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Title: Sounding the Ocean of Air
Author: Rotch, A. Lawrence
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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                       _THE ROMANCE OF SCIENCE._
          --------------------------------------------------

                               SOUNDING
                          THE OCEAN OF AIR

                         _BEING SIX LECTURES_

             DELIVERED BEFORE THE LOWELL INSTITUTE OF BOSTON
                           IN DECEMBER 1898


                                  BY
                     A. LAWRENCE ROTCH, S.B., A.M.,

          DIRECTOR OF THE BLUE HILL METEOROLOGICAL OBSERVATORY,
                 MASSACHUSETTS, U.S.A., AND MEMBER
         OF THE INTERNATIONAL CLOUD AND AERONAUTICAL COMMITTEES.

               PUBLISHED UNDER THE DIRECTION OF THE GENERAL
                         LITERATURE COMMITTEE.


                               LONDON:
               SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE,
    NORTHUMBERLAND AVENUE, W.C.; 43, QUEEN VICTORIA STREET, E.C.

                      BRIGHTON: 129, NORTH STREET.

                     NEW YORK: E. & J. B. YOUNG & CO.

                                 1900



               This little Book is gratefully Dedicated

                                  TO

                     The late AUGUSTUS LOWELL, Esq.

                                  OF

                            BOSTON, U.S.A.

                 WHO, AS TRUSTEE OF THE LOWELL INSTITUTE,
                ENABLED SCIENTIFIC MEN OF TWO CONTINENTS TO
                PRESENT THE RESULTS OF THEIR INVESTIGATIONS
                             TO THE PUBLIC



CORRIGENDA


 Page 50, line 11, _for_ "isolation" _read_ "insolation."

 Page 59, line 21, _before_ "direction" _insert_ "opposite."

 Page 112, line 2; page 115, lines 2 and 15, and Index, pages
   175 and 183, _for_ "Viollé" _read_ "Violle."

 Page 112, line 23; page 113, line 6, and Index, page 181, _for_
   "Muntz" _read_ "Müntz."

 Page 123, last line, _for_ "1889" _read_ "1891."

 Index, page 177, _for_ "Cotte (T.)" _read_ "Cotte (L.)."

 Index, page 179, _after_ "Hann (J.), 36," _add_ "173."

 Index, page 179, _for_ "Hellman (G.)" _read_ "Hellmann (G.)."

 Index, page 181, _after_ "Langley (S. P.)" _insert_ "28."


   [Transcriber's Note: These corrections have been applied to the
   current version]



 CONTENTS


 CHAP.                                                          PAGE

   I. THE ATMOSPHERE--ANCIENT AND MODERN
        KNOWLEDGE--METHODS OF INVESTIGATION                        9

  II. CLOUDS--FORMATION AND CLASSIFICATION--MEASUREMENTS
        AT BLUE HILL--THE INTERNATIONAL OBSERVATIONS              38

 III. BALLOONS--NOTABLE ASCENTS AND RESULTS
        OBTAINED--CAPTIVE BALLOONS                                68

  IV. _BALLONS-SONDES_ FOR GREAT ALTITUDES--THE INTERNATIONAL
        ASCENTS                                                   98

   V. KITES--HISTORY AND APPLICATION TO METEOROLOGICAL
        PURPOSES AT BLUE HILL AND ELSEWHERE                      117

  VI. RESULTS OF THE KITE-FLIGHTS AT BLUE HILL--FUTURE WORK      145

      INDEX                                                      175



 LIST OF ILLUSTRATIONS


                                                                PAGE

    PLATE I. Comparative Altitudes                                22

   PLATE II. Optical Phenomena showing the Height of the
               Atmosphere                                         26

  PLATE III. Temperature at different Latitudes and Altitudes     31

     FIG. 1. Nephoscope at Blue Hill Observatory.                 52

     FIG. 2. Cloud Theodolite at Blue Hill Observatory            54

   PLATE IV. Heights and Velocities of Clouds                     57

    PLATE V. Atmospheric Circulation at different Heights in
               Cyclones and Anti-cyclones                         61

     FIG. 3. German Balloon equipped for Meteorological
               Observations                                       87

   PLATE VI. Temperatures observed in Four High Balloon Ascents   91

     FIG. 4. German Kite-balloon.                                 95

     FIG. 5. Baro-thermograph of Richard                         102

     FIG. 6. The _Aérophile_ rising                              103

  PLATE VII. Heights and Temperatures recorded in eight Ascents
               of the _Cirrus_                                   109

     FIG. 7. Oriental tailless Kites.                            119

     FIG. 8. Eddy tailless Kite                                  129

     FIG. 9. Hargrave Kite                                       130

    FIG. 10. Modified Hargrave Kites at Blue Hill                133

    FIG. 11. Lamson's Aero-curve Kite                            135

    FIG. 12. Meteorograph lifted by Kites at Blue Hill           138

 PLATE VIII. Meteorogram from the Kite-flight of Oct. 8, 1896,
               at Blue Hill                                      148

   PLATE IX. Mean Changes with Height, and Changes during the
               Kite-flight of Oct. 8, 1896                       150

    PLATE X. Changes with Height recorded by Kites at Blue Hill  155

   PLATE XI. Kite Observations at Blue Hill, Sept. 5-11, 1897    163

  PLATE XII. Automatic Records during a High Kite-flight
               at Blue Hill                                      166

 PLATE XIII. Results of Kite-flights at Blue Hill during an
               Anti-cyclone and a Cyclone                        168



                               SOUNDING
                           THE OCEAN OF AIR

                              ----------

                               CHAPTER I

   THE ATMOSPHERE--ANCIENT AND MODERN KNOWLEDGE--METHODS OF
   INVESTIGATION


Concerning this most important element in which we live and move and
have our being, Pliny, in the first century of our era, wrote as
follows: "It is time to consider the other marvels of the heavens;
thus our fathers called that immense space where flows the vital fluid
to which we give the name of air, and which is not apparent to the
senses because of its great rarity. There clouds form, thunder and
lightning also; it is the region of tempests and of whirlwinds; from
there fall rains, hail, and hoar frost; from there come all those
phenomena, astonishing and often disastrous, which follow the combat
of Nature with herself.... The sun's rays strike the earth on all
sides, warming and strengthening it; they are reflected and detach
all the particles they can carry away; vapours descend and rise again;
the winds come empty and return laden with spoil; animals breathe in
from above this vital fluid which animates them, and the earth sends
it back to its source as if she would fill the void by this means. So,
by Nature acting everywhere and in all directions there results an
apparent discord from which is born the harmony of the Universe; it is
this general movement which puts all things in their places; some are
preserved by the destruction of others; all move, all act, the
struggle is continual, if it ceased an instant everything would fall
into chaos...."

From the earliest times, as far back as history extends, we find
mankind interested in meteorological phenomena. This appears natural
if we consider the importance of the weather to the ancient pastoral
nations, which, from the open-air life and keen perceptive faculties
of their people, were well fitted to study natural phenomena. The
beauty and grandeur of many of the phenomena occurring in the
atmosphere, and the curiosity excited concerning their causes,
probably contributed to interest people in them. Meteorology appears
to have been first treated systematically, as distinct from astronomy
and astrology, by the Greek philosopher, Aristotle, more than 2000
years ago. The word "meteor," derived from the Greek "elevated," was
applied to certain phenomena having their origin in the atmosphere.
These were classified into aërial, aqueous, and luminous meteors, and
were all included in the term Meteorology. In his treatise by this
name Aristotle gave a more detailed account of them than any preceding
or contemporary writer, and Theophrastus, his pupil, wrote two books
on the winds and on the signs of rain, which have been translated into
Latin and English. About the same period Aratus incorporated the
current weather proverbs in his poem, _Diosemeia_. The Greek
historians and poets frequently alluded to atmospheric phenomena, and
their example was followed by the Romans, of whom Pliny has been
quoted.

No doubt the desire to ascend into the air always possessed man, but
owing to the awe with which mountains seem to have inspired the
ancients, there is rarely mention in their writings of climbing
mountains, or of the physiological effects which could hardly have
failed to be apparent upon high summits. Citing one of the few
existing narratives, Aristotle relates: "Those which ascend to the top
of the mountain Olympus could not keep themselves alive without
carrying with them wet sponges, by whose assistance they could respire
in that air otherwise too thin for respiration." This mountain of
less than 10,000 feet was said to be so high that it never rained on
its summit, where, it was supposed, the air was always still. A still
higher mountain, easily accessible to the ancient world, and which we
know was ascended, is Etna.

