    8 "   3 "  56.7 "  P.M.   13 "  22 "  40.7 "
  Oct.  "    8 "   4 "  11.1 "  A.M.   13 "  23 "  58.2 "
  Nov.  "    6 "   3 "  48.1 "  P.M.   14 "   5 "   1.3 "
  Dec.  "    6 "   2 "  51.9 "  A.M.   14 "  12 "  48.4 "
  Jan. 1912  4 "   1 "  99.7 "  P.M.   14 "  21 "  40.3 "


The astronomer who dealt with this difficulty was HIPPARCHUS (about
190-120 B.C.), who was born at Nicæa, in Bithynia, but made most of his
astronomical observations in Rhodes.  He attempted to explain these
irregularities in the motions of the Sun and Moon by supposing that
though they really moved uniformly in their orbits, yet the centre of
their orbits was not the centre of the Earth, but was situated a little
distance from it.  This point was called "+the excentric+," and the
line from the excentric to the Earth was called "+the line of apsides+."

But when he tried to deal with the movements of the planets, he found
that there were not enough good observations available for him to build
up any satisfactory theory.  He therefore devoted himself to the work
of making systematic determinations of the places of the planets that
he might put his successors in a better position to deal with the
problem than he was.  His great successor was CLAUDIUS PTOLEMY of {25}
Alexandria, who carried the work of astronomical observation from about
A.D. 127 to 150.  He was, however, much greater as a mathematician than
as an observer, and he worked out a very elaborate scheme, by which he
was able to represent the motions of the planets with considerable
accuracy.  The system was an extremely complex one, but its principle
may be represented as follows: If we suppose that a planet is moving
round the Earth in a circle at a uniform rate, and we tried to compute
the place of the planet on this assumption for regular intervals of
time, we should find that the planet gradually got further and further
away from the predicted place.  Then after a certain time the error
would reach a maximum, and begin to diminish, until the error vanished
and the planet was in the predicted place at the proper time.  The
error would then begin to fall in the opposite direction, and would
increase as before to a maximum, subsequently diminishing again to
zero.  This state of things might be met by supposing that the planet
was not itself carried by the circle round the earth, but by an
+epicycle+--_i.e._ a circle travelling upon the first circle--and by
judiciously choosing the size of the epicycle and the time of
revolution the bulk of the errors in the planet's place might be
represented.  But still there would be smaller errors going through
their own period, and these, again, would have to be met by imagining
that the first epicycle carried a second, and it might be that the
second carried a third, and so on.

The Ptolemaic system was more complicated than this brief summary would
suggest, but it is not possible here to do more than indicate the
general principles upon which it was founded, and the numerous other
systems or modifications of them produced in the {26} five centuries
from Eudoxus to Ptolemy must be left unnoticed.  The point to be borne
in mind is that one fundamental assumption underlay them all, an
assumption fundamental to all science--the assumption that like causes
must always produce like effects.  It was apparent to the ancient
astronomers that the stars--that is to say, the great majority of the
heavenly bodies--do move round the Earth in circles, and with a perfect
uniformity of motion, and it seemed inevitable that, if one body moved
round another, it should thus move.  For if the revolving body came
nearer to the centre at one time and receded at another, if it moved
faster at one time and slower at another, then, the cause remaining the
same, the effect seemed to be different.  Any complexity introduced by
superposing one epicycle upon another seemed preferable to abandoning
this great fundamental principle of the perfect uniformity of the
actings of Nature.

For more than 1300 years the Ptolemaic system remained without serious
challenge, and the next great name that it is necessary to notice is
that of COPERNICUS (1473-1543).  Copernicus was a canon of Frauenburg,
and led the quiet, retired life of a student.  The great work which
made him immortal, _De Revolutionibus_, was the result of many years'
meditation and work, and was not printed until he was on his deathbed.
In this work Copernicus showed that he was one of those great thinkers
who are able to look beyond the mere appearance of things and to grasp
the reality of the unseen.  Copernicus realised that the appearance
would be just the same whether the whole starry vault rotated every
twenty-four hours round an immovable Earth from east to west or the
Earth rotated from west to east in the midst of the starry sphere; and,
as the {27} stars are at an immeasurable distance, the latter
conception was much the simpler.  Extending the idea of the Earth's
motion further, the supposition that, instead of the Sun revolving
round a fixed Earth in a year, the Earth revolved round a fixed Sun,
made at once an immense simplification in the planetary motions.  The
reason became obvious why Mercury and Venus were seen first on one side
of the Sun and then on the other, and why neither of them could move
very far from the Sun; their orbits were within the orbit of the Earth.
The stationary points and retrogressions of the planets were also
explained; for, as the Earth was a planet, and as the planets moved in
orbits of different sizes, the outer planets taking a longer time to
complete a revolution than the inner, it followed, of necessity, that
the Earth in her motion would from time to time be passed by the two
inner planets, and would overtake the three outer.  The chief of the
Ptolemaic epicycles were done away with, and all the planets moved
continuously in the same direction round the Sun.  But no planet's
motion could be represented by uniform motion in a single circle, and
Copernicus had still to make use of systems of epicycles to account for
the deviations from regularity in the planetary motions round the Sun.
The Earth having been abandoned as the centre of the universe, a
further sacrifice had to be made: the principle of uniform motion in a
circle, which had seemed so necessary and inevitable, had also to be
given up.

For the time came when the instruments for measuring the positions of
the stars and planets had been much improved, largely due to TYCHO
BRAHE (1546-1601), a Dane of noble birth, who was the keenest and most
careful observer that astronomy had yet produced.  {28} His
observations enabled his friend and pupil, JOHANN KEPLER, (1571-1630),
to subject the planetary movements to a far more searching examination
than had yet been attempted, and he discovered that the Sun is in the
plane of the orbit of each of the planets, and also in its +line of
apsides+--that is to say, the line joining the two points of the orbit
which are respectively nearest and furthest from the Sun.  Copernicus
had not been aware of either of these two relations, but their
discovery greatly strengthened the Copernican theory.

Then for many years Kepler tried one expedient after another in order
to find a combination of circular motions which would satisfy the
problem before him, until at length he was led to discard the circle
and try a different curve--the oval or ellipse.  Now the property of a
circle is that every point of it is situated at the same distance from
the centre, but in an ellipse there are two points within it, the
"foci," and the sum of the distances of any point on the circumference
from these two foci is constant.  If the two foci are at a great
distance from each other, then the ellipse is very long and narrow; if
the foci are close together, the ellipse differs very little from a
circle; and if we imagine that the two foci actually coincide, the
ellipse becomes a circle.  When Kepler tried motion in an ellipse
instead of motion in a circle, he found that it represented correctly
the motions of all the planets without any need for epicycles, and that
in each case the Sun occupied one of the foci.  And though the planet
did not move at a uniform speed in the ellipse, yet its motion was
governed by a uniform law, for the straight line joining the planet to
the Sun, the "+radius vector+," passed over equal areas of space in
equal periods of time.

{29}

These two discoveries are known as Kepler's First and Second Laws.  His
Third Law connects all the planets together.  It was known that the
outer planets not only take longer to revolve round the Sun than the
inner, but that their actual motion in space is slower, and Kepler
found that this actual speed of motion is inversely as the square root
of its distance from the Sun; or, if the square of the speed of a
planet be multiplied by its distance from the Sun, we get the same
result in each case.  This is usually expressed by saying that the cube
of the distance is proportional to the square of the time of
revolution.  Thus the varying rate of motion of each planet in its
orbit is not only subject to a single law, but the very different
speeds of the different planets are also all subject to a law that is
the same for all.

Thus the whole of the complicated machinery of Ptolemy had been reduced
to three simple laws, which at the same time represented the facts of
observation much better than any possible development of the Ptolemaic
mechanism.  On his discovery of his third law Kepler had written: "The
book is written to be read either now or by posterity--I care not
which; it may well wait a century for a reader, as God has waited 6000
years for an observer."  Twelve years after his death, on Christmas Day
1642 (old style), near Grantham, in Lincolnshire, the predestined
"reader" was born.  The inner meaning of Kepler's three laws was
brought to light by ISAAC NEWTON.



{30}

CHAPTER III

THE LAW OF GRAVITATION

The fundamental thought which, recognised or not, had lain at the root
of the Ptolemaic system, as indeed it lies at the root of all science,
was that "like causes must always produce like effects."  Upon this
principle there seemed to the ancient astronomers no escape from the
inference that each planet must move at a uniform speed in a circle
round its centre of motion.  For, if there be any force tending to
alter the distance of the planet from that centre, it seemed inevitable
that sooner or later it should either reach that centre or be
indefinitely removed from it.  If there be no such force, then the
planet's distance from that centre must remain invariable, and if it
move at all, it must move in a circle; move uniformly, because there is
no force either to hasten or retard it.  Uniform motion in a circle
seemed a necessity of nature.

But all this system, logical and inevitable as it had once seemed, had
gone down before the assault of observed facts.  The great example of
uniform circular motion had been the daily revolution of the star
sphere; but this was now seen to be only apparent, the result of the
rotation of the Earth.  The planets revolved round the Sun, but the Sun
was not in the centre of their motion; they moved, not in circles, but
in ellipses; not at a uniform speed, but at a speed which diminished
with the increase of their distance from {31} the Sun.  There was need,
therefore, for an entire revision of the principles upon which motion
was supposed to take place.

The mistake of the ancients had been that they supposed that continued
motion demanded fresh applications of force.  They noticed that a ball,
set rolling, sooner or later came to a stop; that a pendulum, set
swinging, might swing for a good time, but eventually came to rest;
and, as the forces that were checking the motion--that is to say, the
friction exercised by the ground, the atmosphere, and the like--did not
obtrude themselves, they were overlooked.

Newton brought out into clear statement the true conditions of motion.
A body once moving, if acted upon by no force whatsoever, must continue
to move forward in a straight line at exactly the same speed, and that
for ever.  It does not require any maintaining force to keep it going.
If any change in its speed or in its direction takes place, that change
must be due to the introduction of some further force.

This principle, that, if no force acts on a body in motion, it will
continue to move uniformly in a straight line, is Newton's First Law of
Motion.  His Second lays it down that, if force acts on a body, it
produces a change of motion proportionate to the force applied, and in
the same direction.  And the Third Law states that when one body exerts
force upon another, that second body reacts with equal force upon the
first.  The problem of the motions of the planets was, therefore, not
what kept them moving, but what made them deviate from motion in a
straight line, and deviate by different amounts.

It was quite clear, from the work of Kepler, that the force deflecting
the planets from uniform motion in a {32} straight line lay in the Sun.
The facts that the Sun lay in the plane of the orbits of all the
planets, that the Sun was in one of the foci of each of the planetary
ellipses, that the straight line joining the Sun and planet moved for
each planet over equal areas in equal periods of time, established this
fact clearly.  But the amount of deflection was very different for
different planets.  Thus the orbit of Mercury is much smaller than that
of the Earth, and is travelled over in a much shorter time, so that the
distance by which Mercury is deflected in a course of an hour from
movement in a straight line is much greater than that by which the
Earth is deflected in the same time, Mercury falling towards the Sun by
about 159 miles, whilst the fall of the Earth is only about 23.9 miles.
The force drawing Mercury towards the Sun is therefore 6.66 times that
drawing the Earth, but 6.66 is the square of 2.58, and the Earth is
2.58 times as far from the Sun as Mercury.  Similarly, the fall in an
hour of Jupiter towards the Sun is about 0.88 miles, so that the force
drawing the Earth is 27 times that drawing Jupiter towards the Sun.
But 27 is the square of 5.2, and Jupiter is 5.2 times as far from the
Sun as the Earth.  Similarly with the other planets.  The force,
therefore, which deflects the planets from motion in a straight line,
and compels them to move round the Sun, is one which varies inversely
as the square of the distance.

But the Sun is not the only attracting body of which we know.  The old
Ptolemaic system was correct to a small extent; the Earth is the centre
of motion for the Moon, which revolves round it at a mean distance of
238,800 miles, and in a period of 27 d. 7 h. 43 m.  Hence the
circumference of her orbit is 1,500,450 miles, and the length of the
straight line which she would travel {33} in one second of time, if not
deflected by the Earth, is 2828 feet.  In this distance the deviation
of a circle from a straight line is one inch divided by 18.66.  But we
know from experiment that a stone let fall from a height of 193 inches
above the Earth's surface will reach the ground in exactly one second
of time.  The force drawing the stone to the Earth, therefore, is 193 x
18.66; _i.e._ 3601 times as great as that drawing the Moon.  But the
stone is only 1/330 of a mile from the Earth's surface, while the Moon
is 238,800 miles away--more than 78 million times as far.  The force,
therefore, would seem not to be diminished in the proportion that the
distance is increased--much less in the proportion of its square.

But Newton proved that a sphere of uniform density, or made up of any
number of concentric shells of uniform density, attracted a body
outside itself, just as if its entire mass was concentrated at its
centre.  The distance of the stone from the Earth must therefore be
measured, not from the Earth's surface, but from its centre; in other
words, we must consider the stone as being distant from the Earth, not
some 16 feet, but 3963 miles.  This is very nearly one-sixtieth of the
Moon's distance, and the square of 60 is 3600.  The Earth's pull upon
the Moon, therefore, is almost exactly in the inverse square of the
distance as compared with its pull on the stone.