Concerning the progress of meteorology, from the time of the ancient
Romans to the revival of knowledge in Europe, there is little to say
except that during the middle ages meteorology, like other learning,
was confined to the monasteries. Speculations were current as to the
extent of the atmosphere until, in the middle of the eleventh century,
Alhazen, a learned Arab, computed from the duration of twilight that
the atmosphere extended nineteen leagues above the earth. The same
method was applied with more precision by Tycho Brahe, Kepler, and
other astronomers of the sixteenth and seventeenth centuries. The
earliest weather chronicles were probably noted by monks from time to
time in almanacks or missals, although when this was done first we do
not know. The oldest daily chronicles of the weather extant are those
kept by William Merle in Oxford from 1337 to 1344. We owe it to the
late Mr. Symons, the English meteorologist and bibliophile, that this
MS. and many other old records have been brought to light and
published. Dr. Hellmann has done even more in Germany, and this
historical research is evidence of the growing importance of the
science of meteorology.

With the advent of the age of geographical discovery it was seen that
the climatic features of our globe depend chiefly upon distance from
the equator, proximity to the ocean, and height above it. In the
tropics especially, the luxuriant vegetation, which diminishes on
mountain slopes and higher up gives place to snow, must have been
visible proof of the decrease of temperature with altitude, for, as
Professor Daniell remarked, mountains are a gigantic registering
thermometer having for the freezing-point the line of perpetual snow.
The invention of instruments for measuring temperature and barometric
pressure made possible the quantitative observations that have
supplied the data for deducing the laws governing the atmosphere. The
oldest meteorological instrument is, no doubt, the weather or
wind-vane, which had its origin before the Christian era. The next
oldest is the hygrometer, or instrument for measuring moisture in the
air, the form which acts by absorption dating from the middle of the
fifteenth century, and the condensation hygrometer being a century
younger. Next in chronological order comes the rain-gauge, which
appears to have been used by Castelli, a friend of Galileo, in the
year 1639. The history of that important instrument, the thermometer,
is obscure, but it is certain that Galileo in Padua used an
air-thermometer in the latter part of the sixteenth century, which
Rey, a French physician, filled with liquid in 1631. This thermometer,
as well as other physical instruments, was perfected by members of the
Accademia del Cimento at Florence. These instruments are described in
_Saggi di Naturali Esperienza_, written in 1666, and translated into
Latin and English. The Florentine thermometers had one fixed point,
that of freezing water, and contained either spirits or mercury. In
1724 Fahrenheit, in Danzig, fixed three points on the scale of the
mercurial thermometer, viz. the cold produced by ice and sal-ammoniac
which he called 0°, freezing water or 32°, and the heat of the human
blood which he assumed to be 96°. This thermometric scale, having 180°
between freezing and boiling water, and that of Celsius, with 100°,
are the only ones in scientific use to-day. It is a remarkable fact in
the history of thermometers that neither of these thermometers
remained in the country where it was invented; thus the thermometer of
Fahrenheit, a German, came into use exclusively in England and her
colonies, while that of Celsius, a Swede, is now used on the continent
of Europe except in Germany, where the thermometer of Réaumur, a
Frenchman, is still in popular use. Of the four fundamental
meteorological instruments, the barometer was the last invented.
Aristotle had suspected that air had weight, but it was not
demonstrated until the middle of the seventeenth century, when the old
axiom "that Nature abhors a vacuum" was replaced by the rational
explanation, given by Galileo and Torricelli, his pupil, why water
will not rise in a suction pump more than thirty-two feet. In 1643
Torricelli executed this famous experiment: he took a glass tube,
sealed at one end, and filled it with mercury, then, closing the open
end with his finger, he inverted it in a basin of mercury. The mercury
fell to about thirty inches, which was recognized to be the weight of
a column having the area of the tube and of the height of the
atmosphere. The application of the barometer was due to Blaise Pascal,
who repeated at Rouen Torricelli's experiment with a much longer tube
filled with water, which being thirteen times lighter than mercury,
stood thirteen times higher, or thirty-two feet, in the tube. Pascal,
being himself at Paris in 1648, got his brother-in-law Perier to carry
a barometric tube filled with mercury to the top of the Puy de Dôme, a
mountain in Auvergne rising about 3500 feet above the city of
Clermont. The mercury fell in the tube with the ascent, and at the top
of the mountain it stood some three inches lower than at the base,
showing that the lower layers of the atmosphere are denser than the
upper. Pascal repeated the experiment on the Tower of St. Jacques in
Paris, and it is interesting to note that more than two hundred years
afterwards, meteorological stations were established both there and on
the Puy de Dôme. It was soon perceived that not only did the level of
the mercury in the tube change with height, but that it oscillated
continually at the same place, and from its observed relation to the
state of the weather its name "weather-glass" is derived. In 1650 the
weight of the air was demonstrated in another manner by Otto von
Guericke, burgomaster of Magdeburg, who by means of an air-pump of his
invention performed the experiment, which Aristotle had tried
unsuccessfully, of weighing a vessel full of air and the same vessel
exhausted of air. He also showed the pressure of the air in all
directions by the famous experiment of the Magdeburg hemispheres,
which, being hollow, were placed together, and after the air was
exhausted from the sphere so formed sixteen horses were unable to pull
them apart. Soon afterwards Robert Boyle experimented further upon the
weight and "spring of the air," as he called it, and gave the name to
the barometer. Both Boyle in England and Mariotte in France discovered
the law, bearing indifferently their names, that the pressure of gases
is proportional to their density. Halley, a few years later, showed
that the rate of decrease in pressure differed from the rate of
increase in height, and developed formulæ for measuring heights by the
barometer, which were afterwards perfected by Laplace. Knowing the
heights of the barometer at a high and at a low-level station, and the
mean temperature of the air lying between them, it is possible to
compute accurately the difference of height of the two stations, or,
conversely, given this height, the difference in barometric pressure
can be calculated. By the middle of the seventeenth century the most
important meteorological instruments had been invented, and not only
can Italy claim to be their birthplace, but the Grand Duke Ferdinand
II., whose brother Leopold founded the Accademia del Cimento,
distributed the new instruments in Italy and even beyond the Alps, so
that in 1654 observations several times a day were begun at a dozen
stations. The observations in Florence from 1650 to 1670 were
preserved and constitute the commencement of instrumental meteorology.

It was the conquest of Peru which, by leading men over the high passes
of the Andes, first brought them to great heights, but although we
find mention in the history of the expeditions of the so-called
mountain sickness, caused by fatigue as well as by cold and rarefied
air, it does not appear that scientific observations were made.
Therefore, while it must be assumed that the atmospheric conditions at
considerable altitudes were familiar to travellers, yet not until the
middle of the last century did Bouguer, one of three French
Academicians sent to Peru on a geodetic mission, fix the height of the
freezing point in various latitudes, after observing that the
temperature fell below freezing at night upon the mountains near the
equator. During the latter part of the century, Kirwan, an English
chemist, calculated the temperature for various parallels of latitude,
and in 1817 Alexander von Humboldt, after a voyage around the world,
published his isothermal lines, or lines of equal temperature on the
surface of the globe, by which he showed that the deviation from the
normal, or calculated, temperature arose from the distribution of land
and water, and from the geographical relief of the former. This work
of von Humboldt formed the basis of all subsequent studies in
comparative climatology. Meanwhile chemistry had kept pace with
physics, and in 1774 the old theory, that air was one of the four
elements from which all things originated, was rendered untenable by
Priestley, who proved that oxygen gas, which he discovered, was a
constituent part of air. The other constituent, nitrogen, formerly
called azote from its destructiveness to life, was discovered soon
afterwards, and its proportion in the air determined by the French
chemist, Lavoisier.

In 1783 man became possessed of the long-sought-for means of rising
freely in the air, and he speedily availed himself of it. The first
balloons, filled with heated air, were called _Montgolfières_ from the
inventors, the brothers Montgolfier, living in Annonay, France. After
animals had been sent up attached to one, Pilâtre de Rozier ventured
to ascend in the aerostatic machine, first tethered captive but then
set free, and before the close of the year a balloon, filled with
hydrogen gas, or "inflammable air" as it was called, carried M.
Charles 9000 feet above Paris. During more than a century the balloon
has been the most important agent for the exploration of the
atmosphere, and yet, notwithstanding the courage and devotion to
science of the early aeronauts, their ascents with unsuitable
instruments furnished much discordant and erroneous data. Some of the
most remarkable balloon voyages and the modern methods of sounding and
dredging the atmosphere, to borrow terms from the exploration of the
ocean, will be described in two future chapters.

Perhaps the chief reason for the slow progress of meteorology to the
status of a science is the variable character of its phenomena with
the place of observation. In this respect it differs from astronomy,
which was more easily cultivated in the restricted ancient world. Only
after many years of observation at different places had contributed a
foundation for climatology was it realized that man, in his relation
to the atmosphere, resembled marine organisms confined to the bottom
of the ocean, and that in order to discover the true conditions of the
atmosphere it was necessary to observe them at considerable heights.
In the last century the highest point at which physical observations
had been made was the summit of Mont Blanc, less than 16,000 feet
above the sea. The ascent of this mountain was first accomplished in
1787 by H. B. De Saussure and his guides with much difficulty and
suffering, and the observations, abridged and rendered less accurate
by the fatigue and sickness of De Saussure, were also influenced by
the proximity of the mountain itself. In 1802 von Humboldt and
Bonpland reached a height of about 18,000 feet in the Andes, where
they made important observations. The ascent of man was rapid during
the first years of the nineteenth century, for in 1804 Gay-Lussac rose
in a balloon, without exertion or discomfort, to the height of 23,000
feet, and there made observations which were assumed to give the true
atmospheric conditions. After an active campaign the conquest of the
air by balloons was temporarily abandoned, and the field was left free
to the mountaineer. But to-day supremacy rests with the aeronaut, for
no one has succeeded in getting higher than 24,000 feet on a mountain,
while the aeronaut has exceeded this altitude by a mile without great
hardship, and lately has sent his unmanned balloons twice as high as
the loftiest mountains.

Plate I., headed The Exploration of the Atmosphere, represents a
vertical section of the lower portion of our atmosphere. On the right
is a scale of miles above the sea, and on the left is a scale of
barometric pressures corresponding to the height. The right-hand half
of the diagram shows the eastern hemisphere with the Himalaya
mountains, the left-hand half the western hemisphere with the Andes.
There are seen the heights of the different kinds of clouds, measured
at Blue Hill, as described in the next chapter; the highest
meteorological stations, those on Mont Blanc and El Misti in Peru; the
highest permanently inhabited place, which is a monastery in Thibet;
and the greatest height to which man has climbed, namely, in the
Andes. The heights at which observations have been made in balloons,
carrying observers, or only recording instruments, may be compared
with the height attained by the Blue Hill kites, to be described
hereafter. Other altitudes can be noted, such as the height of the
snow-line on various mountains, and as a thousand-foot measure, the
Eiffel Tower in Paris, the tallest structure erected by man, may be
used.

   [Illustration: PLATE I.--COMPARATIVE ALTITUDES.]

The development of meteorological knowledge to the commencement of the
present century has now been traced, but before beginning the
consideration of the methods of exploring the atmosphere that form the
subject of the book, let us, in order to understand this work better,
review the general knowledge which we possess of our atmosphere as
regards its origin, composition, extent, and conditions of heat and
moisture. First, then, regarding the =Origin of the Atmosphere=, or
vapour envelope which the name means. According to the nebular
hypothesis of Laplace, our earth, like all existing suns and planets,
was condensed from clouds of nebulous matter and became a
highly-heated globular mass rotating, like every celestial body, from
west to east. As the earth cooled, a crust was formed, and many of the
substances that now exist in the earth were suspended as clouds in the
cooler atmosphere surrounding it. Eventually, these substances were
condensed upon the crust; the oxygen, especially, must have been
diminished by combining with the rocks, while the lighter gases, such
as hydrogen, may have escaped from the earth's atmosphere. No doubt,
when vegetable and animal life began, the earth's atmosphere was
denser than now and much richer in carbonic acid, which, during the
carboniferous period, was absorbed by plants, and is now imprisoned in
coal and limestone. Within historic times, however, there is no
evidence of any change in the composition of our atmosphere, or the
climatic conditions as a whole.

M. Jourdanet, a distinguished French physiologist, maintained that man
appeared on the earth at the close of the tertiary period, when the
barometric pressure at sea-level was, he supposed, about forty-three
inches, or nearly a half more than it is to-day, and owing to the
greater density of the air its temperature was also considerably
higher. Under these circumstances he believed that man first occupied
the high regions of Central Asia, and only emigrated to lower levels
when the climatic conditions became ameliorated. In other words, M.
Jourdanet believed in a literal "descent of man," but if this be true,
many of the race have returned to their birthplace, for to-day
millions of people dwell on the great Asiatic plateau, and on the
South American Cordillera, at an average altitude of 10,000 feet,
while a few live throughout the year at extreme heights of 15,000
feet.

=Composition of the Atmosphere.=--Dry air is a mixture of about
one-fifth of a volume of oxygen to four-fifths of a volume of
nitrogen, besides a very small quantity (3/10,000) of carbonic acid,
traces of ammonia, ozone, argon, and other recently discovered gases.
The oxygen consumed, and the carbonic acid given off by animal life
and by combustion, are maintained in this fixed proportion in the free
air by the absorption of the carbonic acid, and the setting free of
oxygen by vegetation. By diffusion and the mobility of the air, a
thorough mixture is effected, with the result that the fundamental
composition of our atmosphere is everywhere nearly the same. In the
lower atmosphere the vapour of water is present in a varying quantity,
in the average about one per cent. in weight, with a volume depending
on the temperature. Dust is always suspended in the atmosphere; the
coarser particles settle, but the finer ones, that come from
volcanoes, may float for a long time in the high atmosphere. Dust is
an important factor in the production of clouds and rain, and
occasions many optical phenomena.

   [Illustration: PLATE II.--OPTICAL PHENOMENA SHOWING THE HEIGHT
   OF THE ATMOSPHERE. ]

=Extent of the Atmosphere.=--If the atmosphere were incompressible and
had throughout the density that it has at the earth, its height would
be about five miles only, but actually it is composed of gases that
follow Boyle's law and vary in volume inversely as the pressure upon
them. Since the pressure decreases with height in a geometrical
progression, it would be halved for each three and a half miles of
ascent were the temperature constant, but as the temperature also
decreases with height, the successive intervals, beginning with three
and a half miles, become shorter because the volume of a gas depends
on its temperature as well as on the pressure upon it. The decrease of
pressure with increasing height above the earth is shown by the
left-hand scale of Plate I., already described, and the subsequent
diminution of density to the limits of our measurable atmosphere is
indicated on the right of Plate II., Optical Phenomena showing the
Height of the Atmosphere. The gases composing the atmosphere probably
extend to heights proportional to their density; viz. oxygen to about
thirty miles and nitrogen to thirty-five miles, although water-vapour
nearly disappears at twelve miles. From these considerations it is
supposed that the atmosphere, as measurable by the barometer, vanishes
at about thirty-eight miles, and this is about the height indicated by
twilight, which is the reflected light of the sun when 18° below the
horizon. After the great eruption of the volcano Krakatoa in the South
Seas in 1883, the brilliant sunset glows and the longer twilight
showed that the dust emitted by the eruption remained for more than a
year suspended at a height of at least sixty miles. The so-called
"luminous clouds" seen at night during the same period, and which were
probably these same dust particles still illumined by the sun, were
found by trigonometrical measurements to have about the same altitude.
Although it is computed that at a height of seventy miles the air has
less than one-millionth of its density at sea-level--which is about
the density of the air remaining in the exhausted bulb of an
incandescent electric lamp--it is there sufficiently dense to render
meteors luminous by friction after they with great velocity enter our
atmosphere. The height of these meteors has been found, from
simultaneous trigonometrical measures at two stations, sometimes to
exceed one hundred miles, and if we suppose the aurora borealis to be
an electrical discharge in highly rarefied air, measures made in the
same way indicate as great a height for our atmosphere. The height of
the aurora varies enormously, but the average altitude of it and of
the other phenomena described, with the corresponding computed density
of the air, are shown in the preceding diagram, in which the depth of
the ocean of air may be compared with the deepest seas and the highest
mountains. While, as Professor Young says, it cannot be asserted that
the atmosphere has any defined upper limit, yet the kinetic theory of
gases seems to afford evidence that the molecules of oxygen and
nitrogen do not escape from the earth's attraction, and therefore the
hypothesis of Professor Förster is unwarranted, that interplanetary
space is filled with _Himmelsluft_, or very thin air.