Kepler's book had found its "reader."  His three laws were but three
particular aspects of Newton's great discovery that the planets moved
under the influence of a force, lodged in the Sun, which varied
inversely as the square of their distances from it.  But Newton's work
went far beyond this, for he showed that the same law governed the
motion of the Moon round the {34} Earth and the motions of the
satellites revolving round the different planets, and also governed the
fall of bodies upon the Earth itself.  It was universal throughout the
solar system.  The law, therefore, is stated as of universal
application.  "Every particle of matter in the universe attracts every
other particle with a force varying inversely as the square of the
distance between them, and directly as the product of the masses of the
two particles."  And Newton further proved that if a body, projected in
free space and moving with any velocity, became subject to a central
force acting, like gravitation, inversely as the square of the
distance, it must revolve in an ellipse, or in a closely allied curve.

These curves are what are known as the "+conic sections+"--that is,
they are the curves found when a cone is cut across in different
directions.  Their relation to each other may be illustrated thus.  If
we have a very powerful light emerging from a minute hole, then, if we
place a screen in the path of the beam of light, and exactly at right
angles to its axis, the light falling on the screen will fill an exact
circle.  If we turn the screen so as to be inclined to the axis of the
beam, the circle will lengthen out in one direction, and will become an
ellipse.  If we turn the screen still further, the ellipse will
lengthen and lengthen, until at last, when the screen has become
parallel to one of the edges of the beam of light, the ellipse will
only have one end; the other will be lost.  For it is clear that that
edge of the beam of light which is parallel to the screen can never
meet it.  The curve now shown on the screen is called a +parabola+, and
if the screen is turned further yet, the boundaries of the light
falling upon it become divergent, and we have a fourth curve, the
+hyperbola+.  Bodies moving under the influence of {35} gravitation can
move in any of these curves, but only the circle and ellipse are closed
orbits.  A particle moving in a parabola or hyperbola can only make one
approach to its attracting body; after such approach it continually
recedes from it.  As the circle and parabola are only the two extreme
forms of an ellipse, the two foci being at the same point for the
circle and at an infinite distance apart for the parabola, we may
regard all orbits under gravitation as being ellipses of one form or
another.

From his great demonstration of the law of gravitation, Newton went on
to apply it in many directions.  He showed that the Earth could not be
truly spherical in shape, but that there must be a flattening of its
poles.  He showed also that the Moon, which is exposed to the
attractions both of the Earth and of the Sun, and, to a sensible
extent, of some of the other planets, must show irregularities in her
motion, which at that time had not been noticed.  The Moon's orbit is
inclined to that of the Earth, cutting its plane in two opposite
points, called the "+nodes+."  It had long been observed that the
position of the nodes travelled round the ecliptic once in about
nineteen years.  Newton was able to show that this was a consequence of
the Sun's attraction upon the Moon.  And he further made a particular
application of the principle thus brought out, for, the Earth not being
a true sphere, but flattened at the poles and bulging at the equator,
the equatorial belt might be regarded as a compact ring of satellites
revolving round the Earth's equator.  This, therefore, would tend to
retrograde precisely as the nodes of a single satellite would, so that
the axis of the equatorial belt of the Earth--in other words, the axis
of the Earth--must revolve round the pole of the ecliptic.  {36}
Consequently the pole of the heavens appears to move amongst the stars,
and the point where the celestial equator crosses the equator
necessarily moves with it.  This is what we know as the "+Precession of
the Equinoxes+," and it is from our knowledge of the fact and the
amount of precession that we are able to determine roughly the date
when the first great work of astronomical observation was accomplished,
namely, the grouping of the stars into constellations by the
astronomers of the prehistoric age.

The publication of Newton's great work, the _Principia_ (_The
Mathematical Principles of Natural Philosophy_), in which he developed
the Laws of Motion, the significance of Kepler's Three Planetary Laws,
and the Law of Universal Gravitation, took place in 1687, and was due
to his friend EDMUND HALLEY, to whom he had confided many of his
results.  That he was the means of securing the publication of the
_Principia_ is Halley's highest claim to the gratitude of posterity,
but his own work in the field which Newton had opened was of great
importance.  Newton had treated +comets+ as moving in parabolic orbits,
and Halley, collecting all the observations of comets that were
available to him, worked out the particulars of their orbits on this
assumption, and found that the elements of three were very closely
similar, and that the interval between their appearances was nearly the
same, the comets having been seen in 1531, 1607, and 1682.  On further
consulting old records he found that comets had been observed in 1456,
1378, and 1301.  He concluded that these were different appearances of
the same object, and predicted that it would be seen again in 1758, or,
according to a later and more careful computation, in 1759.  As the
time for its return drew near, CLAIRAUT {37} computed with the utmost
care the retardation which would be caused to the comet by the
attractions of Jupiter and Saturn.  The comet made its predicted
nearest approach to the Sun on March 13, 1759, just one month earlier
than Clairaut had computed.  But in its next return, in 1835, the
computations effected by PONTÉCOULANT were only two days in error, so
carefully had the comet been followed during its unseen journey to the
confines of the solar system and back again, during a period of
seventy-five years.  Pontécoulant's exploit was outdone at the next
return by Drs. COWELL and CROMMELIN, of Greenwich Observatory, who not
only computed the time of its perihelion passage--that is to say, its
nearest approach to the Sun--for April 16, 1910, but followed the comet
back in its wanderings during all its returns to the year 240 B.C.
Halley's Comet, therefore, was the first comet that was known to travel
in a closed orbit and to return to the neighbourhood of the Sun.  Not a
few small or telescopic comets are now known to be "periodic," but
Halley's is the only one which has made a figure to the naked eye.
Notices of it occur not a few times in history; it was the comet "like
a flaming sword" which Josephus described as having been seen over
Jerusalem not very long before the destruction by Titus.  It was also
the comet seen in the spring of the year when William the Conqueror
invaded England, and was skilfully used by that leader as an omen of
his coming victory.

The law of gravitation had therefore enabled men to recognise in
Halley's Comet an addition to the number of the primary bodies in the
solar system--the first addition that had been made since prehistoric
times.  On March 13, 1781, Sir WILLIAM HERSCHEL {38} detected a new
object, which he at first supposed to be a comet, but afterwards
recognised as a planet far beyond the orbit of Saturn.  This planet, to
which the name of Uranus was finally given, had a mean distance from
the Sun nineteen times that of the Earth, and a diameter four times as
great.  This was a second addition to the solar system, but it was a
discovery by sight, not by deduction.

The first day of the nineteenth century, January 1, 1801, was
signalised by the discovery of a small planet by PIAZZI.  The new
object was lost for a time, but it was redetected on December 31 of the
same year.  This planet lay between the orbits of Mars and Jupiter--a
region in which many hundreds of other small bodies have since been
found.  The first of these "+minor planets+" was called Ceres; the next
three to be discovered are known as Pallas, Juno, and Vesta.  Beside
these four, two others are of special interest: one, Eros, which comes
nearer the Sun than the orbit of Mars--indeed at some oppositions it
approaches the Earth within 13,000,000 miles, and is therefore, next to
the Moon, our nearest neighbour in space; the other, Achilles, moves at
a distance from the Sun equal to that of Jupiter.

Ceres is much the largest of all the minor planets; indeed is larger
than all the others put together.  Yet the Earth exceeds Ceres 4000
times in volume, and 7000 times in mass, and the entire swarm of minor
planets, all put together, would not equal in total volume one-fiftieth
part of the Moon.

The search for these small bodies rendered it necessary that much
fuller and more accurate maps of the stars should be made than had
hitherto been attempted, and this had an important bearing on the next
great event in the development of gravitational astronomy.

{39}

The movements of Uranus soon gave rise to difficulties.  It was found
impossible, satisfactorily, to reconcile the earlier and later
observations, and in the tables of Uranus, published by BOUVARD in
1821, the earlier observations were rejected.  But the discrepancies
between the observed and calculated places for the planet soon began to
reappear and quickly increase, and the suggestion was made that these
discrepancies were due to an attraction exercised by some planet as yet
unknown.  Thus Mrs. Somerville in a little book on the connection of
the physical sciences, published in 1836, wrote, "Possibly it (that is,
Uranus) may be subject to disturbances from some unseen planet
revolving about the Sun beyond the present boundaries of our system.
If, after the lapse of years, the tables formed from a combination of
numerous observations should still be inadequate to represent the
motions of Uranus, the discrepancies may reveal the existence, nay,
even the mass and orbit of a body placed for ever beyond the sphere of
vision."  In 1843 JOHN C. ADAMS, who had just graduated as Senior
Wrangler at Cambridge, proceeded to attack the problem of determining
the position, orbit, and mass of the unknown body by which on this
assumption Uranus was disturbed, from the irregularities evident in the
motion of that planet.  The problem was one of extraordinary intricacy,
but by September 1845 Adams had obtained a first solution, which, he
submitted to AIRY, the Astronomer Royal.  As, however, he neglected to
reply to some inquiries made by Airy, no search for the new planet was
instituted in England until the results of a new and independent worker
had been published.  The same problem had been attacked by a well-known
and very gifted French mathematician, U. J. J. LEVERRIER, and {40} in
June 1846 he published his position for the unseen planet, which proved
to be in close accord with that which Adams had furnished to Airy nine
months before.  On this Airy stirred up Challis, the Director of the
Cambridge Observatory, which then possessed the most powerful telescope
in England, to search for the planet, and Challis commenced to make
charts, which included more than 3000 stars, in order to make sure that
the stranger should not escape his net.  Leverrier, on the other hand,
communicated his result to the Berlin Observatory, where they had just
received some of the star charts prepared by Dr. Bremiker in connection
with the search for minor planets.  The Berlin observer, Dr. Galle, had
therefore nothing to do but to compare the stars in the field, upon
which he turned his telescope, with those shown on the chart; a star
not in the chart would probably be the desired stranger.  He found it,
therefore, on the very first evening, September 23, 1846, within less
than four diameters of the Moon of the predicted place.  The same
object had been observed by Challis at Cambridge on August 4 and 12,
but he was deferring the reduction of his observations until he had
completed his scrutiny of the zone, and hence had not recognised it as
different from an ordinary star.

This discovery of the planet now known as Neptune, which had been
disturbing the movement of Uranus, has rightly been regarded as the
most brilliant triumph of gravitational astronomy.  It was the
legitimate crown of that long intellectual struggle which had commenced
more than 2000 years earlier, when the first Greek astronomers set
themselves to unravel the apparently aimless wanderings of the planets
in the assured faith that they would find them obedient unto law.  {41}
But of what use was all this effort?  What is the good of astronomy?
The question is often asked, but it is the question of ignorance.  The
use of astronomy is the development which it has given to the
intellectual powers of man.  Directly the problem of the planetary
motions was first attempted, it became necessary to initiate
mathematical processes in order to deal with it, and the necessity for
the continued development of mathematics has been felt in the same
connection right down to the present day.  When the Greek astronomers
first began their inquiries into the planetary movements they hoped for
no material gain, and they received none.  They laboured; we have
entered into their labours.  But the whole of our vast advances in
mechanical and engineering science--advances which more than anything
else differentiate this our present age from all those which have
preceded it--are built upon our command of mathematics and our
knowledge of the laws of motion--a command and a knowledge which we owe
directly to their persevering attempts to advance the science of
astronomy, and to follow after knowledge, not for any material rewards
which she had to offer, but for her own sake.



{42}

CHAPTER IV

ASTRONOMICAL MEASUREMENTS

The old proverb has it that "Science is measurement," and of none of
the sciences is this so true as of the science of astronomy.  Indeed
the measurement of time by observation of the movements of the heavenly
bodies was the beginning of astronomy.  The movement of the Sun gave
the day, which was reckoned to begin either at sunrise or at sunset.
The changes of the Moon gave the month, and in many languages the root
meaning of the word for _Moon_ is "measurer."  The apparent movement of
the Sun amongst the stars gave a yet longer division of time, the year,
which could be determined in a number of different ways, either from
the Sun alone, or from the Sun together with the stars.  A very simple
and ancient form of instrument for measuring this movement of the Sun
was the obelisk, a pillar with a pointed top set up on a level
pavement.  Such obelisks were common in Egypt, and one of the most
celebrated, known as Cleopatra's Needle, now stands on the Thames
Embankment.  As the Sun moved in the sky, the shadow of the pillar
moved on the pavement, and midday, or noon, was marked when the shadow
was shortest.  The length of the shadow at noon varied from day to day;
it was shortest at mid-summer, and longest at midwinter, _i.e._ at the
summer and winter solstices.  Twice in the year the shadow of the
pillar pointed due west at sunrise, and due east at {43} sunset--that
is to say, the shadow at the beginning of the day was in the same
straight line as at its end.  These two days marked the two equinoxes
of spring and autumn.

The obelisk was a simple means of measuring the height and position of
the Sun, but it had its drawbacks.  The length of the shadow and its
direction did not vary by equal amounts in equal times, and if the
pavement upon which the shadow fell was divided by marks corresponding
to equal intervals of time for one day of the year, the marks did not
serve for all other days.

But if for the pillar a triangular wall was substituted--a wall rising
from the pavement at the south and sloping up towards the north at such
an angle that it seemed to point to the invisible pivot of the heavens,
round which all the stars appeared to revolve--then the shadow of the
wall moved on the pavement in the same manner every day, and the
pavement if marked to show the hours for one day would show them for
any day.  The sundials still often found in the gardens of country
houses or in churchyards are miniatures of such an instrument.

But the Greek astronomers devised other and better methods for
determining the positions of the heavenly bodies.  Obelisks or dials
were of use only with the Sun and Moon which cast shadows.  To
determine the position of a star, "sights" like those of a rifle were
employed, and these were fixed to circles which were carefully divided,
generally into 360 "degrees."  As there are 365 days in a year, and as
the Sun makes a complete circuit of the Zodiac in this time, it moves
very nearly a degree in a day.  The twelve Signs of the Zodiac are
therefore each 30° in length, and each {44} takes on the average a
double-hour to rise or set.  While the Sun and Moon are each about half
a degree in diameter, _i.e._ about one-sixtieth of the length of a
Sign, and therefore take a double-minute to rise or set.  Each degree
of a circle is therefore divided into 60 minutes, and each minute may
be divided into 60 seconds.

As the Sun or Moon are each about half a degree, or, more exactly, 32
minutes in diameter, it is clear that, so long as astronomical
observations were made by the unaided sight, a minute of arc (written
1') was the smallest division of the circle that could be used.  A cord
or wire can indeed be detected when seen projected against a moderately
bright background if its thickness is a second of arc (written 1")--a
sixtieth of a minute--but the wire is merely perceived, not properly
defined.

Tycho Brahe had achieved the utmost that could be done by the naked
eye, and it was the certainty that he could not have made a mistake in
an observation in the place of the planet Mars amounting to as much as
8 minutes of arc--that is to say, of a quarter the apparent diameter of
the Moon--that made Kepler finally give up all attempts to explain the
planetary movements on the doctrine of circular orbits and to try
movements in an ellipse.  But a contemporary of Kepler, as gifted as he
was himself, but in a different direction, was the means of increasing
the observing power of the astronomer.  GALILEO GALILEI (1564-1642), of
a noble Florentine family, was appointed Lecturer in Mathematics at the
University of Pisa.  Here he soon distinguished himself by his
originality of thought, and the ingenuity and decisiveness of his
experiments.  Up to that time it had been taught that of {45} two
bodies the heavier would fall to the ground more quickly than the
lighter.  Galileo let fall a 100-lb.  weight and a 1-lb. weight from
the top of the Leaning Tower, and both weights reached the pavement
together.  By this and other ingenious experiments he laid a firm
foundation for the science of mechanics, and he discovered the laws of
motion which Newton afterwards formulated.  He heard that an instrument
had been invented in Holland which seemed to bring distant objects
nearer, and, having himself a considerable knowledge of optics, it was
not long before he made himself a little telescope.  He fixed two
spectacle glasses, one for long and one for short sight, in a little
old organ-pipe, and thus made for himself a telescope which magnified
three times.  Before long he had made another which magnified thirty
times, and, turning it towards the heavenly bodies, he discovered dark
moving spots upon the Sun, mountains and valleys on the Moon, and four
small satellites revolving round Jupiter.  He also perceived that Venus
showed "+phases+"--that is to say, she changed her apparent shape just
as the Moon does--and he found the Milky Way to be composed of an
immense number of small stars.  These discoveries were made in the
years 1609-11.

A telescope consists in principle of two parts--an +object-glass+, to
form an image of the distant object, and an +eye-piece+, to magnify it.
The rays of light from the heavenly body fall on the object-glass, and
are so bent out of their course by it as to be brought together in a
point called the focus.  The "light-gathering power" of the telescope,
therefore, depends upon the size of the object-glass, and is
proportional to its area.  But the size of the image depends upon the
focal length of the telescope, _i.e._ upon the distance that the focus
{46} is from the object-glass.  Thus a small disc, an inch in
diameter--such as a halfpenny--will exactly cover the full Moon if held
up nine feet away from the eye; and necessarily the image of the full
Moon made by an object-glass of nine-feet focus will be an inch in
diameter.  The eye-piece is a magnifying-glass or small microscope
applied to this image, and by it the image can be magnified to any
desired amount which the quality of the object-glass and the steadiness
of the atmosphere may permit.

This little image of the Moon, planet, or group of stars lent itself to
measurement.  A young English gentleman, GASCOIGNE, who afterwards fell
at the Battle of Marston Moor, devised the "micrometer" for this
purpose.  The micrometer usually has two frames, each carrying one or
more very thin threads--usually spider's threads--and the frames can be
moved by very fine screws, the number of turns or parts of a turn of
each screw being read off on suitable scales.  By placing one thread on
the image of one star, and the other on the image of another, the
apparent separation of the two can be readily and precisely measured.

Within the last thirty years photography has immensely increased the
ease with which astronomical measurements can be made.  The sensitive
photographic plate is placed in the focus of the telescope, and the
light of Sun, Moon, or stars, according to the object to which the
telescope is directed, makes a permanent impression on the plate.  Thus
a picture is obtained, which can be examined and measured in detail at
any convenient time afterwards; a portion of the heavens is, as it
were, brought actually down to the astronomer's study.

It was long before this great advance was effected.  {47} The first
telescopes were very imperfect, for the rays of different colour
proceeding from any planet or star came to different foci, so that the
image was coloured, diffused, and ill-defined.  The first method by
which this difficulty was dealt with was by making telescopes of
enormously long focal length; 80, 100, or 150 feet were not uncommon,
but these were at once cumbersome and unsteady.  Sir Isaac Newton
therefore discarded the use of object-glasses, and used curved mirrors
in order to form the image in the focus, and succeeded in making two
telescopes on this principle of reflection.  Others followed in the
same direction, and a century later Sir WILLIAM HERSCHEL was most
skilful and successful in making "+reflectors+," his largest being 40
feet in focal length, and thus giving an image of the Moon in its focus
of nearly 4-½ inches diameter.

But in 1729 CHESTER MOOR HALL found that by combining two suitable
lenses together in the object-glass he could get over most of the
colour difficulty, and in 1758 the optician DOLLOND began to make
object-glasses that were almost free from the colour defect.  From that
time onward the manufacture of "+refractors+," as object-glass
telescopes are called, has improved; the glass has been made more
transparent and more perfect in quality, and larger in size, and the
figure of the lens improved.  The largest refractor now in use is that
of the Yerkes Observatory, Wisconsin, U.S.A., and is 40 inches in
aperture, with a focal length of 65 feet, so that the image of the Moon
in its focus has a diameter of more than 7 inches.  At present this
seems to mark the limit of size for refractors, and the difficulty of
getting good enough glass for so large a lens is very great indeed.
Reflectors have therefore come again into favour, as mirrors can be
made larger {48} than any object-glass.  Thus Lord Rosse's great
telescope was 6 feet in diameter; and the most powerful telescope now
in action is the great 5-foot mirror of the Mt. Wilson Observatory,
California, with a focal length, as sometimes used, of 150 feet.  Thus
its light-gathering power is about 60,000 times that of the unaided
eye, and the full Moon in its focus is 17 inches in diameter; such is
the enormous increase to man's power of sight, and consequently to his
power of learning about the heavenly bodies, which the development of
the telescope has afforded to him.

The measurement of time was the first purpose for which men watched the
heavenly bodies; a second purpose was the measurement of the size of
the Earth.  If at one place a star was observed to pass exactly
overhead, and if at another, due south of it, the same star was
observed to pass the meridian one degree north of the zenith, then by
measuring the distance between the two places the circumference of the
whole Earth would be known, for it would be 360 times that amount.  In
this way the size of the Earth was roughly ascertained 2000 years
before the invention of the telescope.  But with the telescope measures
of much greater precision could be made, and hence far more difficult
problems could be attacked.

One great practical problem was that of finding out the position of a
ship when out of sight of land.  The ancient Phoenician and Greek
navigators had mostly confined themselves to coasting voyages along the
shores of the Mediterranean Sea, and therefore the quick recognition of
landmarks was the first requisite for a good sailor.  But when, in
1492, Columbus had brought a new continent to light, and long voyages
were freely taken across the great oceans, it became an urgent {49}
necessity for the navigator to find out his position when he had been
out of sight of any landmark for weeks.

This necessity was especially felt by the nations of Western Europe,
the countries facing the Atlantic with the New World on its far-distant
other shore.  Spain, France, England, and Holland, all were eager
competitors for a grasp on the new lands, and therefore were earnest in
seeking a solution of the problem of navigation.

The latitude of the ship could be found out by observing the height of
the Sun at noon, or of the Pole Star at night, or in several other
ways.  But the longitude was more difficult.  As the Earth turns on its
axis, different portions of its surface are brought in succession under
the Sun, and if we take the moment when the Sun is on the meridian of
any place as its noon, as twelve o'clock for that place, then the
difference of longitude between any two places is essentially the
difference in their local times.

It was possible for the sailor to find out when it was local noon for
him, but how could he possibly find out what time it was at that moment
at the port from which he had sailed, perhaps several weeks before?

The Moon and stars supplied eventually the means for giving this
information.  For the Moon moves amongst the stars, as the hand of a
clock moves amongst the figures of a dial, and it became possible at
length to predict for long in advance exactly where amongst the stars
the Moon would be, for any given time, of any selected place.

When this method was first suggested, however, neither the motion of
the Moon nor the places of the principal stars were known with
sufficient accuracy, and it was to remedy this defect, and put
navigation upon {50} a sound basis, that CHARLES II. founded Greenwich
Observatory in the year 1675, and appointed FLAMSTEED the first
Astronomer Royal.  In the year 1767 MASKELYNE, the fifth Astronomer
Royal, brought out the first volume of the _Nautical Almanac_, in which
the positions of the Moon relative to certain stars were given for
regular intervals of Greenwich time.  Much about the same period the
problem was solved in another way by the invention of the chronometer,
by JOHN HARRISON, a Yorkshire carpenter.  The +chronometer+ was a large
watch, so constructed that its rate was not greatly altered by heat or
cold, so that the navigator had Greenwich time with him wherever he
went.

The new method in the hands of CAPTAIN COOK and other great navigators
led to a rapid development of navigation and the discovery of Australia
and New Zealand, and a number of islands in the Pacific.  The building
up of the vast oceanic commerce of Great Britain and of her great
colonial empire, both in North America and in the Southern Oceans, has
arisen out of the work of the Royal Observatory, Greenwich, and has had
a real and intimate connection with it.

To observe the motions of the Moon, Sun, and planets, and to determine
with the greatest possible precision the places of the stars have been
the programme of Greenwich Observatory from its foundation to the
present time.  Other great national observatories have been Copenhagen,
founded in 1637; Paris, in 1667; Berlin, in 1700; St. Petersburg, in
1725, superseded by that of Pulkowa, in 1839; and Washington, in 1842;
while not a few of the great universities have also efficient
observatories connected with them.

Of the directly practical results of astronomy, the {51} promotion of
navigation stands in the first rank.  But the science has never been
limited to merely utilitarian inquiries, and the problem of measuring
celestial distances has followed on inevitably from the measurement of
the Earth.

The first distance to be attacked was that of the nearest companion to
the Earth, _i.e._ the Moon.  It often happens on our own planet that it
is required to find the distance of an object beyond our reach.  Thus a
general on the march may come to a river and need to know exactly how
broad it is, that he may prepare the means for bridging it.  Such
problems are usually solved on the following principle.  Let A be the
distant object.  Then if the direction of A be observed from each of
two stations, B and C, and the distance of B from C be measured, it is
possible to calculate the distances of A from B and from C.  The
application of this principle to the measurement of the Moon's distance
was made by the establishment of an observatory at the Cape of Good
Hope, to co-operate with that of Greenwich.  It is, of course, not
possible to see Greenwich Observatory from the Cape, or vice versa, but
the stars, being at an almost infinite distance, lie in the same
direction from both observatories.  What is required then is to measure
the apparent distance of the Moon from the same stars as seen from
Greenwich and as seen from the Cape, and, the distance apart of the two
observatories being known, the distance of the Moon can be calculated.

This was a comparatively easy problem.  The next step in celestial
measurement was far harder; it was to find the distance of the Sun.
The Sun is 400 times as far off as the Moon, and therefore it seems to
be practically in the same direction as seen from each of {52} the two
observatories, and, being so bright, stars cannot be seen near it in
the telescope.  But by carefully watching the apparent movements of the
planets their _relative_ distances from the Sun can be ascertained, and
were known long before it was thought possible that we should ever know
their real distances.  Thus Venus never appears to travel more than 47°
15' from the Sun.  This means that her distance from the Sun is a
little more than seven-tenths of that of the Earth.  If, therefore, the
distance of one planet from the Sun can be measured, or the distance of
one planet from the Earth, the actual distances of all the planets will
follow.  We know the proportions of the parts of the solar system, and,
if we can fix the scale of one of the parts, we fix the scale of all.

It has been found possible to determine the distance of Mars, of
several of the "minor planets," and especially of Eros, a very small
minor planet that sometimes comes within 13,000,000 miles of the Earth,
or seven times nearer to us than is the Sun.

From the measures of Eros, we have learned that the Sun is separated
from us by very nearly 93,000,000 miles--an unimaginable distance.
Perhaps the nearest way of getting some conception of this vast
interval is by remembering that there are only 31,556,926 seconds of
time in a year.  If, therefore, an express train, travelling 60 miles
an hour--a mile a minute--set out for the Sun, and travelled day and
night without cease, it would take more than 180 years to accomplish
the journey.

But this astronomical measure has led on to one more daring still.  The
earth is on one side of the Sun in January, on the other in July.  At
these two dates, therefore, we are occupying stations 186,000,000 miles
{53} apart, and can ascertain the apparent difference in direction of
the stars as viewed from the two points But the astonishing result is
that this enormous change in the position of the Earth makes not the
slightest observable difference in the position of most of the stars.
A few, a very few, do show a very slight difference.  The nearest star
to us is about 280,000 times as far from us as the Sun; this is Alpha
Centauri, the brightest star in the constellation of the Centaur and
the third brightest star in the sky.  Sirius, the brightest star, is
twice this distance.  Some forty or fifty stars have had their
distances roughly determined; but the stars in general far transcend
all our attempts to plumb their distances.  But, from certain indirect
hints, it is generally supposed that the mass of stars in the Milky Way
are something like 300,000,000 times as far from us as we are from our
Sun.