=Temperature of the Atmosphere.=--The warmth of the atmosphere is
derived chiefly from the sun's rays which, arrested by the earth's
surface, are partly reflected and partly radiated back through the
atmosphere. Not more than seventy-five per cent.--Professor Langley
says only sixty per cent.--of the heat of the sun, which is received
vertically on the upper surface of the atmosphere, penetrates to the
earth, and very much less than this when the angle of the sun is low.
The reason why temperature diminishes as we ascend, is partly owing to
the greater loss of heat by radiation through the thinner envelope of
the upper strata, and partly owing to the greater absorption of the
heat given off from the earth by the lower and denser strata. In
general, it may be said that there is a diminution of 1° Fahrenheit
for each three hundred and thirty feet that we rise vertically, but,
this rate varies greatly at different heights, places, and times. For
instance, the decrease is not the same on mountains as it is in the
free air, and in the northern hemisphere it is greater on the south
than on the north sides of mountains; it is usually greatest near the
ground, and is faster in summer than in winter. But in the average,
the temperature falls as much for three hundred and thirty feet of
elevation as it does for a change of seventy miles on the earth's
surface north or south of the equator. When dry air rises, because it
is heated and thereby is made lighter, the laws of thermo-dynamics
show that, by reason of its expansion, its temperature is decreased 1°
Fahrenheit for each one hundred and eighty-three feet that it ascends,
and, by compression, its temperature is increased as much if it is
made to descend the same distance. This is called the "adiabatic rate
of change of temperature," because it is produced by an alteration in
the density of the air, due to variation in pressure, without the
addition or loss of heat. In the course of this book there will be
occasion frequently to refer to this law of heating and cooling. The
adiabatic rate of change is seldom observed on mountains because of
their influence upon the currents of air in contact with their flanks,
or even in balloons, on account of imperfect measurements, but, as
will be explained in the closing chapter, the adiabatic change of
temperature is confirmed by the observations with kites, which furnish
the best method of obtaining the temperature of the free air up to
moderate heights. The adiabatic cooling of rising currents of air is
another reason for the rapid decrease of temperature with height up to
a mile or more. The upper air alters its temperature from diurnal and
seasonal causes much more slowly than the lower air, and a mile above
the earth the daily change of temperature, apart from the passage of
"warm and cold waves," is less than one degree. At a height of six
miles above the earth a temperature much below zero constantly
prevails, while, at ten miles, 80° below zero has been recorded in a
balloon--this is approximately the temperature prevailing winter and
summer above pole and equator. These facts are expressed graphically
in Plate III., Temperature at Different Latitudes and Altitudes,
which represents half of a section of the earth, from the north pole
to the equator, with the superincumbent atmosphere.

   [Illustration: PLATE III.--TEMPERATURE AT DIFFERENT LATITUDES
   AND ALTITUDES.]

Perhaps it should be explained, that whereas the curvature of the
earth with respect to the height of the atmosphere in the previous
diagram was not exaggerated, in the present diagram the height of the
atmosphere over the radius of the earth is enormously increased. At
the north pole the mean annual temperature is about 0° Fahrenheit, and
at the equator it is about 80°. It is seen that the atmospheric layer
having a temperature of 50° (here represented in section by a line)
touches the earth at 45° latitude, but is about two miles above the
equator. In the same way the line of freezing (32°) leaves the earth's
surface at 58° latitude and rises to about three and a half miles over
the equator; the line of 0° rises from the pole to about seven miles
at the equator. This is familiarly illustrated by the fact that only
the highest mountains in the tropics are snow-capped, while within the
Arctic circle the snow-line descends nearly to sea-level. The lines in
the diagram show the mean annual temperatures, but the isothermal
surfaces rise in summer and sink in winter, the change of altitude
being greatest in northern regions and near the ground. Frequently
there is an inversion of temperature, that is to say, it is warmer
above than below. Notably, in Siberia, where the winter temperature is
60° below zero, there can be no immediate decrease of temperature with
height, and it is probable that there is a warmer layer of air
interposed between the very cold earth and the still colder upper air,
so that the temperature first rises rapidly with elevation and then
falls slowly to the limits of the atmosphere. In temperate latitudes
it often happens, with a high barometric pressure, in winter that the
mountain stations enjoy a long period of still and relatively warm
weather, as compared to that experienced in the valleys. But the
subject of inversions of temperature will be discussed at length in
considering the results of the balloon and kite observations.

The observations from balloons at great heights are neither
sufficiently numerous nor accurate to enable us to form an opinion as
to what is the temperature of interplanetary space, which the kinetic
theory of gases places at 460° Fahrenheit below zero. This temperature
is called "the absolute zero," and is calculated from the fact that
air under a constant pressure contracts 1/490 of its volume for each
degree Fahrenheit it is cooled below the temperature of freezing
water, and consequently under no pressure it should have an infinite
volume and a temperature of about 490° below freezing, or 458° below
zero. There are other hypotheses regarding the temperature of space,
but since it can never be measured directly, it will probably remain a
matter of speculation. It is certain, however, that if the earth were
deprived of its atmosphere, the temperature would fall very low, and
even with our atmosphere as a blanket our earth would be
uninhabitable were it not for the aqueous vapour which controls the
selective absorption of the solar rays, transforming them into obscure
rays so that they cannot escape from the atmosphere. Water-vapour,
then, is a very important factor in the physics of the atmosphere, but
it can only be considered briefly here.

=Moisture of the Atmosphere.=--The air is constantly absorbing
moisture from the water on the earth, but the tension of this aqueous
vapour decreases with elevation much faster than does the atmospheric
pressure. At the height of about a mile and a quarter half the
quantity of water-vapour is below, while we must rise about three and
a half miles to reduce the quantity of air one-half, as may be seen in
Plate I. The relative humidity, or the percentage of moisture in the
air, as compared to the amount which it could contain at that
temperature, is nearly the reverse at low and at high levels. It is
found from the kite-observations at Blue Hill, that up to the height
of a mile or two the air is drier during winter and at night, and
damper during summer and in the day-time than it is near the ground.
At great heights probably the air is always very dry. The condensation
of the invisible vapour into a visible form is considered in the next
chapter on clouds.

It is apparent that our observational knowledge of the atmosphere is
gained by two general methods of exploring it, viz. observations made
from the earth upon clouds and optical phenomena at a distance, and
observations made directly in the air itself. Although it was realized
at the beginning of this century that meteorological observations were
almost all conducted at the very bottom of our atmosphere--"in the
shoals and shallows of the ocean of air," von Humboldt said--yet only
within the past thirty years was it thought necessary to replace the
occasional observations on mountains by systematic and long-continued
ones, comparable to those so generally carried on at low levels. It is
an evidence of the zeal in America to advance the young science of
meteorology, that the first mountain-top station in the world was
established in 1871 upon Mount Washington, and that both this exposed
post of observation, 6300 feet above the sea, and the one more than
twice as high on Pike's Peak, which was for a long time the highest in
the world, were maintained for many years by the United States Signal
Service. The present highest station in the world is maintained by the
Harvard Observatory upon El Misti in Peru, where, at a height
exceeding 19,000 feet, a combination of self-recording instruments was
constructed by my assistant, Mr. Fergusson, to operate during three
months without attention. It must be admitted, however, that the
addition to our knowledge of the physics of the atmosphere afforded by
the American stations has been slight and incommensurate with the
expense incurred. More has been gained from the mountain stations in
Europe, notably from those in the Austrian Alps, which have furnished
data for Dr. Hann's splendid discussions of the thermo-dynamics of the
atmosphere. While mountain stations present the only means of
obtaining continuous observations at a considerable and constant
height, still they have serious drawbacks. Not only is the
distribution of mountains over our globe irregular, but since they
form part of the earth's crust, terrestrial influences affect all
observations made upon them. In the case of plateaux this was at once
admitted, but by placing the stations on the summits of high and
isolated peaks, it was hoped to approximate to the conditions of the
free air. It is now recognized that the equilibrium of the atmosphere
is so delicate that for its dynamical study exact and minute
measurements of temperature, moisture, and currents are required, and
the methods which will be described are intended to give the values of
these elements free from terrestrial disturbances.

Clouds, balloons, and kites naturally supplement one another. While
clouds indicate the direction and velocity of the air at different
heights, yet the lower clouds often conceal the upper strata, or there
may be no clouds at all, in which case balloons or kites will aid us
to determine the drift of the currents. When there is little wind at
the ground, or to reach heights of several miles, we must employ
balloons, but otherwise kites are preferable in most cases. The
thermal and hygrometric conditions of the free air can be ascertained
only by personal observations in balloons, or by raising
self-recording instruments with balloons and kites, and this latter
method it is predicted will be the path of greatest progress.