Thus far, then, astronomy has led us in the direction or measurement.
It has enabled us to measure the size of the Earth upon which we live,
and to find out the position of a ship in the midst of the trackless
ocean.  It has also enabled us to cast a sounding-line into space, to
show how remote and solitary the earth moves through the void, and to
what unimaginable lengths the great stellar universe, of which it forms
a secluded atom, stretches out towards infinity.



{54}

CHAPTER V

THE MEMBERS OF THE SOLAR SYSTEM

Astronomical measurement has not only given us the distances of the
various planets from the Sun; it has also furnished us, as in the
annexed table, with their real diameters, and, as a consequence of the
law of gravitation, with their densities and weights, and the force of
gravity at their surfaces.  And these numerical details are of the
first importance in directing us as to the inferences that we ought to
draw as to their present physical conditions.

The theory of Copernicus deprived the Earth of its special position as
the immovable centre of the universe, but raised it to the rank of a
planet.  It is therefore a heavenly body, yet needs no telescope to
bring it within our ken; bad weather does not hide it from us, but
rather shows it to us under new conditions.  We find it to be a globe
of land and water, covered by an atmosphere in which float changing
clouds; we have mapped it, and we find that the land and water are
always there, but their relations are not quite fixed; there is give
and take between them.  We have found of what elements the land and
water consist, and how these elements combine with each other or
dissociate.  In a word, the Earth is the heavenly body that we know the
best, and with it we must compare and contrast all the others.

Before the invention of the telescope there were but {55} two other
heavenly bodies--the Sun and the Moon--that appeared as orbs showing
visible discs, and even in their cases nothing could be satisfactorily
made out as to their conditions.  Now each of the five planets known to
the ancients reveals to us in the telescope a measurable disc, and we
can detect significant details on their surfaces.

THE MOON is the one object in the heavens which does not disappoint a
novice when he first sees it in the telescope.  Every detail is hard,
clear-cut, and sharp; it is manifest that we are looking at a globe, a
very rough globe, with hills and mountains, plains and valleys, the
whole in such distinct relief that it seems as if it might be touched.
No clouds ever conceal its details, no mist ever softens its outlines;
there are no half-lights, its shadows are dead black, its high lights
are molten silver.  Certain changes of illumination go on with the
advancing age of the Moon, as the crescent broadens out to the half,
the half to the full, and the full, in its turn, wanes away; but the
lunar day is nearly thirty times as long as that of the Earth, and
these changes proceed but slowly.

The full Moon, as seen by the naked eye, shows several vague dark
spots, which most people agree to fancy as like the eyes, nose, and
mouth of a broad, sorrowful face.  The ordinary astronomical telescope
inverts the image, so the "eyes" of the Moon are seen in the lower part
of the field of the telescope as a series of dusky plains stretching
right across the disc.  But in the upper part, near the left-hand
corner of the underlip, there is a bright, round spot, from which a
number of bright streaks radiate--suggesting a peeled orange with its
stalk, and the lines marking the sections radiating from it.  This
bright spot has been called after the great {56}

                          Mean distance from Sun.    Period       Velocity
  Class.       Name.      Earth's    In millions  of revolution.  in orbit.   Eccentricity.
                          distance   of miles.      In years.     Miles per
                            =1.                                     sec.

  Terrestrial  Mercury    0.387        36.0           0.24          29.7      0.2056
  Planets      Venus      0.723        67.2           0.62          21.9      0.0068
               Earth      1.000        92.9           1.00          18.5      0.0168
               Mars       1.524       141.5           1.88          15.0      0.0933

  Minor        Eros       1.458       135.5           1.76          15.5      0.2228
  Planets      Ceres      2.767       257.1           4.60          11.1      0.0763
               Achilles   5.253       488.0          12.04           8.1      0.0509

  Major        Jupiter    5.203       483.3          11.86           8.1      0.0483
  Planets      Saturn     9.539       886.6          29.46           6.0      0.0561
               Uranus    19.183      1781.9          84.02           4.2      0.0463
               Neptune   30.055      2791.6         164.78           3.4      0.0090

{57}

                           Mean diameter.       Surface.     Volume.       Mass.
     Name.    Symbol.   In miles.  [Earth]=1.  [Earth]=1.  [Earth]=1.   [Earth]=1.

  Sun         [Sun]     866400     109.422     11973.      1310130.     332000.
  Moon        [Moon]      2163       0.273         0.075         0.02        0.012

  Mercury     [Mercury]   3030       0.383         0.147         0.06        0.048
  Venus       [Venus]     7700       0.972         0.945         0.92        0.820
  Earth       [Earth]     7918       1.000         1.000         1.00        1.000
  Mars        [Mars]      4230       0.534         0.285         0.15        0.107

  Jupiter     [Jupiter]  86500      10.924       119.3        1304.        317.7
  Saturn      [Saturn]   73000       9.219        85.0         783.         94.8
  Uranus      [Uranus]   31900       4.029        16.2          65.         14.6
  Neptune     [Neptune]  34800       4.395        19.3          85.         17.0

{58}

                                                  Light
                                 Gravity.       and heat                      Albedo;
                Density.              Fall in   received                      _i.e._ re-
            [Earth]   Water  [Earth]  feet per   from Sun.  Time of rotation  flecting
  Name.        =1.     =1.     =1.      sec.    [Earth]=1.      on axis.      power.

                                                            d. h.  m.
  Sun         0.25    1.39    27.65   444.60      ...       25  4  48 ±       ...
  Moon        0.61    3.39     0.17     2.73      1.00      27  7  43         0.17

                                                            d. h.  m.  s.
  Mercury     0.85    4.72     0.43     6.91      6.67      88            (?) 0.14
  Venus       0.89    4.94     0.82    13.19      1.91         23  21  23 (?) 0.76
  Earth       1.00    5.55     1.00    16.08      1.00         23  56   4     0.50 (?)
  Mars        0.71    3.92     0.38     6.11      0.43         24  37  23     0.22

                                                                   h.  m.
  Jupiter     0.24    1.32     2.65    42.61      0.037        9   55      ±  0.62
  Saturn      0.13    0.72     1.18    18.97      0.011       10   14      ±  0.72
  Uranus      0.22    1.22     0.90    14.47      0.003        9   30     (?) 0.60
  Neptune     0.20    1.11     0.89    14.31      0.001                   (?) 0.52

{59} Danish astronomer, "Tycho," and is one of the most conspicuous
objects of the full Moon.

The contrasts of the Moon are much more pronounced when she is only
partly lit up.  Then the mountains and valleys stand out in the
strongest relief, and it becomes clear that the general type of
formation on the Moon is that of rings--rings of every conceivable
size, from the smallest point that the telescope can detect up to some
of the great dusky plains themselves, hundreds of miles in diameter.
These rings are so numerous that Galileo described the Moon as looking
as full of "eyes" as a peacock's tail.

The "right eye" of the moonface, as we see it in the sky, is formed by
a vast dusky plain, nearly as large as France and Germany put together,
to which has been given the name of the "Sea of Rains" (_Mare
Imbrium_), and just below this (as seen in the telescope) is one of the
most perfect and beautiful of all the lunar rings--a great ring-plain,
56 miles in diameter, called after the thinker who revolutionised men's
ideas of the solar system, "Copernicus."  "Copernicus," like "Tycho,"
is the centre of a set of bright streaks; and a neighbouring but
smaller ring, bearing the great name of "Kepler," stands in a like
relation to another set.

The most elevated region of the Moon is immediately in the
neighbourhood of the great "stalk of the orange," "Tycho."  Here the
rings are crowded together as closely as they can be packed; more
closely in many places, for they intrude upon and overlap each other in
the most intricate manner.  A long chain of fine rings stretches from
this disturbed region nearly to the centre of the disc, where the great
Alexandrian astronomer is commemorated by a vast walled plain, {60}
considerably larger than the whole of Wales, and known as "Ptolemæus."

The distinctness of the lunar features shows at once that the Moon is
in an altogether different condition from that of the Earth.  Here the
sky is continually being hidden by cloud, and hence the details of the
surface of the Earth as viewed from any other planet must often be
invisible, and even when actual cloud is absent there is a more
permanent veil of dust, which must greatly soften and confuse
terrestrial outlines.  The clearness, therefore, with which we perceive
the lunar formations proves that there is little or no atmosphere
there.  Nor is there any sign upon it of water, either as seas or lakes
or running streams.

Yet the Moon shows clearly that in the past it has gone through great
and violent changes.  The gradation is so complete from the little
craterlets, which resemble closely, in form and size, volcanic craters
on the Earth, up to the great ring-plains, like "Copernicus" or
"Tycho," or formations larger still, that it seems natural to infer not
only that the smaller craters were formed by volcanic eruption, like
the similar objects with which we are acquainted on our own Earth, but
that the others, despite their greater sizes, had a like origin.  In
consequence of the feebler force of gravity on the Moon, the same
explosive force there would carry the material of an eruption much
further than on the Earth.

The darker, low-lying districts of the Moon give token of changes of a
different order.  It is manifest that the material of which the floors
of these plains is composed has invaded, broken down, and almost
submerged many of the ring-formations.  Sometimes half {61} of a ring
has been washed away; sometimes just the outline of a ring can still be
traced upon the floor of the sea; sometimes only a slight breach has
been made in the wall.  So it is clear that the Moon was once richer in
the great crater-like formations than it is to-day, but a lava-flood
has overflowed at least one-third of its area.  More recent still are
the bright streaks, or rays, which radiate in all directions from
"Tycho," and from some of the other ring-plains.

It is evident from these different types of structure on the Moon, and
from the relations which they bear to each other, that the lunar
surface has passed through several successive stages, and that its
changes tended, on the whole, to diminish in violence as time went on;
the minute crater pits with which the surface is stippled having been
probably the last to form.

But the 300 years during which the Moon has been watched with the
telescope have afforded no trace of any continuance of these changes.
She has had a stormy and fiery past; but nothing like the events of
those bygone ages disturbs her serenity to-day.

And yet we must believe that change does take place on the Moon even
now, because during the 354 hours of its long day the Sun beats down
with full force on the unprotected surface, and during the equally long
night that surface is exposed to the cold of outer space.  Every part
of the surface must be exposed in turn to an extreme range of
temperature, and must be cracked, torn, and riven by alternate
expansion and contraction.  Apart from this slow, wearing process, and
a very few rather doubtful cases in which a minute alteration of some
surface detail has been suspected, our sister planet, the Moon, shows
herself as changeless and inert, without any appreciable trace of air
or water or any sign {62} of life--a dead world, with all its changes
and activities in the past.

MARS, after the Moon, is the planet whose surface we can study to best
advantage.  Its orbit lies outside that of the Earth, so that when it
is nearest to us it turns the same side to both the Sun and Earth, and
we see it fully illuminated.  Mercury and Venus, on the contrary, when
nearest us are between us and the Sun, and turn their dark sides to us.
When fully illuminated they are at their greatest distance, and appear
very small, and, being near the Sun, are observed with difficulty.
These three are intermediate in size between the Moon and the Earth.

In early telescopic days it was seen that Mars was an orange-coloured
globe with certain dusky markings upon it, and that these markings
slowly changed their place--that, in short, it was a world rotating
upon its axis, and in a period not very different from that of the
Earth.  The rotation period of Mars has indeed been fixed to the
one-hundredth part of a second of time; it is 24 h. 37 m. 22.67 s.  And
this has been possible because some of the dusky spots observed in the
seventeenth century can be identified now in the twentieth.  Some of
the markings on Mars, like our own continents and seas, and like the
craters on the Moon, are permanent features; and many charts of the
planet have been constructed.

Other markings are variable.  Since the planet rotates on its axis, the
positions of its poles and equator are known, its equator being
inclined to its orbit at an angle of 24° 50', while the angle in the
case of the Earth is 23° 27'.  The times when its seasons begin and end
are therefore known; and it is found that the spring of its northern
hemisphere lasts 199 of our {63} days, the summer 183, the autumn 147,
and the winter 158.  Round the pole in winter a broad white cap forms,
which begins to shrink as spring comes on, and may entirely disappear
in summer.  No corresponding changes have been observed on the Moon,
but it is easy to find an analogy to them on the Earth.  Round both our
poles a great cap of ice and snow is spread--a cap which increases in
size as winter comes on, and diminishes with the advance of summer--and
it seems a reasonable inference to suppose that the white polar caps of
Mars are, like our own, composed of ice and snow.

From time to time indications have been observed of the presence on
Mars of a certain amount of cloud.  Familiar dark markings have, for a
short time, been interrupted, or been entirely hidden, by white bands,
and have recovered their ordinary appearance later.  With rotation on
its axis and succession of seasons, with atmosphere and cloud, with
land and water, with ice and snow, Mars would seem to be a world very
similar to our own.

This was the general opinion up to the year 1877, when SCHIAPARELLI
announced that he had discovered a number of very narrow, straight,
dark lines on the planet--lines to which he gave the name of
"canali"--that is, "channels."  This word was unfortunately rendered
into English by the word "+canals+," and, as a canal means a waterway
artificially made, this mistranslation gave the idea that Mars was
inhabited by intelligent beings, who had dug out the surface of the
planet into a network of canals of stupendous length and breadth.

The chief advocate of this theory is LOWELL, an American observer, who
has given very great attention {64} to the study of the planet during
the last seventeen years.  His argument is that the straight lines, the
canals, which he sees on the planet, and the round dots, the "+oases+,"
which he finds at their intersections, form a system so obviously
_un_natural, that it must be the work of design--of intelligent beings.
The canals are to him absolutely regular and straight, like lines drawn
with ruler and pen-and-ink, and the oases are exactly round.  But, on
the one hand, the best observers, armed with the most powerful
telescopes, have often been able to perceive that markings were really
full of irregular detail, which Lowell has represented as mere hard
straight lines and circular dots, and, on the other hand, the straight
line and the round dot are the two geometric forms which all very
minute objects must approach in appearance.  That we cannot see
irregularities in very small and distant objects is no proof at all
that irregularities do not exist in them, and it has often happened
that a marking which appeared a typical "canal" when Mars was at a
great distance lost that appearance when the planet was nearer.

Astronomers, therefore, are almost unanimous that there is no reason
for supposing that any of the details that we see on the surface of
Mars are artificial in their origin.  And indeed the numerical facts
that we know about the planet render it almost impossible that there
should be any life upon it.

If we turn to the table, we see that in size, volume, density, and
force of gravity at its surface, Mars lies between the Moon and the
Earth, but is nearer the Moon.  This has an important bearing as to the
question of the planet's atmosphere.  On the Earth we pass through half
the atmosphere by ascending a mountain {65} that is three and a third
miles in height; on Mars we should have to ascend nearly nine miles.
If the atmospheric pressure at the surface of Mars were as great as it
is at the surface of the Earth, his atmosphere would be far deeper than
ours and would veil the planet more effectively.  But we see the
surface of Mars with remarkable distinctness, almost as clearly, when
its greater distance is allowed for, as we see the Moon.  It is
therefore accepted that the atmospheric pressure at the surface of Mars
must be very slight, probably much less than at the top of our very
highest mountains, where there is eternal snow, and life is completely
absent.

But Mars compares badly with the Earth in another respect.  It receives
less light and heat from the Sun in the proportion of three to seven.
This we may express by saying that Mars, on the whole, is almost as
much worse off than the Earth as a point on the Arctic Circle is worse
off than a point on the Equator.  The mean temperature of the Earth is
taken as about 60° of the Fahrenheit thermometer (say, 15° Cent.); the
mean temperature of Mars must certainly be considerably below
freezing-point, probably near 0° F.  Here on our Earth the
boiling-point of water is 212°, and, since the mean temperature is 60°
and water freezes at 32°, it is normally in the liquid state.  On Mars
it must normally be in the solid state--ice, snow, or frost, or the
like.  But with so rare an atmosphere water will boil at a low
temperature, and it is not impossible that under the direct rays of the
Sun--that is to say, at midday of the torrid zone of Mars--ice may not
only melt, but water boil by day, condensing and freezing again during
the night.  NEWCOMB, the foremost astronomer of his day, concluded
"that during {66} the night of Mars, even in the equatorial regions,
the surface of the planet probably falls to a lower temperature than
any we ever experienced on our globe.  If any water exists, it must not
only be frozen, but the temperature of the ice must be far below the
freezing point....  The most careful calculation shows that if there
are any considerable bodies of water on our neighbouring planet, they
exist in the form of ice, and can never be liquid to a depth of more
than one or two inches, and that only within the torrid zone and during
a few hours each day."  With regard to the snow caps of Mars, Newcomb
thought it not possible that any considerable fall of snow could ever
take place.  He regarded the white caps as simply due to a thin deposit
of hoar frost, and it cannot be deemed wonderful that such should
gradually disappear, when it is remembered that each of the two poles
of Mars is in turn presented to the Sun for more than 300 consecutive
days.  Newcomb's conclusion was: "Thus we have a kind of Martian
meteorological changes, very slight indeed, and seemingly very
different from those of our Earth, but yet following similar lines on
their small scale.  For snowfall substitute frostfall; instead of (the
barometer reading) feet or inches say fractions of a millimetre, and
instead of storms or wind substitute little motions of an air thinner
than that on the top of the Himalayas, and we shall have a general
description of Martian meteorology."

We conclude, then, that Mars is not so inert a world as the Moon, but,
though some slight changes of climate or weather take place upon it, it
is quite unfitted for the nourishment and development of the different
forms of organic life.

Of MERCURY we know very little.  It is smaller than Mars but larger
than the Moon, but it differs from them {67} both in that it is much
nearer the Sun, and receives, therefore, many times the light and heat,
surface for surface.  We should expect, therefore, that water on
Mercury would exist in the gaseous state instead of in the solid state
as on Mars.  The little planet reflects the sunlight only feebly, and
shows no evidence of cloud.  A few markings have been made out on its
surface, and the best observers agree that it appears to turn the same
face always to the Sun.  This would imply that the one hemisphere is in
perpetual darkness and cold, the other, exposed to an unimaginable
fiery heat.

VENUS is nearly of the same size as the Earth, and the conditions as to
the arrangement of its atmosphere, the force of gravity at its surface,
must be nearly the same as on our own world.  But we know almost
nothing of the details of its surface; the planet is very bright,
reflecting fully seven-tenths of the sunlight that falls upon it.  It
would seem that, in general, we see nothing of the actual details of
the planet, but only the upper surface of a very cloudy atmosphere.
Owing to the fact that Venus shows no fixed definite marking that we
can watch, it is still a matter of controversy as to the time in which
it rotates upon its axis.  Schiaparelli and some other observers
consider that it rotates in the same time as it revolves round the Sun.
Others believe that it rotates in a little less than twenty-four hours.
If this be so, and there is any body in the solar system other than the
Earth, which is adapted to be the home of life, then the planet Venus
is that one.

THE SUN, like the Moon, presents a visible surface to the naked eye,
but one that shows no details.  In the telescope the contrast between
it and the Moon is very great, and still greater is the contrast which
is brought {68} out by the measurements of its size, volume, and
weight.  But the really significant difference is that the Sun is a
body giving out light and heat, not merely reflecting them.  Without
doubt this last difference is connected most closely with the
difference in size.  The Moon is cold, dead, unchanging, because it is
a small world; the Sun is bright, fervent, and undergoes the most
violent change, because it is an exceedingly large world.

The two bodies--the Sun and Moon--appear to the eye as being about the
same size, but since the Sun is 400 times as far off as the Moon it
must be 400 times the diameter.  That means that it has 400 times 400,
or 160,000 times the surface and 400 times 400 times 400, or 64,000,000
times the volume.  The Sun and Moon, therefore, stand at the very
extremes of the scale.

The heat of the Sun is so great that there is some difficulty in
observing it in the telescope.  It is not sufficient to use a dark
glass in order to protect the eye, unless the telescope be quite a
small one.  Some means have to be employed to get rid of the greater
part of the heat and light.  The simplest method of observing is to fix
a screen behind the eyepiece of a telescope and let the image of the
Sun be projected upon the screen, or the sensitive plate may be
substituted for the screen, and a photograph obtained, which can be
examined at leisure afterwards.

As generally seen, the surface of the Sun appears to be mottled all
over by a fine irregular stippling.  This stippling, though everywhere
present, is not very strongly marked, and a first hasty glance might
overlook it.  From time to time, however, dark spots are seen, of
ever-changing form and size.  By watching these, Galileo proved that
the Sun rotated on its axis in a little more than twenty-five days, and
in the {69} nineteenth century SCHWABE proved that the sunspots were
not equally large and numerous at all times, but that there was a kind
of cycle of a little more than eleven years in average length.  At one
time the Sun would be free from spots; then a few small ones would
appear; these would gradually become larger and more numerous; then a
decline would follow, and another spotless period would succeed about
eleven years after the first.  As a rule, the increase in the spots
takes place more quickly than the decline.

Most of the spot-groups last only a very few days, but about one group
in four lasts long enough to be brought round by the rotation of the
Sun a second time; in other words, it continues for about a month.  In
a very few cases spots have endured for half a year.

An ordinary form for a group of spots is a long stream drawn out
parallel to the Sun's equator, the leading spot being the largest and
best defined.  It is followed by a number of very small irregular and
ill-developed spots, and the train is brought up by a large spot,
sometimes even larger than the leader, but by no means so regular in
form or so well defined.  The leading spot for a short time moves
forward much faster than its followers, at a speed of about 8000 miles
per day.  The small middle spots then gradually die out, or rather seem
to be overflowed by the bright material of the solar surface, the
"+photosphere+," as it is called; the spot in the rear breaks up a
little later, and the leader, which is now almost circular, is left
alone, and may last in this condition for some weeks.  Finally, it
slowly contracts or breaks up, and the disturbance comes to an end.
This is the course of development of many long-lived spot-groups, but
all do not conform to the same type.  {70} The very largest spots are
indeed usually quite different in their appearance and history.

In size, sunspots vary from the smallest dot that can be discovered in
the telescope up to huge rents with areas that are to be counted by
thousands of millions of square miles; the great group of February 1905
had an area of 4,000,000,000 square miles, a thousand times the area of
Europe.

Closely associated with the _maculæ_, as the spots were called by the
first observers, are the "+faculæ+"--long, branching lines of bright
white light, bright as seen even against the dazzling background of the
Sun itself, and looking like the long lines of foam of an incoming
tide.  These are often associated with the spots; the spots are formed
between their ridges, and after a spot-group has disappeared the broken
waves of faculæ will sometimes persist in the same region for quite a
long time.

The faculæ clearly rise above the ordinary solar surface; the spots as
clearly are depressed a little below it; because from time to time we
see the bright material of the surface pour over spots, across them,
and sometimes into them.  But there is no reason to believe that the
spots are deep, in proportion either to the Sun itself or even to their
own extent.

Sunspots are not seen in all regions of the Sun.  It is very seldom
that they are noted in a higher solar latitude than 40°, the great
majority of spots lying in the two zones between 5° and 25° latitude on
either side of the equator.  Faculæ, on the other hand, though most
frequent in the spot zones, are observed much nearer the two poles.

It is very hard to find analogies on our Earth for sunspots and for
their peculiarities of behaviour.  Some {71} of the earlier astronomers
thought they were like terrestrial volcanoes, or rather like the
eruptions from them.  But if there were a solid nucleus to the Sun, and
the spots were eruptions from definite areas of the nucleus, they would
all give the same period of rotation.  But sunspots move about freely
on the solar surface, and the different zones of that surface rotate in
different times, the region of the equator rotating the most quickly.
This alone is enough to show that the Sun is essentially not a solid
body.  Yet far down below the photosphere something approaching to a
definite structure must already be forming.  For there is a well-marked
progression in the zones of sunspots during the eleven-year cycle.  At
a time when spots are few and small, known as +the sunspot minimum+,
they begin to be seen in fairly high latitudes.  As they get more
numerous, and many of them larger, they frequent the medium zones.
When the Sun is at its greatest activity, known as +the sunspot
maximum+, they are found from the highest zone right down to the
equator.  Then the decline sets in, but it sets in first in the highest
zones, and when the time of minimum has come again the spots are close
to the equator.  Before these have all died away, a few small spots,
the heralds of a new cycle of activity, begin to appear in high
latitudes.

This law, called after SPÖRER, its discoverer, indicates that the
origin and source of sunspot activity lie within the Sun.  At one time
it was thought that sunspots were due to some action of Jupiter--for
Jupiter moves round the Sun in 11.8 years, a period not very different
from the sunspot cycle--or to some meteoric stream.  But Spörer's Law
could not be imposed by some influence from without.  Still sunspots,
once formed, may be influenced by the Earth, and perhaps by other {72}
planets also, for MRS. WALTER MAUNDER has shown that the numbers and
areas of spots tend to be smaller on the western half of the disc, as
seen from the Earth, than on the eastern, while considerably more
groups come into view at the east edge of the Sun than pass out of view
at the west edge, so that it would appear as if the Earth had a damping
effect upon the spots exposed to it.

But the Sun is far greater than it ordinarily appears to us.  Twice
every year, and sometimes oftener, the Moon, when new, comes between
the Earth and the Sun, and we have an +Eclipse of the Sun+, the dark
body of the Moon hiding part, or all, of the greater light.  The Sun
and Moon are so nearly of the same apparent size that an eclipse of the
Sun is total only for a very narrow belt of the Earth's surface, and,
as the Moon moves more quickly than the Sun, the eclipse only remains
total for a very short time--seven minutes at the outside, more usually
only two or three.  North or south of that belt the Moon is projected,
so as to leave uncovered a part of the Sun north or south of the Moon.
A total eclipse, therefore, is rare at any particular place, and if a
man were able to put himself in the best possible position on each
occasion, it would cost him thirty years to secure an hour's
accumulated duration.

Eclipses of the Moon are visible over half the world at one time, for
there is a real loss to the Moon of her light.  Her eclipses are
brought about when, in her orbit, she passes behind the Earth, and the
Earth, being between the Sun and the Moon, cuts off from the latter
most of the light falling upon her; not quite all; a small portion
reaches her after passing through the thickest part of the Earth's
atmosphere, so that the {73} Moon in an eclipse looks a deep copper
colour, much as she does when rising on a foggy evening.

Total eclipses of the Sun have well repaid all the efforts made to
observe them.  It is a wonderful sight to watch the blackness of
darkness slowly creeping over the very fountain of light until it is
wholly and entirely hidden; to watch the colours fade away from the
landscape and a deathlike, leaden hue pervade all nature, and then to
see a silvery, star-like halo, flecked with bright little rose-coloured
flames, flash out round the black disc that has taken the place of the
Sun.

These rose-coloured flames are the solar "+prominences+," and the halo
is the "+corona+," and it is to watch these that astronomers have made
so many expeditions hither and thither during the last seventy years.
The "prominences," or red flames, can be observed, without an eclipse,
by means of the spectroscope, but, as the work of the spectroscope is
to form the subject of another volume of this series, it is sufficient
to add here that the prominences are composed of various glowing gases,
chiefly of hydrogen, calcium, and helium.