                               CHAPTER II

   CLOUDS--FORMATION AND CLASSIFICATION--MEASUREMENTS AT BLUE
   HILL--THE INTERNATIONAL OBSERVATIONS


Clouds must have been among the earliest observed natural phenomena,
and they were used from time immemorial as weather signs. Yet their
every-day occurrence was very likely the reason why their origin was
not studied until about a century ago. Father Cotte, in his classic
work on meteorology, published in 1774, devotes only a couple of
paragraphs to clouds, but Abbé Richard, in his contemporary _Histoire
Naturelle de l'Air_, discusses the appearance and theories of clouds
in ten chapters. The cause of evaporation was unknown in the last
century, and it was not until its close that Dalton, the English
chemist, proved that water-vapour exists independently in the air, and
Hutton explained that precipitation was produced by the contact of a
current of saturated air with a colder one. Although there remains
much to be learned about cloud formation, yet it is now pretty well
established that its most effective cause is the ascent, and
consequent cooling by expansion of the air, rather than the mixture of
masses of air having different temperatures. The ascent of the air may
result from its being forced up a mountain slope by its horizontal
movement, or from its being drawn up in a vortex, but most commonly
the air rises from its lessened specific gravity when warmed. If the
temperature of the quiescent air decreases faster than 1° for each 183
feet of height, which is the adiabatic rate of cooling for dry air, as
explained in the last chapter, air warmed locally will rise and cool
at this rate until the dew-point is reached. Then the vapour in the
air will be condensed upon particles of dust, which Aitken found to be
more numerous in clouds than outside them. The most conspicuous of the
clouds formed by rising currents is the cumulus, or rounded summer
cloud, which has been aptly termed "the visible capital of an
invisible column of air." Saturated air cools as it rises more slowly
than dry air, consequently the upward motion is maintained through the
cloud mass, causing the swelling up of the tops of the cumulus clouds,
which reach their highest development in the thunder clouds, or
cumulo-nimbus, as they are called. The lower limit of the cloud region
is determined therefore by the height at which the rising currents
reach their dew-point, and the altitude of the cloud formation depends
upon the humidity of the ascending current, the drier it is, so much
the higher must it rise to have its vapour condensed. In storms the
rising current mingles with the stronger horizontal current above,
which carries with it the upper portion of the cloud, and covers the
whole sky with a uniform sheet. The wave, or ripple cloud, has been
explained by von Helmholtz and von Bezold to be due to the undulations
in a horizontal current producing alternate rarefaction and
condensation of its water-vapour through changes of temperature. Still
another cause of low-lying clouds is the cooling of the air to its
dew-point by contact with a cold surface, such as the earth when
cooled by radiation during a clear night, or the polar currents of the
ocean. Fog is often formed in this way, which we call stratus cloud
when it rises above us. The highest clouds consist of ice crystals,
because the temperature of the air where they are is much below that
of freezing water. Although it is possible to cool drops of water
considerably below 32° Fahrenheit without congelation, yet it can be
told with certainty that the clouds are composed of ice if the sun and
moon when seen through them are surrounded by the large rings or
halos, which the theory of optics shows can only result from
refraction of light by ice crystals, whereas water drops in the clouds
produce the smaller coloured rings, which are called coronæ. The old
question, why clouds float unless their particles are hollow, is
easily answered, for they do not float, and always tend to sink if
they are not supported by the currents of air. In sinking into warmer
air the particles are vapourized and become invisible, but others
rising are condensed and take their places, so that the cloud
persists, although its particles change. This is illustrated by the
"cloud banners," which frequently stream from mountain peaks, and are
caused by the rise of air up the mountain side. Even in a strong wind
the cloud remains attached to the peak, showing that its particles are
being renewed continually; but if, as is often the case, the wind
descends on the leeward side of the mountain, the cloud particles
disappear.

Lamarck, the celebrated naturalist, in the opening year of the present
century, first proposed a classification of cloud forms. Two years
later Luke Howard, a London merchant, published his epoch-making essay
on _The Modifications of Clouds_. The theories there advanced and the
nomenclature proposed have been accepted generally to our day,
notwithstanding the more complete classifications devised by Poëy,
Ley, and others. Howard believed that clouds are formed by the aqueous
vapour which rises from the earth, and that the globules which compose
them are solid, and are not filled with hydrogen gas as had been
maintained by Deluc and De Saussure. Howard classified the clouds as
we do to-day, according to their appearance, into three principal
types, viz. stratus, cumulus, and cirrus, which represented also low,
middle, and high clouds. Stratus is the sheet of low-lying cloud which
forms at night, and commonly rests on the earth; cumulus is the
heaped-up cloud of the day-time; and cirrus is the curl cloud of the
high atmosphere. These three types were further divided into four
intermediate types, viz. nimbus, cumulo-stratus, cirro-stratus, and
cirro-cumulus. Howard's nomenclature was used almost exclusively,
until in 1889 the International Meteorological Conference that met at
Paris recommended the adoption of another classification, based on
Howard's, but modified by two experts, Abercromby of England and
Hildebrandsson of Sweden. This classification also disregarded the
origin of clouds, and was based only on their appearance. The next
year an atlas, with coloured pictures of the clouds, separated
according to the new nomenclature, with descriptive text, was prepared
by Dr. Hildebrandsson, assisted by Drs. Neumayer and Köppen of the
Deutsche Seewarte, or German National Meteorological Observatory. This
atlas was adopted by the principal meteorological institutions on the
continent of Europe for their observers. The preface contained the
following statement: "The study of the forms of clouds is daily
increasing in importance, both from the standpoints of theory and of
weather prediction. Observations taken at the bottom of the
atmospheric ocean are plainly insufficient to determine its
circulation. The clouds, however, furnish information about the
condition and motion of the air at various levels. But, a comparison
of the observations of different observers is only possible when the
same ideas are connected with the same expressions. It is hardly
possible to give a sufficient verbal description of such indeterminate
and changeable forms as those of the clouds; graphical representations
are therefore necessary, with the help of a short description, in
order to enable an observer to connect what he sees in the sky with
what he finds in the instructions. In order that a cloud picture may
be intelligible to non-specialists, the clouds and the blue sky must,
at least, be plainly distinguishable from each other."

The meeting of the directors of the meteorological institutions in
different parts of the world, which was held at Munich in 1891,
decided to adopt the classification of Abercromby and Hildebrandsson,
and a committee was appointed to prepare an Atlas of Clouds, which
should be cheaper than the preceding one. This committee, of which the
writer has the honour to be the American member, met at Upsala in
1894. It defined the various forms of clouds, selected typical
pictures to illustrate them, and drew up instructions for observing.
This atlas, which was published in 1896, is the recognized authority
on cloud forms.

Meanwhile the United States Weather Bureau had issued a plate of
clouds, printed in one colour, to familiarize its observers with the
new system. The Navy Department has also an interest in clouds, for
several thousand seamen in various parts of the world send their
special logs to the United States Hydrographic Office. The
Hydrographer, a few years ago, was Captain Sigsbee, who, long before
he became known to the public as commander of the ill-fated _Maine_,
had achieved scientific reputation from his investigations upon the
depths and the currents of the ocean. Captain Sigsbee desired to
render comparable the observations of clouds which were being made all
over the world, and to this end he resolved to publish a coloured
atlas of the international cloud types which should be intelligible to
seamen, and yet not too costly for his office to supply. After two
years of experimenting, during which the writer and his assistant, Mr.
Clayton, were frequently consulted, the _Illustrative Cloud Forms_,
with and without descriptive text, were issued in 1897 by the
Hydrographic Office, and in several respects this atlas is the best.
Still, it is impossible for anything but a photograph from the cloud
itself to show the extreme delicacy of certain forms. Perhaps it
should be explained, however, that as the blue sky and the white
clouds act with almost equal actinic effect upon the sensitized plate,
in order to obtain the proper contrast between sky and cloud it is
necessary either to polarize the light from the sky, or, as is most
commonly done, to separate the coloured rays by allowing them to pass
through a yellow screen, and to fall upon autochromatic plates.

Before defining the ten principal types of cloud it should be
explained that two general classes of clouds are distinguished,
separate or globular masses, which are most frequently seen in dry
weather, and forms which are widely extended or completely cover the
sky, which are typical of wet weather. Both these classes of clouds
are found at all heights.

=Cirrus= are thin, fibrous, detached, and feather-like clouds formed
of ice-crystals. They are the highest of all the clouds, and move
with the greatest velocity.

=Cirro-stratus= form a thin whitish veil, more or less fibrous, which
often produces halos around the sun and moon and other optical
phenomena.

=Cirro-cumulus= are flocks of small detached fleecy clouds, generally
white and without shadows.

=Alto-stratus= is a grey or bluish veil through which the sun and moon
are faintly visible, occasionally giving rise to coronæ. Its altitude
is only about half that of Cirro-stratus.

=Alto-cumulus= are flocks of larger, more or less rounded, white or
partially shaded masses, often touching one another, and frequently
arranged in lines in one or more directions.

=Strato-cumulus= are large globular masses or rolls of dark cloud,
frequently covering the whole sky, especially in winter.

=Cumulus= are piled clouds with conical or hemispherical tops and flat
bases. They are formed by rising currents of heated air, and are
therefore most common in summer and in tropical regions. When broken
up by strong winds the detached portions are called Fracto-cumulus.

=Cumulo-nimbus= is the massive thunder shower cloud rising in the form
of mountains or turrets, and generally having above a screen of
fibrous appearance (False Cirrus), and underneath a mass of cloud
similar to Nimbus from which rain falls.