These and other gases form a shell round the Sun, about 3000 miles in
depth, to which the name "+chromosphere+" has been given.  It is out of
the chromosphere that the prominences arise as vast irregular jets and
clouds.  Ordinarily they do not exceed 40 or 50 thousand miles in
height, but occasionally they extend for 200 or even 300 thousand miles
from the Sun.  Their changes are as remarkable as their dimensions;
huge jets of 50 or 100 thousand miles have been seen to form, rise, and
disappear within an hour or less, and movements have been chronicled of
200 or 300 miles in a single second of time.

Prominences are largest and most frequent when {74} sunspots and faculæ
are most frequent, and fewest when those are fewest.  The corona, too,
varies with the sunspots.  At the time of maximum the corona sends
forth rays and streamers in all directions, and looks like the
conventional figure of a star on a gigantic scale.  At minimum the
corona is simpler in form, and shows two great wings, east and west, in
the direction of the Sun's equator, and round both of his poles a
number of small, beautiful jets like a crest of feathers.

Some of the streamers or wings of the corona have been traced to an
enormous distance from the Sun.  Mrs. Walter Maunder photographed one
ray of the corona of 1898 to a distance of 6 millions of miles.
LANGLEY, in the clear air of Pike's Peak, traced the wings of the
corona of 1878 with the naked eye to nearly double this distance.

But the rapid changes of sunspots and the violence of some of the
prominence eruptions are but feeble indications of the most wonderful
fact concerning the Sun, _i.e._ the enormous amount of light and heat
which it is continually giving off.  Here we can only put together
figures which by their vastness escape our understanding.  Sunlight is
to moonlight as 600,000 is to 1, so that if the entire sky were filled
up with full moons, they would not give us a quarter as much light as
we derive from the Sun.  The intensity of sunlight exceeds by far any
artificial light; it is 150 times as bright as the calcium light, and
three or four times as bright as the brightest part of the electric arc
light.  The amount of heat radiated by the Sun has been expressed in a
variety of different ways; C. A. YOUNG very graphically by saying that
if the Sun were encased in a shell of ice 64 feet deep, its heat would
melt the shell in one minute, and that if a bridge of ice could be {75}
formed from the Earth to the Sun, 2-½ miles square in section and 93
millions of miles long, and the entire solar radiation concentrated
upon it, in one second the ice would be melted, in seven more
dissipated into vapour.

The Earth derives from the Sun not merely light and heat, but, by
transformation of these, almost every form of energy manifest upon it;
the energy of the growth of plants, the vital energy of animals, are
only the energy received from the Sun, changed in its expression.

The question naturally arises, "If the Sun, to which the Earth is
indebted for nearly everything, passes through a change in its activity
every eleven years or so, how is the Earth affected by it?"  It would
seem at first sight that the effect should be great and manifest.  A
sunspot, like that of February 1905, one thousand times as large as
Europe, into which worlds as large as our Earth might be poured, like
peas into a saucer, must mean, one might think, an immense falling off
of the solar heat.

Yet it is not so.  For even this great sunspot was but small as
compared with the Sun as a whole.  Had it been dead black, it would
have stopped out much less than 1 per cent. of the Sun's heat; and even
the darkest sunspot is really very bright.  And the more spots there
are, the more numerous and brighter are the faculæ; so that we do not
know certainly which of the two phases, maximum or minimum, means the
greater radiation.  If the weather on the Earth answers to the sunspot
cycle, the connection is not a simple one; as yet no connection has
been proved.  Thus two of the worst and coldest summers experienced in
England fell the one in 1860, the other in 1879, _i.e._ at {76} maximum
and minimum respectively.  So, too, the hot summers of 1893 and 1911
were also, the one at maximum and the other at minimum; and ordinary
average years have fallen at both the phases just the same.

Yet there is an answer on the part of the Earth to these solar changes.
The Earth itself is a kind of magnet, possessing a magnetism of which
the intensity and direction is always changing.  To watch these
changes, very sensitive magnets are set up, and a slight daily
to-and-fro swing is noticed in them; this swing is more marked in
summer than in winter, but it is also more marked at times of the
sunspot maximum than at minimum, showing a dependence upon the solar
activity.

Yet more, from time to time the magnetic needle undergoes more or less
violent disturbance; in extreme cases the electric telegraph
communication has been disturbed all over the world, as on September
25, 1909, when the submarine cables ceased to carry messages for
several hours.  In most cases when such a "magnetic storm" occurs,
there is an unusually large or active spot on the Sun.  The writer was
able in 1904 to further prove that such "storms" have a marked tendency
to recur when the same longitude of the Sun is presented again towards
the Earth.  Thus in February 1892, when a very large spot was on the
Sun, a violent magnetic storm broke out.  The spot passed out of sight
and the storm ceased, but in the following month, when the spot reached
exactly the same apparent place on the Sun's disc, the storm broke out
again.  Such magnetic disturbances are therefore due to streams of
particles driven off from limited areas of the Sun, probably in the
same way that the long, {77} straight rays of the corona are driven
off.  Such streams of particles, shot out into space, do not spread out
equally in all directions, like the rays of light and heat, but are
limited in direction, and from time to time they overtake the Earth in
its orbit, and, striking it, cause a magnetic storm, which is felt all
over the Earth at practically the same moment.

JUPITER is, after the Sun, much the largest member of the solar system,
and it is a peculiarly beautiful object in the telescope.  Even a small
instrument shows the little disc striped with many delicately coloured
bands or belts, broken by white clouds and dark streaks, like a "windy
sky" at sunset.  And it changes while being watched, for, though
400,000,000 miles away from us, it rotates so fast upon its axis that
its central markings can actually be seen to move.

This rapid rotation, in less than ten hours, is the most significant
fact about Jupiter.  For different spots have different rotation
periods, even in the same latitude, proving that we are looking down
not upon any solid surface of Jupiter, but upon its cloud envelope--an
envelope swept by its rapid rotation and by its winds into a vast
system of parallel currents.

One object on Jupiter, the great "+Red Spot+," has been under
observation since 1878, and possibly for 200 years before that.  It is
a large, oval object fitted in a frame of the same shape.  The spot
itself has often faded and been lost since 1878, but the frame has
remained.  The spot is in size and position relative to Jupiter much as
Australia is to the Earth, but while Australia moves solidly with the
rest of the Earth in the daily rotation, neither gaining on South
America nor losing on Africa, the Red Spot on Jupiter sees many other
spots and clouds pass it by, and does not even {78} retain the same
rate of motion itself from one year to another.

No other marking on Jupiter is so permanent as this.  From time to time
great round white clouds form in a long series as if shot up from some
eruption below, and then drawn into the equatorial current.  From time
to time the belts themselves change in breadth, in colour, and
complexity.  Jupiter is emphatically the planet of change.

And such change means energy, especially energy in the form of heat.
If Jupiter possessed no heat but that it derived from the Sun, it would
be colder than Mars, and therefore an absolutely frozen globe.  But
these rushing winds and hurrying clouds are evidences of heat and
activity--a native heat much above that of our Earth.  While Mars is
probably nearer to the Moon than to the Earth in its condition, Jupiter
has probably more analogies with the Sun.

The one unrivalled distinction of SATURN is its Ring.  Nothing like
this exists elsewhere in the solar system.  Everywhere else we see
spherical globes; this is a flat disc, but without its central portion.
It surrounds the planet, lying in the plane of its equator, but touches
it nowhere, a gap of 7000 miles intervening.  It appears to be
circular, and is 42,000 miles in breadth.

Yet it is not, as it appears to be, a flat continuous surface.  It is
in reality made up of an infinite number of tiny satellites, mere dust
or pebbles for the most part, but so numerous as to look from our
distance like a continuous ring, or rather like three or four
concentric rings, for certain divisions have been noticed in it--an
inner broad division called after its discoverer, CASSINI, and an
outer, fainter, narrower one discovered by ENCKE.  The innermost part
of the ring is dusky, fainter {79} than the planet or the rest of the
ring, and is known as the "crape-ring."

Of Saturn itself we know little; it is further off and fainter than
Jupiter, and its details are not so pronounced, but in general they
resemble those of Jupiter.  The planet rotates quickly--in 10 h. 14
m.--its markings run into parallel belts, and are diversified by spots
of the same character as on Jupiter.  Saturn is probably possessed of
no small amount of native heat.

URANUS and NEPTUNE are much smaller bodies than Jupiter and Saturn,
though far larger than the Earth.  But their distance from the Earth
and Sun makes their discs small and faint, and they show little in the
telescope beyond a hint of "belts" like those of Jupiter; so that, as
with that planet, the surfaces that they show are almost certainly the
upper surfaces of a shell of cloud.

In general, therefore, the rule appears to hold good throughout the
solar system that a very large body is intensely hot and in a condition
of violent activity and rapid change; that smaller bodies are less hot
and less active, until we come down to the smallest, which are cold,
inert, and dead.  Our own Earth, midway in the series, is itself cold,
but is placed at such a distance from the Sun as to receive from it a
sufficient but not excessive supply of light and heat, and the changes
of the Earth are such as not to prohibit but to nourish and support the
growth and development of the various forms of life.

The smallest members of the solar system are known as METEORS.  These
are often no more than pebbles or particles of dust, moving together in
associated orbits round the Sun.  They are too small and too scattered
to be seen in open space, and become visible to us only {80} when their
orbits intersect that of the earth, and the earth actually encounters
them.  They then rush into our atmosphere at a great speed, and become
highly heated and luminous as they compress the air before them; so
highly heated that most are vapourised and dissipated, but a few reach
the ground.  As they are actually moving in parallel paths at the time
of one of these encounters, they appear from the effect of perspective
to diverge from a point, hence called the "+radiant+."  Some showers
occur on the same date of every year; thus a radiant in the
constellation Lyra is active about April 21, giving us meteors, known
as the "Lyrids"; and another in Perseus in August, gives us the
"Perseids."  Other radiants are active at intervals of several years;
the most famous of all meteoric showers, that of the "Leonids," from a
radiant in Leo, was active for many centuries every thirty-third year;
and another falling in the same month, November, came from a radiant in
Andromeda every thirteen years.  In these four cases and in some others
the meteors have been found to be travelling along the same path as a
comet.  It is therefore considered that meteoric swarms are due to the
gradual break up of comets; indeed the comet of the Andromeda shower,
known from one of its observers as "Biela's," was actually seen to
divide into two in December 1845, and has not been observed as a comet
since 1852, though the showers connected with it, giving us the meteors
known as the "Andromedes," have continued to be frequent and rich.
Meteors, therefore, are the smallest, most insignificant, of all the
celestial bodies; and the shining out of a meteor is the last stage of
its history--its death; after death it simply goes to add an
infinitesimal trifle to the dust of the earth.



{81}

CHAPTER VI

THE SYSTEM OF THE STARS

The first step towards our knowledge of the starry heavens was made
when the unknown and forgotten astronomers of 2700 B.C. arranged the
stars into constellations, for it was the first step towards
distinguishing one star from another.  When one star began to be known
as "the star in the eye of the Bull," and another as "the star in the
shoulder of the Giant," the heavens ceased to display an indiscriminate
crowd of twinkling lights; each star began to possess individuality.

The next step was taken when Hipparchus made his catalogue of stars
(129 B.C.), not only giving its name to each star, but measuring and
fixing its place--a catalogue represented to us by that of Claudius
Ptolemy (A.D. 137).

The third step was taken when BRADLEY, the third Astronomer Royal,
made, at Greenwich, a catalogue of more than 3000 star-places
determined with the telescope.

A century later ARGELANDER made the great Bonn Zone catalogue of
330,000 stars, and now a great photographic catalogue and chart of the
entire heavens have been arranged between eighteen observatories of
different countries.  This great chart when complete will probably
present 30 millions of stars in position and brightness.

{82}

The question naturally arises, "Why so many stars?  What conceivable
use can be served by catalogues of 30 millions or even of 3000 stars?"
And so far as strictly practical purposes are concerned, the answer
must be that there is none.  Thus MASKELYNE, the fifth Astronomer
Royal, restricted his observations to some thirty-six stars, which were
all that he needed for his _Nautical Almanac_, and these, with perhaps
a few additions, would be sufficient for all purely practical ends.

But there is in man a restless, resistless passion for knowledge--for
knowledge for its own sake--that is always compelling him to answer the
challenge of the unknown.  The secret hid behind the hills, or across
the seas, has drawn the explorer in all ages; and the secret hid behind
the stars has been a magnet not less powerful.  So catalogues of stars
have been made, and made again, and enlarged and repeated; instruments
of ever-increasing delicacy have been built in order to determine the
positions of stars, and observations have been made with
ever-increasing care and refinement.  It is knowledge for its own sake
that is longed for, knowledge that can only be won by infinite patience
and care.

The chief instrument used in making a star catalogue is called a
transit circle; two great stone pillars are set up, each carrying one
end of an axis, and the axis carries a telescope.  The telescope can
turn round like a wheel, in one direction only; it points due north or
due south.  A circle carefully divided into degrees and fractions of a
degree is attached to the telescope.

In the course of the twenty-four hours every star above the horizon of
the observatory must come at least once within the range of this
telescope, and at that moment the observer points the telescope to the
{83} star, and notes the time by his clock when the star crossed the
spider's threads, which are fitted in the focus of his eye-piece.  He
also notes the angle at which the telescope was inclined to the horizon
by reading the divisions of his circle.  For by these two--the time
when the star passed before the telescope and the angle at which the
telescope was inclined--he is able to fix the position of the star.

"But why should catalogues be repeated?  When once the position of a
star has been observed, why trouble to observe it again?  Will not the
record serve in perpetuity?"

The answers to these questions have been given by star catalogues
themselves, or have come out in the process of making them.  The Earth
rotates on its axis and revolves round the Sun.  But that axis also has
a rolling motion of its own, and gives rise to an apparent motion of
the stars called +Precession+.  Hipparchus discovered this effect while
at work on his catalogue, and our knowledge of the amount of Precession
enables us to fix the date when the constellations were designed.

Similarly, Bradley discovered two further apparent motions of the
stars--+Aberration+ and +Nutation+.  Of these, the first arises from
the fact that the light coming from the stars moves with an
inconceivable speed, but does not cross from star to Earth instantly;
it takes an appreciable, even a long, time to make the journey.  But
the Earth is travelling round the Sun, and therefore continually
changing its direction of motion, and in consequence there is an
apparent change in the direction in which the star is seen.  The change
is very small, for though the Earth moves 18-½ miles in a second, light
travels 10,000 times as fast.  Stars therefore are deflected from their
true positions by Aberration, by {84} an extreme amount of 20.47" of
arc, that being the angle shown by an object that is slightly more
distant than 10,000 times its diameter.

The axis of the Earth not only rolls on itself, but it does so with a
slight staggering, nodding motion, due to the attractions of the Sun
and Moon, known as +Nutation+.  And the axis does not remain fixed in
the solid substance of the Earth, but moves about irregularly in an
area of about 60 feet in diameter.  The positions of the north and
south poles are therefore not precisely fixed, but move, producing what
is known as the +Variation of Latitude+.  Then star-places have to be
corrected for the effect of our own atmosphere, _i.e._ refraction, and
for errors of the instruments by which their places are determined.
And when all these have been allowed for, the result stands out that
different stars have real movement of their own--their +Proper Motions+.

No stars are really "fixed"; the name "+fixed stars+" is a tradition of
a time when observation was too rough to detect that any of the
heavenly bodies other than the planets were in motion.  But nothing is
fixed.  The Earth on which we stand has many different motions; the
stars are all in headlong flight.

And from this motion of the stars it has been learned that the Sun too
moves.  When Copernicus overthrew the Ptolemaic theory and showed that
the Earth moves round the Sun, it was natural that men should be
satisfied to take this as the centre of all things, fixed and
immutable.  It is not so.  Just as a traveller driving through a wood
sees the trees in front apparently open out and drift rapidly past him
on either hand, and then slowly close together behind him, so Sir
WILLIAM HERSCHEL showed that the stars in one {85} part of the heavens
appear to be opening out, or slowly moving apart, while in the opposite
part there seems to be a slight tendency for them to come together, and
in a belt midway between the two the tendency is for a somewhat quicker
motion toward the second point.  And the explanation is the same in the
one case as in the other--the real movement is with the observer.  The
Sun with all its planets and smaller attendants is rushing onward,
onward, towards a point near the borders of the constellations Lyra and
Hercules, at the rate of about twelve miles per second.

Part of the Proper Motions of the stars are thus only apparent, being
due to the actual motion of the Sun--the "+Sun's Way+," as it is
called--but part of the Proper Motions belong to the stars themselves;
they are really in motion, and this not in a haphazard, random manner.
For recently KAPTEYN and other workers in the same field have brought
to light the fact of +Star-Drift+, _i.e._ that many of the stars are
travelling in associated companies.  This may be illustrated by the
seven bright stars that make up the well-known group of the "Plough,"
or "Charles's Wain," as country people call it.  For the two stars of
the seven that are furthest apart in the sky are moving together in one
direction, and the other five in another.

Another result of the close study of the heavens involved in the making
of star catalogues has been the detection of DOUBLE STARS--stars that
not only appear to be near together but are really so.  Quite a
distinct and important department of astronomy has arisen dealing with
the continual observation and measurement of these objects.  For many
double stars are in motion round each other in obedience to the law of
gravitation, and their orbits have been computed.  {86} Some of these
systems contain three or even four members.  But in every case the
smaller body shines by its own light; we have no instance in these
double stars of a sun attended by a planet; in each case it is a sun
with a companion sun.  The first double star to be observed as such was
one of the seven stars of the Plough.  It is the middle star in the
Plough handle, and has a faint star near it that is visible to any
ordinarily good sight.

Star catalogues and the work of preparing them have brought out another
class--VARIABLE STARS.  As the places of stars are not fixed, so
neither are their brightnesses, and some change their brightness
quickly, even as seen by the naked eye.  One of these is called
+Algol+, _i.e._ the Demon Star, and is in the constellation Perseus.
The ancient Greeks divided all stars visible to the naked eye into six
classes, or "+magnitudes+," according to their brightness, the
brightest stars being said to be of the first magnitude, those not
quite so bright of the second, and so on.  Algol is then usually
classed as a star of the second magnitude, and for two days and a half
it retains its brightness unchanged.  Then it begins to fade, and for
four and a half hours its brightness declines, until two-thirds of it
has gone.  No further change takes place for about twenty minutes,
after which the light begins to increase again, and in another four and
a half hours it is as bright as ever, to go through the same changes
again after another interval of two days and a half.

Algol is a double star, but, unlike those stars that we know under that
name, the companion is dark, but is nearly as large as its sun, and is
very close to it, moving round it in a little less than three days.  At
one point of its orbit it comes between Algol and the Earth, {87} and
Algol suffers, from our point of view, a partial eclipse.

There are many other cases of variable stars of this kind in which the
variation is caused by a dark companion moving round the bright star,
and eclipsing it once in each revolution; and the diameters and
distances of some of these have been computed, showing that in some
cases the two stars are almost in contact.  In some instances the
companion is a dull but not a dark star; it gives a certain amount of
light.  When this is the case there is a fall of light twice in the
period--once when the fainter star partly eclipses the brighter, once
when the brighter star partly eclipses the fainter.

But not all variable stars are of this kind.  There is a star in the
constellation Cetus which is sometimes of the second magnitude, at
which brightness it may remain for about a fortnight.  Then it will
gradually diminish in brightness for nine or ten weeks, until it is
lost to the unassisted sight, and after six months of invisibility it
reappears and increases during another nine or ten weeks to another
maximum.  "Mira," _i.e._ wonderful star, as this variable is called, is
about 1000 times as bright at maximum as at minimum, but some maxima
are fainter than others; neither is the period of variation always the
same.  It is clear that variation of this kind cannot be caused by an
eclipse, and though many theories have been suggested, the
"+long-period variables+," of which Mira is the type, as yet remain
without a complete explanation.

More remarkable still are the "NEW STARS"--stars that suddenly burst
out into view, and then quickly fade away, as if a beacon out in the
stellar depths had suddenly been fired.  One of these suggested to
Hipparchus the need for a catalogue of the {88} stars; another, the
so-called "Pilgrim Star," in the year 1572 was the means of fixing the
attention of Tycho Brahe upon astronomy; a third in 1604 was observed
and fully described by Kepler.  The real meaning of these "new," or
"temporary," stars was not understood until the spectroscope was
applied to astronomy.  They will therefore be treated in the volume of
this series to be devoted to that subject.  It need only be mentioned
here that their appearance is evidently due to some kind of collision
between celestial bodies, producing an enormous and instantaneous
development of light and heat.

These New Stars do not occur in all parts of the heavens.  Even a hasty
glance at the sky will show that the stars are not equally scattered,
but that a broad belt apparently made up of an immense number of very
small stars divides them into two parts.

THE MILKY WAY, or GALAXY, as this belt is called, bridges the heavens
at midnight, early in October, like an enormous arch, resting one foot
on the horizon in the east, and the other in the west, and passing
through the "+Zenith+," _i.e._ the point overhead.  It is on this belt
of small stars--on the Milky Way--that New Stars are most apt to break
out.

The region of the Milky Way is richer in stars than are the heavens in
general.  But it varies itself also in richness in a remarkable degree.
In some places the stars, as seen on some of the wonderful photographs
taken by E. E. BARNARD, seem almost to form a continuous wall; in other
places, close at hand, barren spots appear that look inky black by
contrast.  And the +Star Clusters+, stars evidently crowded together,
are frequent in the Milky Way.

And yet again beside the stars the telescope reveals {89} to us the
NEBULÆ.  Some of these are the Irregular Nebulæ--wide-stretching,
cloudy, diffused masses of filmy light, like the Great Nebula in Orion.
Others are faint but more defined objects, some of them with small
circular discs, and looking like a very dim Uranus, or even like
Saturn--that is to say, like a planet with a ring round its equator.
This class are therefore known as "+Planetary Nebulæ+," and, when
bright enough to show traces of colour, appear green or greenish blue.

These are, however, comparatively rare.  Other of these faint, filmy
objects are known as the "+White Nebulæ+," and are now counted by
thousands.  They affect the spiral form.  Sometimes the spiral is seen
fully presented; sometimes it is seen edgewise; sometimes more or less
foreshortened, but in general the spiral character can be detected.
And these White Nebulæ appear to shun the Galaxy as much as the
Planetary Nebula; and Star Clusters prefer it; indeed the part of the
northern heavens most remote from the Milky Way is simply crowded with
them.

It can be by no accident or chance that in the vast edifice of the
heavens objects of certain classes should crowd into the belt of the
Milky Way, and other classes avoid it; it points to the whole forming a
single growth, an essential unity.  For there is but one belt in the
heavens, like the Milky Way, a belt in which small stars, New Stars,
and Planetary Nebulæ find their favourite home; and that belt encircles
the entire heavens; and similarly that belt is the only region from
which the White Nebulæ appear to be repelled.  The Milky Way forms the
foundation, the strong and buttressed wall of the celestial building;
the White Nebulæ close in the roof of its dome.

{90}

And how vast may that structure be--how far is it from wall to wall?

That, as yet, we can only guess.  But the stars whose distances we can
measure, the stars whose drifting we can watch, almost infinitely
distant as they are, carry us but a small part of the way.  Still, from
little hints gathered here and there, we are able to guess that, though
the nearest star to us is nearly 300,000 times as far as the Sun, yet
we must overpass the distance of that star 1000 times before we shall
have reached the further confines of the Galaxy.  Nor is the end in
sight even there.

This is, in briefest outline, the Story of Astronomy.  It has led us
from a time when men were acquainted with only a few square miles of
the Earth, and knew nothing of its size and shape, or of its relation
to the moving lights which shone down from above, on to our present
conception of our place in a universe of suns of which the vastness,
glory, and complexity surpass our utmost powers of expression.  The
science began in the desire to use Sun, Moon, and stars as timekeepers,
but as the exercise of ordered sight and ordered thought brought
knowledge, knowledge began to be desired, not for any advantage it
might bring, but for its own sake.  And the pursuit itself has brought
its own reward in that it has increased men's powers, and made them
keener in observation, clearer in reasoning, surer in inference.  The
pursuit indeed knows no ending; the questions to be answered that lie
before us are now more numerous than ever they have been, and the call
of the heavens grows more insistent:

  "LIFT UP YOUR EYES ON HIGH."



{91}

BOOKS TO READ


POPULAR GENERAL DESCRIPTIONS:--

  Sir R. S. Ball.--_Star-Land_. (Cassell.)
  Agnes Giberne.---Sun, Moon and Stars_. (Seeley.)
  W. T. Lynn.--_Celestial Motions_. (Stanford.)
  A. & W. Maunder.---The Heavens and their Story_. (Culley.)
  Simon Newcomb.--_Astronomy for Everybody_. (Isbister.)


FOR BEGINNERS IN OBSERVATION:--

  W. F. Denning.--_Telescopic Work for Starlight Evenings_.
      (Taylor & Francis.)
  E. W. Maunder.--_Astronomy without a Telescope_. (Thacker.)
  Arthur P. Norton.--_A Star Atlas and Telescopic Handbook_.
      (Gall & Inglis.)
  Garrett P. Serviss.--_Astronomy with an Opera-Glass_.
      (Appleton.)


STAR-ATLASES:--

  Rev. J. Gall--_An Easy Guide to the Constellations_. (Gall
      and Inglis.)
  E. M'Clure and H. J. Klein.--_Star-Atlas_. (Society for
      Promoting Christian Knowledge.)
  R. A. Proctor.--_New Star Atlas_. (Longmans.)


ASTRONOMICAL INSTRUMENTS AND METHODS:--

  Sir G. B. Airy.--_Popular Astronomy; Lectures delivered at
      Ipswich_. (Macmillan.)
  E. W. Maunder.--_Royal Observatory, Greenwich; its History
      and Work_.  (Religious Tract Society.)

{92}

GENERAL TEXT-BOOKS:--

  Clerke, Fowler & Gore.--Concise Astronomy. (Hutchinson.)
  Simon Newcomb.--Popular Astronomy. (Macmillan.)
  C. A. Young.--Manual of Astronomy. (Ginn.)


SPECIAL SUBJECTS:--

  Rev. E. Ledger.--_The Sun; its Planets and Satellites_. (Stanford.)
  C. A. Young.--_The Sun_. (Kegan Paul.)
  Mrs. Todd.--_Total Eclipses_. (Sampson Low.)
  Nasmyth and Carpenter.--_The Moon_. (John Murray.)
  Percival Lowell.--_Mars_. (Longmans.)
  Ellen M. Clerke.--_Jupiter_. (Stanford.)
  E. A. Proctor.--_Saturn and its System_. (Longmans.)
  W. T. Lynn.--_Remarkable Comets_. (Stanford.)
  E. W. Maunder.--_The Astronomy of the Bible_. (Hodder and Stoughton.)


HISTORICAL:--

  W. W. Bryant.--_History of Astronomy_. (Methuen.)
  Agnes M. Clerke.--_History of Astronomy in the Nineteenth
      Century_. (A. & C. Black.)
  George Forbes.--_History of Astronomy_. (Watts.)