=Nimbus= is a dense, dark sheet of ragged cloud from which continued
rain or snow generally falls. Broken clouds underneath, forming the
scud of the sailors, are called Fracto-nimbus.

=Stratus= is a thin uniform layer of cloud at a very low level. When
the sheet is broken up into irregular shreds it is called
Fracto-stratus.

Having described the origin and appearance of the different clouds, an
account will now be given of the measurements made at Blue Hill
Observatory and the information which they give about the circulation
of the atmosphere. The work there was taken up in 1887 in consequence
of the interest of the meteorologist, Mr. Clayton, in the study of
clouds; his discussion of the cloud observations, published two years
ago with the Blue Hill observations, has been termed by far the most
thorough study of the kind ever undertaken in America if not in the
world. Most of the conclusions which are stated popularly here have
their scientific expression in his work.

The first investigation related to the amount of cloud at different
hours of the day, and during the various seasons. It is customary to
note the degree of cloudiness on a scale of from 0, when there are no
clouds, to 10, when the whole sky is covered. For twelve years the
amount of cloud at each hour of the day has been recorded at Blue
Hill. The personal observations have been supplemented during the
day-time by an automatic instrument called a sunshine-recorder, for it
has been proved that the cloudiness is very nearly the inverse of the
bright sunshine. Consequently, if, as is usual there, the sun shines
forty-six per cent. of the time when it is above the horizon, the
cloudiness is very nearly fifty-four per cent., which is the average
for the year. The instrument generally used for this purpose is a
glass sphere which acts as a burning-glass, and chars a strip of
cardboard placed concentrically around the lower part of the sphere.
As the sun moves, the image on the card moves in the opposite
direction over the card, burning a line as long as it shines, but
leaving the card untouched when it is cloudy. In a similar way a
record may be obtained on sensitized "blue paper" by allowing the
sun's rays to enter a dark chamber containing the paper. The
maintenance of personal observations at each hour of the night is
arduous, and, therefore, during ten years an automatic instrument has
been used at Blue Hill which deserves to be better known. It is called
the pole-star recorder, and was devised by Professor Pickering,
director of the Harvard College Observatory. The instrument is very
simple, and consists of a telescopic camera focussed on Polaris. This
star is not at the north pole of the heavens but a little more than a
degree distant, and, consequently, it describes a small circle in the
heavens during twenty-four hours. When the sky is clear around Polaris
its trail upon the photographic plate is continuous, but when the sky
is partly or entirely covered with clouds the trail is broken or
obscured. Of course the plate is not exposed until after dark, and a
shutter is closed by a clock before dawn. The only hourly records of
cloudiness at night in the United States are obtained by this
instrument on Blue Hill and at Cambridge. It will be objected,
perhaps, that the cloudiness derived from observations of the sun or
the pole-star is not the amount over the whole sky, but only that in
the region of the luminary. This is true, but it is found that the
average of the records for a month or a year agrees very closely with
the average of estimates of cloudiness over the whole sky during these
periods. The use of the pole-star is preferable to that of the sun,
because in our latitude it gives values at a point about half-way
between the horizon and the zenith; while since the sun travels at a
variable height across the sky, when its altitude is low the same mass
of cloud may intercept more sunlight than when it shines vertically.
From ten years' observations the following deductions have been made
concerning the variation in the amount of cloud at Blue Hill. For all
the months the diurnal amount of cloud is greatest about one o'clock
in the afternoon, on account of the frequency of cumulus clouds near
the warmest part of the day, while the next greatest amount, due to
the frequency of stratus clouds, occurs near sunrise, or at the
coldest time of day. All over the world the least cloudiness is in the
evening, when the sum of the combined effects of radiation and
insolation is least. The annual period in the cloudiness is complex,
because the amount of cloud is connected with changes of humidity at
many different levels in the atmosphere, but in the northern
hemisphere there is most cloud during the first half of the year and
least during the latter half, probably because the increasing warmth
at the earth's surface produces increased ascending currents until
summer, while the chilling of the earth's surface in the autumn
becomes unfavourable for ascending currents. The distribution of cloud
over the globe is intimately connected with the general atmospheric
circulation, being greater where there are rising currents and less
where there are downward currents. The reason, naturally, is that as
descending air becomes warmer and therefore relatively drier, the
clouds in it evaporate and disappear. A cloudy belt encircles the
earth at the equator, and on either side are two belts of less cloud,
but in higher latitudes the cloudiness increases. If we could see our
earth from outside its atmosphere, the light reflected from the upper
surfaces of the cloud-belts would probably make them appear bright.
From the markings on a planet that are known to be caused by
condensation, a French meteorologist, M. Teisserenc de Bort, believes
that the circulation of its atmosphere can be inferred, for wherever
on the surface of the planet bright spots are seen, there the vapour
of rising currents should be condensed. If this be true, there is a
resemblance between Jupiter, as we see it, and the earth as it would
appear from another planet, the bright bands being cloud surfaces, and
the dark patches glimpses of the surface of the planet beneath.

Observations of the direction of motion, and apparent velocity of
clouds at different heights, have been made at Blue Hill several times
a day since 1886. To measure the motion of clouds the nephoscope (Fig.
1) is used. It consists of a horizontal circular mirror with a
concentric circle of azimuths and an eye-piece _C_, movable in a plane
_BD_ at right angles to the mirror and also around it, through which
the image of the cloud is brought to the centre of the mirror _A_. It
can be proved by geometry that the motion of the cloud-image is
proportional to the movement of the cloud itself, so by noting in what
direction and how far the image is displaced in a given time, we have
the true direction of motion of the cloud itself and also its relative
velocity, comparable with the velocity of all clouds having the same
height. If the height is known, then the relative velocity can be
easily converted into absolute velocity, and thus the velocity of
currents at different heights in the atmosphere is accurately
ascertained.

   [Illustration: FIG. 1.--Nephoscope at Blue Hill Observatory.]

The height of clouds seems to have been measured trigonometrically
from two stations as early as 1644 by Riccioli and Grimaldi, two
Jesuits of Bologna, but notwithstanding these measurements and some
conclusions derived from observations on mountains, and in balloons,
the altitudes of the different clouds were not known with any accuracy
until in 1884 Ekholm and Hagström made a series of trigonometrical
measurements upon the different kinds of clouds at Upsala, Sweden.
About the same time attempts were made at Kew Observatory to measure
clouds by photography, and in 1885 probably the first trigonometrical
measurements in America were made at Cambridge, Mass., by Professor W.
M. Davis and Mr. A. McAdie. In 1890-91 the Swedish methods were
employed at Blue Hill by Messrs. Clayton and Fergusson of the
Observatory staff, and until recently the measurements there and at
Upsala comprised all that was known accurately about the heights and
velocities of the various species of clouds.

   [Illustration: FIG. 2.--Cloud Theodolite at Blue Hill
   Observatory.]

The trigonometrical measurements at Blue Hill were made as follows: at
two stations, one at the Observatory, the other at the base of the
hill about a mile distant, two observers determined simultaneously the
angular altitude and azimuth of some point on the cloud which was
agreed upon by telephonic conversation. If, as is generally the case,
the lines of sight did not meet, the trigonometrical formulæ gave the
height of a point midway between the crossing of these lines. Such was
the accuracy of these measurements that the probable error of the
calculated heights of the highest clouds is only a few hundred feet.
Successive observations at the two stations of the position of the
cloud enabled its velocity to be calculated, or, as already explained,
this may be got from the relative velocity measured at one station, if
the height of the cloud be known. Fig. 2 shows the theodolite on the
tower of the Observatory. Five other methods of measuring clouds have
been employed at Blue Hill: (1) The only method of finding the height
of lofty and uniform cloud strata is by means of the light thrown on
them from below, and on Blue Hill the electrical illumination of the
surrounding towns is utilized. The angle which the centre of the
illumination makes with the horizon is measured, and knowing the
distance of the town, the right-angled triangle may be solved. (2) An
accurate method for low and uniform clouds is to send kites into them,
as will be explained in the closing chapter. (3) When the clouds are
low enough to cast shadows on the ground, the angles of the cloud and
sun as seen from the Observatory are measured, and with the distance
of the shadow from the hill-top, ascertained by a map, this triangle
can be solved. The times of passage of the shadow over known points on
the landscape afford another means of calculating its velocity. (4) A
method that was suggested by Espy, the pioneer American meteorologist,
for getting the altitude of the bases of clouds lying within a mile of
the earth, is to find the difference in temperature between the air
and the dew-point at the ground, and to compute the height at which
this difference should disappear. When the temperature of the rising
currents increases, as on warm days, and the level of the dew-point
rises higher, the cloud can be seen to ascend, and, in fact, the
measurements at Blue Hill show that the clouds of moderate altitude
are highest during the warmest part of the day. (5) Finally, very low
stratus or nimbus may be measured by noting the heights of their bases
on the sides of the hill.

   [Illustration: PLATE IV.]

The identity of cloud-forms all over the world has been established,
and as a result of the measurements at Blue Hill, the heights and
speed of all clouds observed there are known. The averages have been
plotted in the five levels into which we separate the clouds in Plate
IV., Heights and Velocities of Clouds, where ordinates represent
heights and abscissæ velocities, and, consequently, the distances of
the various forms of clouds above the horizontal base indicate their
heights, and the distances from the left-hand vertical line their
velocities. For comparison, the velocity of the wind on Blue Hill, a
few hundred feet above the general level of the country, is
represented. The mean height of the cirrus is about 29,000 feet, but
this cloud sometimes reaches 49,000 feet. The mean height of the
cumulus is about a mile, but the tops of the cumulo-nimbus, or
thunder-shower cloud, sometimes penetrate into the cirrus level.
Generally the base of the nimbus, or rain cloud, is only 2300 feet
above the ground, and it frequently sinks below the top of Blue Hill,
which is only 630 feet above the sea. The poetic saying, that "Earth
wraps her garment closer about her in winter," has a scientific
basis, for the average height of all the clouds is greatest in summer
and least in winter. But the reverse is true of their velocity, for
the entire atmosphere moves twice as fast in winter as it does in
summer, and at the lower levels the seasonal change is even greater.
The average velocity of cirriform clouds is ninety miles an hour in
winter, and sixty miles in summer, but occasionally in winter cirrus
have been found to have the enormous velocity of two hundred and
thirty miles an hour. In the average, the velocity of the currents
increases, from the lowest to the highest clouds, at the rate of about
three miles an hour for each 1000 feet of height, but near the ground
the increase with height is faster. It has been found that the
velocity of the lower clouds is less than the velocity of the wind on
a mountain of the same height, which may, perhaps, be explained on the
supposition that the mountain acts like a dam to accelerate the flow
of air over it. The measurements in Sweden showed that the middle and
upper levels of clouds are higher than in America, but that they move
less rapidly. This may be because the surfaces of equal temperature in
the air are higher in the United States than in Sweden, on account of
the direction of the upper currents, while the greater velocity of our
high clouds corresponds with the more rapid movement of areas of low
and high barometric pressure over the United States.

These results are suggestive. For instance, the energy of the upper
half of the mass of the atmosphere, or that portion which lies above
18,000 feet, has been calculated to possess six times the energy of
the lower half in which we live, and as yet, none of this enormous
store of energy is applied to the use of man. While it appears certain
that no navigable balloon or flying machine will ever be able to stem
the enormous velocity of the upper atmosphere, rarified though it is,
perhaps in the future aërial machines will take advantage of the
prevailing currents of the high atmosphere, as our sailing ships do of
the trade winds. The observations of cirrus clouds in various parts of
the world show that they always move from a general westerly
direction, while below this primary drift toward the east occur the
relatively permanent or transient differences of pressure which cause
the deviations from the normal circulation of the atmosphere, and give
rise to the local circulation in storms. In the familiar daily weather
map it will be noticed that there is usually some portion marked
"low," and another portion marked "high." The former is an area of low
barometric pressure, into which the winds at the ground blow spirally
inward in the opposite direction that the clock hands turn; the latter
is an area of high barometer, out of which the winds at the ground
blow in the contrary way. The former when well developed are called
"cyclones," and are usually accompanied by stormy weather, and the
latter, called "anti-cyclones," bring fair weather. From the
observations of the directions from which the clouds move in cyclones
and anti-cyclones, we have found that above the cumulus level (at the
height of about a mile) the inward inclination of the wind in a
cyclone, and the outward inclination in an anti-cyclone, both
disappear, and the general drift from the west prevails. The results
of the observations are shown in Plate V., Atmospheric Circulation in
Cyclones and Anti-cyclones, representing sections of the atmosphere,
concentric to the earth's surface, in the five cloud-levels seen from
above. The arrows fly with the wind and are proportional in length to
its velocity, the dotted arrows indicating the probable flow of the
air through the cyclones and anti-cyclones that are indicated by the
circles, their axes being assumed to be nearly perpendicular to the
earth's surface. Above the cumulus it will be observed that the wind
in the cyclone tends to come from the south-west in front and from the
north-west in the rear, while in the anti-cyclone the contrary is the
case, indicating a deflection of the westerly upper current to the
right in cyclones and to the left in anti-cyclones. This sustains the
theory that the cyclonic circulation is struggling against a general
atmospheric drift from the west which increases with altitude, and
above the height of a mile becomes greater than the cyclonic
influence. Higher than this, the atmospheric circulation is
controlled primarily by the permanent temperature gradient between
equator and pole, by the seasonal temperature gradient between ocean
and continent, and, in the United States, by the passage of "warm and
cold waves." Mr. Clayton's investigations indicate that the motion of
the upper clouds is nearly parallel to the lines of equal temperature
at the earth's surface. A high temperature, by expanding the air
upward, causes in the upper air a high pressure; and a low
temperature, by contracting the air towards the ground, causes in the
upper air a low pressure, so that the lines of equal pressure in the
upper air are parallel to the lower lines of equal temperature, and
since there is little friction in the upper air the motion of the wind
is nearly parallel to the lines of equal pressure. Below the cumulus
level the winds follow the normal cyclonic and anti-cyclonic
circulation. There are two theories of the origin of these areas of
high and low pressure, the "driven theory" which supposes that they
derive their energy and drift from the general atmospheric movement
from west to east, and the "convectional theory" which attributes
their formation and progression to the difference of temperature
between them and the adjacent air. While the observations on mountains
have favoured the driven theory, yet the inward spiral motion of the
cirrus clouds above the anti-cyclone, indicating a lower pressure
than in the surrounding air, contradicts the hypothesis, and the
recent observations with kites at Blue Hill strongly support the
convectional theory of cyclones.

   [Illustration: ATMOSPHERIC CIRCULATION AT DIFFERENT HEIGHTS IN
   CYCLONES ANTI-CYCLONES
   PLATE V.]

The relation of the clouds to weather forecasting has been
investigated at Blue Hill. For instance, it is found, in this region
at least, and contrary to the general opinion, that cirrus clouds do
not indicate rain, but do foretell a change of temperature that is
proportional to the rapidity of motion of the clouds. Alto-cumulus is
followed by rain within twenty-four hours three times in four. Rain
follows the appearance of all high and intermediate clouds most
frequently when the cloud banks are densest toward some westerly point
and when they come from the west. Mr. Sweetland, an assistant, has
studied special forms of cloud in their relation to the succeeding
weather. He concludes that cirrus plumes precede fair weather, while
dense clots of cirro-cumulus are followed by rain. Rounded pendants,
or mammillated clouds, in the lower levels indicate rain, but in the
upper levels fair weather. Of all the forms, the dark sheet of
stratus, and clouds of lenticular shape, are most frequently followed
by rain. Of clouds presaging changes in temperature, the turreted
cumulus, which is connected with thunder-storms, precedes the
greatest fall in temperature, and next in order come lenticular
clouds, flaky cirrus, and alto-cumulus. In general, flat and flaky
clouds, clouds forming and disappearing rapidly, and clouds changing
to forms at a higher level precede dry and cooler weather.

It will be seen that this modern study of clouds as prognostics simply
adds to the weather proverbs that have come down to us from the time
of Theophrastus. It does not appear, however, that cloud forms alone
can usually serve to predict rain for more than twenty-four hours, but
for a few hours in advance the appearance of certain cloud forms
frequently furnishes the observer more trustworthy signs of coming
rain than does the synoptic weather map. To a forecaster in possession
of telegraphic data, the prevalence of rapidly-moving cirrus over a
wide area indicates a rapid storm movement, with sudden and marked
changes of weather and of temperature, while slowly-moving cirrus
indicate slight changes of temperature and dry weather. The direction
of the cirrus movements in front and around a storm centre will
usually point out the future movement of the storm, which tends to
advance in the same general direction.