BIOGRAPHICAL:--

  Sir E. S. Ball.--_Great Astronomers_. (Isbister.)
  Agnes M. Clerke.--_The Herschels and Modern Astronomy_. (Cassell.)
  Sir O. Lodge.--_Pioneers of Science_. (Macmillan.)



{93}

INDEX


  ABERRATION, 83
  "Achilles" (Minor planet), 38
  Adams, John C., 39
  Airy, 39
  "Algol," 86
  "Andromedes" (Meteors), 80
  Apsides, 24, 28
  Argelander, 81


  BARNARD, E. E., 88
  "Bear," The, 14
  Biela's Comet, 80
  Bouvard, 39
  Bradley, 81, 83
  Bremiker, 40


  CATALOGUES (star), 81-83
  Centauri, Alpha, 53
  "Ceres" (Minor planet), 38
  Challis, 40
  Charles II., 50
  Chromosphere, 73
  Chronometer, 50
  Clairaut, 36
  Columbus, 48
  Comets, 36
  Comet, Halley's, 37
  ---- Biela's, 80
  Conic Sections, 34
  Constellations, the, 15
  ---- date of, 16
  Cook, Capt., 50
  Copernicus, 26, 54, 84
  "Copernicus" (Lunar crater), 59, 60
  Corona, 73
  Cowell, 37
  Crommelin, 37


  DEGREES, 43
  Dollond, 47
  Double stars, 85


  EARTH, form of, 16
  ---- size of, 17, 33
  Eclipses, 72
  Ecliptic, 21
  Ellipse, 28
  Epicycle, 25
  Eratosthenes, 17
  "Eros" (Minor planet), 38, 52
  Eudoxus, 21
  Excentric, 24
  Eye-piece, 45


  FACULÆ, 70
  Flamsteed, 50


  GALILEO, 44
  Galle, 40
  Gascoigne, 46
  Gravitation, Law of, 34


  HALL, CHESTER MOOR, 47
  Halley, 36
  Halley's Comet, 37
  Harrison, John, 50
  Herschel, Sir W., 37, 47, 84
  Hipparchus, 24, 81, 83, 87
  Hyperbola, 34


  JOB, Book of, 12, 14
  "Juno" (Minor planet), 38
  Jupiter, 18, 32, 77-78


  KAPTEYN, 85
  Kepler, 28, 44, 88
  Kepler's Laws, 29
  "Kepler" (Lunar crater), 59


  LANGLEY, 74
  Latitude, Variation of, 84
  "Leonids" (Meteors), 80
  Leverrier, 39
  Lowell, 63, 64
  "Lyrids" (Meteors), 80


  MAGNETIC STORM, 76
  Magnetism, Earth's, 76
  Magnitudes of stars, 86
  "Mare Imbrium," 59
  Mars, 18, 52, 62-66
  ---- Canals of, 63
  Maskelyne, 50, 82
  Maunder, Mrs. Walter, 72, 74
  Mercury, 17, 18, 27, 32, 66-67
  Meteors, 79, 80
  Micrometer, 46
  Milky Way, 53, 88
  Minor Planets, 38, 52
  Minutes of arc, 44
  "Mira," 87
  Moon, 11, 14, 21, 32, 33, 49, 55-62
  ---- distance of, 51


  "_Nautical Almanac_," 50, 82
  Navigation, 49
  Nebulæ, 89
  Neptune, 40, 79
  Newcomb, 65
  New stars, 87
  Newton, 29, 31, 47
  Newton's Laws of motion, 31
  Nodes, 35
  Nutation, 83, 84


  "OASES of Mars," 64
  Obelisks, 42
  Object glass, 45
  Observatories, Berlin, 50
  ---- Copenhagen, 50
  ---- Greenwich, 50
  ---- Mt. Wilson, 48
  ---- Paris, 50
  ---- Pulkowa, 50
  ---- St. Petersburg, 50
  ---- Washington, 50
  ---- Yerkes, 47


  "PALLAS" (Minor planet), 38
  Parabola, 34
  "Perseids" (Meteors), 80
  Photography, 46
  Photosphere, 69
  "Pilgrim" star, 88
  Piazzi, 38
  Planets, 17
  Pole of the Heavens, 13
  Pontécoulant, 37
  Precession of the Equinoxes, 36, 83
  "_Principia_," 36
  Prominences, 73
  "Ptolemæus" (Lunar crater), 60
  Ptolemy, 24, 81


  RADIANT POINTS, 80
  Radius Vector, 28
  Reflectors, 47
  Refractors, 47


  SATURN, 18, 78-79
  Schiaparelli, 63
  Schwabe, 69
  Seconds of arc, 44
  Sirius, 53
  Solar System, Tables of, 56-58
  Somerville, Mrs., 89
  Spheres, Planetary, 21
  Spörer, 71
  Spörer's Law, 71
  Star catalogues, 81-83
  ---- clusters, 88
  ---- drift, 85
  Stars, fixed, 84
  ---- proper motions of, 84
  Sun, 11, 12, 14, 21, 32, 67-77
  ---- distance of, 51
  ---- dials, 43
  Sun spots, 69
  ---- spot maximum, 71
  ---- ---- minimum, 71
  "Sun's Way," 85


  TELESCOPE, Invention of, 45
  Transit Circle, 82
  Tycho Brahe, 27, 44, 88
  "Tycho" (Lunar crater), 59, 60, 61


  URANUS, 38, 79


  VARIABLE stars, 86
  ---- ----, Long period, 87
  Venus, 18, 27, 67
  "Vesta" (Minor planet), 38


  YOUNG, C. A., 74


  ZENITH, 17, 88
  Zodiac, Signs of, 14, 15, 16, 43



  Printed by BALLANTYNE, HANSON & Co.
  Edinburgh & London



      *      *      *      *      *



  "We have nothing but the highest praise for these
  little books, and no one who examines them will have
  anything else."--_Westminster Gazette_, 22nd June 1912.


THE PEOPLE'S BOOKS

THE FIRST NINETY VOLUMES

The volumes issued are marked with an asterisk


SCIENCE

  1. The Foundations of Science . . . By W. C. D. Whetham, M.A., F.R.S.
  2. Embryology--The Beginnings of Life . . . By Prof. Gerald Leighton, M.D.
  3. Biology . . . By Prof. W. D. Henderson, M.A.
  4. Zoology: The Study of Animal Life . . . By Prof. E. W. MacBride,
             M.A., F.R.S.
  5. Botany; The Modern Study of Plants . . . By M. C. Stopes, D.Sc.,
             Ph.D., F.L.S.
  6. Bacteriology . . . By W. E. Carnegie Dickson, M.D.
  7. The Structure of the Earth . . . By Prof. T. G. Bonney, F.R.S.
  8. Evolution . . . By E. S. Goodrich, M.A., F.R.S.
  9. Darwin . . . By Prof. W. Garstang, M.A., D.Sc.
  10. Heredity . . . By J. A. S. Watson, B.Sc.
  11. Inorganic Chemistry . . . By Prof. E. C. C. Baly, F.R.S.
  12. Organic Chemistry . . . By Prof. J. B. Cohen, B.Sc., F.R.S.
  13. The Principles of Electricity . . . By Norman K. Campbell, M.A.
  14. Radiation . . . By P. Phillips, D.Sc.
  15. The Science of the Stars . . . By E. W. Maunder, F.R.A.S.
  16. The Science of Light . . . By P. Phillips, D.Sc.
  17. Weather Science . . . By R. G. K. Lempfert, M.A.
  18. Hypnotism and Self-Education . . . By A. M. Hutchison, M.D.
  19. The Baby: A Mother's Book . . . By a University Woman.
  20. Youth and Sex--Dangers and Safeguards for Boys and Girls . . .
             By Mary Scharlieb, M.D., M.S., and F. Arthur Sibly, M.A., LL.D.
  21. Marriage and Motherhood . . . By H. S. Davidson, M.B., F.R.C.S.E.
  22. Lord Kelvin . . . By A. Russell, M.A., D.Sc., M.I.E.E.
  23. Huxley . . . By Professor G. Leighton, M.D.
  24. Sir William Huggins and Spectroscopic Astronomy . . .
             By E. W. Maunder, F.R.A.S., of the Royal Observatory, Greenwich.
  62. Practical Astronomy . . . By H. Macpherson, Jr., F.R.A.S.
  63. Aviation . . . By Sydney F. Walker, R.N.
  64. Navigation . . . By William Hall, R.N., B.A.
  65. Pond Life . . . By E. C. Ash, M.R.A.C.
  66. Dietetics . . . By Alex. Bryce, M.D., D.P.H.

PHILOSOPHY AND RELIGION

  25. The Meaning of Philosophy . . . By Prof. A. E. Taylor, M.A., F.B.A.
  26. Henri Bergson . . . By H. Wildon Carr, Litt.D.
  27. Psychology . . . By H. J. Watt, M.A., Ph.D., D.Phil.
  28. Ethics . . . By Canon Rashdall, D.Litt., F.B.A.
  29. Kant's Philosophy . . . By A. D. Lindsay, M.A.
  30. The Teaching of Plato . . . By A. D. Lindsay, M.A.
  67. Aristotle . . . By Prof. A. E. Taylor, M.A., F.B.A.
  68. Friedrich Nietzsche . . . By M. A. Mügge.
  69. Eucken: A Philosophy of Life . . . By A. J. Jones, M.A., B.Sc., Ph.D.
  70. The Experimental Psychology of Beauty . . . By C. W. Valentine,
             B.A., D.Phil.
  71. The Problem of Truth . . . By H. Wildon Carr, Litt.D.
  31. Buddhism . . . By Prof. T. W. Rhys Davids, M.A., F.B.A.
  32. Roman Catholicism . . . By H. B. Coxon.  Preface, Mgr. R. H. Benson.
  33. The Oxford Movement . . . By Wilfrid Ward.
  34. The Bible and Criticism . . . By W. H. Bennett, D.D., Litt.P.,
             and W. F. Adeney, D.D.
  35. Cardinal Newman . . . By Wilfrid Meynell.
  72. The Church of England . . . By Rev. Canon Masterman.
  73. Anglo-Catholicism . . . By A. E. Manning Foster.
  74. The Free Churches . . . By Rev. Edward Shillito, M.A.
  75. Judaism . . . By Ephraim Levine, M.A.
  76. Theosophy . . . By Annie Besant.

HISTORY

  36. The Growth of Freedom . . . By H. W. Nevinson.
  37. Bismarck and the Origin of the German Empire . . .
             By Professor F. M. Powicke.
  38. Oliver Cromwell . . . By Hilda Johnstone, M.A.
  39. Mary Queen of Scots . . . By E. O'Neill, M.A.
  40. Cecil John Rhodes, 1853-1902 . . . By Ian D. Colvin.
  41. Julius Cæsar . . . By Hilary Hardinge.
  42. England in the Making . . . By Prof. F. J. C. Hearnshaw, M.A., LL.D.
  43. England in the Middle Ages . . . By E. O'Neill, M.A.
  44. The Monarchy and the People . . . By W. T. Waugh, M.A.
  45. The Industrial Revolution . . . By Arthur Jones, M.A.
  46. Empire and Democracy . . . By G. S. Veitch, M.A., Litt.D.
  61. Home Rule . . . By L. G. Redmond Howard.
             Preface by Robert Harcourt, M.P.
  77. Nelson . . . By H. W. Wilson.
  78. Wellington and Waterloo . . . By Major G. W. Redway.

SOCIAL AND ECONOMIC

  47. Women's Suffrage . . . By M. G. Fawcett, LL.D.
  48. The Working of the British System
             of Government to-day . . . By Prof. Ramsay Muir, M.A.
  49. An Introduction to Economic Science . . . By Prof H. O. Meredith. M.A.
  50. Socialism . . . By B. B. Kirkman, B.A.
  79. Mediæval Socialism . . . By Bede Jarrett, O.P., M.A.
  80. Syndicalism . . . By J. H. Harley, M.A.
  81. Labour and Wages . . . By H. M. Hallsworth, M.A., B.Sc.
  82. Co-operation . . . By Joseph Clayton.
  83. Insurance as a Means of Investment . . . By W. A. Robertson, F.F.A.
  92. The Training of the Child . . . By G. Spiller

LETTERS

  51. Shakespeare . . . By Prof. C. H. Herford, Litt.D.
  52. Wordsworth . . . By Rosaline Masson.
  53. Pure Gold--A Choice of Lyrics and Sonnets . . . by H. C. O'Neill
  54. Francis Bacon . . . By Prof. A. R. Skemp, M.A.
  55. The Brontës . . . By Flora Masson.
  56. Carlyle . . . By L. MacLean Watt.
  57. Dante . . . By A. G. Ferrers Howell.
  58. Ruskin . . . By A. Blyth Webster, M.A.
  59. Common Faults in Writing English . . . By Prof. A. R. Skemp, M.A.
  60. A Dictionary of Synonyms . . . By Austin K. Gray, B.A.
  84. Classical Dictionary . . . By Miss A. E. Stirling
  85. A History of English Literature . . . By A. Compton-Rickett, LL.D.
  86. Browning . . . By Prof. A. R. Skemp, M.A.
  87. Charles Lamb . . . By Flora Masson.
  88. Goethe . . . By Prof. C. H. Herford, Litt.D.
  89. Balzac . . . By Frank Harris
  90. Rousseau . . . By F. B. Kirkman, B.A.
  91. Ibsen . . . By Hilary Hardinge.
  93. Tennyson . . . By Aaron Watson


  LONDON AND EDINBURGH: T. C. & E. C. JACK
  NEW YORK: DODGE PUBLISHING CO.



[Transcriber's Note:

Italicized text is indicated with _underscores_.

Bold text is indicated with +plus signs+.

Numbers inside curly braces, e.g. {99} are page numbers.]





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