The work done at Blue Hill shows the importance of cloud observations
to elucidate the general movements of the atmosphere, as well as the
circulation of the air above barometric maxima and minima, which can
result practically in making accurate weather forecasts possible a day
or two in advance. The systematic observation of the upper currents
was brought to the attention of the International Meteorological
Committee by Dr. Hildebrandsson in 1885, and at the meeting of the
International Cloud Committee in 1894, besides the adoption of the
nomenclature of clouds and instructions for observing them, it was
decided that observations of their motion, as well as measurements of
their height, should be made in various parts of the world.
Accordingly, the year commencing May 1, 1896, was designated as the
"International Cloud-Year," and observations with nephoscopes of the
direction of motion and relative velocity of clouds were begun at many
stations in Europe and Asia, and at fifteen stations in the United
States. Trigonometrical measures of the heights of clouds were
undertaken at stations in Norway and Sweden, Russia, Finland, Prussia,
and France, as well as at Toronto, Manila, and Batavia; in the United
States the measurements already described were recommenced at Blue
Hill, and the Weather Bureau equipped a similar station in Washington.
In Europe it is thought that the determination of heights by
photogrammeters, as the theodolites with attached photographic
cameras are called, possesses advantages over the visual theodolites,
and it is true that not only is the kind of cloud recorded on the
plates, but there are available for calculation as many points on the
cloud as can be identified on the two plates exposed simultaneously at
both stations. On the other hand, in the case of nearly uniform or
dark cloud-strata, it is easier to see points for measurement on the
cloud than to fix them on the photographic plates. For this reason,
and from the difficulty of manipulating the photogrammeter, visual
instruments were adopted both at Blue Hill and at Washington. The work
was successfully carried on until May 1, 1897, and the observations
and measurements were reduced at Blue Hill according to the plan
prescribed by the Committee. Already the observations and measurements
made at Upsala, Manila, and Blue Hill are published, and the others
will follow. The discussion of the correlated data from the various
countries will probably increase our knowledge of the circulation of
the atmosphere, which is certainly one of the most interesting and
important questions in the physics of the globe. The result will have
been reached by international co-operation, of which the benefits to
science are everywhere manifest to-day. But for the whole problem to
be solved, it is necessary, not only to know the movement of the air,
but, as far as possible, to ascertain its conditions of heat and
moisture. This may be accomplished by the use of balloons and kites,
to be described in the remaining chapters.



                               CHAPTER III

   BALLOONS--NOTABLE ASCENTS AND RESULTS OBTAINED--CAPTIVE BALLOONS


In the first chapter the invention of the hot-air and the hydrogen
balloon was chronicled, and it was stated that on December 1, 1783,
Charles rose from Paris to a height of 9000 feet. Public interest in
France was greatly excited by this wonderful extension of the realm of
man, and numerous ascensions with _Montgolfières_ and _Charlières_, as
the hot-air and hydrogen balloons were respectively called, took place
in Paris and the provinces. The uses of the balloon seemed
innumerable, and Lavoisier was instructed by the Academy of Sciences
to draw up a report on the value of the new discovery. After having
described in detail the ascensions which he had witnessed, the great
chemist stopped, appalled at the multitude of problems which the
balloon could solve. History has shown, however, that no commercial
application of the balloon was possible, and that aside from its
spectacular attractions, its chief use has been for scientific
observations.

The first persons in England who devoted themselves to aërial
navigation were foreigners. Two of them were Italians, the philosopher
Tiberius Cavallo, who already in 1782 had showed to a London assembly
that soap-bubbles filled with hydrogen will rise, and therefore had
almost anticipated the invention of the hydrogen balloon, and the
diplomatist Vincent Lunardi, who made some daring balloon ascents in
1784. But the honour of making the first scientific balloon voyage is
due to a Bostonian, Dr. John Jeffries. Dr. Jeffries graduated at
Harvard College in 1763 and then practised medicine in England, where
he became a loyalist, and during the Revolution was with the British
troops. In London he interested himself in aerostation, and, aided by
the Royal Society, ascended in a balloon because, he said, "I wished
to see the following points more clearly determined: first, the power
of ascending or descending at pleasure, while suspended and floating
in the air; secondly, the effect which oars or wings might be made to
produce towards the purpose and in directing the course of the
balloon; thirdly, the state and temperature of the atmosphere at
different heights from the earth; and fourthly, by observing the
varying course of the currents of air, or winds, at certain
elevations, to throw some new light on the theory of winds in
general." A French professional aeronaut named Blanchard had made
three ascents in France and one in England, and Dr. Jeffries paid one
hundred guineas to accompany Blanchard on his fifth ascent, which was
made from London November 30, 1784. He took with him a thermometer, a
barometer, a hygrometer, an electrometer, and a mariner's compass,
also several numbered bottles, filled with water and provided with
glass stoppers, which were to be emptied and corked up at different
heights in the atmosphere. It was arranged to record the observations
on ruled paper with a silver pen, because the doctor would not trust a
common pen or pencil as liable to accident. He also had a map of
England to determine the direction which the balloon took. Jeffries'
English sentiments are shown by this quotation from his narrative: "I
had provided a handsome British flag, invidiously represented the next
day in one of the public papers to have been the flag of the American
States." The barometer and thermometer were observed every few
minutes, and the hygrometer occasionally. The electrometer did not
change its indications. Samples of air were obtained and sent to the
Royal Society, but it does not appear that they were ever analyzed.
The balloon rose nearly two miles, and descended safely in Kent after
an hour and a half. Jeffries' observations compare favourably with
those made until recently; indeed, for nearly a century there was
little improvement in the apparatus. The decrease of temperature which
Jeffries found, viz. 1° for 360 feet rise, and the decreasing humidity
with height agree very well with later observations.

Jeffries and Blanchard undertook a more perilous voyage on January 7,
1785, from Dover across the Channel, landing in the province of
Artois, after, so runs the announcement, "we were suspended and
floating in the atmosphere two hours over the sea and forty-seven
minutes over the land of France." The voyagers were cordially
welcomed, and were entertained lavishly in Paris as being, Jeffries
says, "the first who passed across the sea from England into France by
the route of the air." No instruments but a barometer and a compass
were carried, and the only scientific result worthy of notice was that
the balloon seemed to lose buoyancy over the sea, due to what Jeffries
thought might be "the power of attraction over the water." The height
of the balloon was measured trigonometrically by French officers in
Calais, who found by angular measures, when the balloon was midway
across the Channel, that its height was 4500 feet. Jeffries' voyages
have been described somewhat at length because the first scientific
balloon voyage is generally attributed to the Belgian physicist,
Robertson, who ascended from Hamburg in 1803 to the improbable height
of 24,000 feet. Robertson made his third ascent the next year from St.
Petersburg, accompanied by the Academician Sacharoff. This was a
scientific voyage, instituted at the request of the Russian Academy,
to ascertain the physical state of the atmosphere and the component
parts of it at different heights, also the difference between the
results of vertical ascents and the observations of Deluc, De
Saussure, von Humboldt and others on mountains, which it was rightly
concluded could not be so free from terrestrial influences as those
made in the open air. Among the experiments which the Academy proposed
were the following: change of rate of evaporation of fluids, decrease
or increase in the magnetic force, inclination of the magnetic needle,
increase of heat of the solar rays, fainter colours in the spectrum,
influence of rarefaction of the air on the human body, as well as some
other chemical and philosophical experiments. A height of about two
miles was reached, and many interesting observations were made, but
since the instruments were not easily used in the basket of the
balloon, the results were unsatisfactory and required repetition to be
conclusive.

The Academy of Sciences of Paris now took up the investigation with
the special object of proving whether the magnetic force decreased as
Robertson in a balloon and De Saussure in the Alps had supposed. Two
young physicists, Biot and Gay-Lussac, were chosen to carry out the
investigations. They ascended from Paris on August 24, 1804, provided
with all necessary instruments, but the balloon was too small to rise
higher than 13,000 feet. Gay-Lussac ascended alone to a height of
23,000 feet on September 16, 1804, in a balloon filled with hydrogen.
His observations confirmed those which he had made with Biot, that
there was no change in the magnetic force, and from samples of air
collected he proved that the chemical constitution of the air is
invariable. His observations of temperature seemed to confirm the
theory of a decline of temperature of 1° in 300 feet of elevation. The
air was found to be very dry, and Gay-Lussac noticed that at the
highest altitude the clouds were still far above him.

Passing over several notable ascents in other countries, it was not
until 1850 that scientific ballooning was begun again in the land
where the balloon originated. Then MM. Barral and Bixio made two
ascents from Paris in rainy weather to the heights of 19,000 and
23,000 feet respectively, although they had expected to attain twice
these altitudes. Their most interesting observations were the great
thickness of the cloud mass, which in one case amounted to 15,000
feet, and the sudden fall of temperature in it from +15° to -39°. Some
curious optical phenomena were connected with the floating ice
crystals, and although the light of the sky was found to be strongly
polarized, the light reflected from the clouds was not polarized.

The field of operations was now transferred to England, where, under
the auspices of the British Association, four ascents were made by
John Welsh of the Kew Observatory in the great _Nassau_ balloon
managed by Green, the veteran aeronaut. The special object of these
investigations, like those in France, was the determination of the
temperature and hygrometric condition of the air at different
elevations. Besides this, samples of air at different heights were
collected for analysis and the light reflected from clouds was
examined for polarization. Recognizing that on account of the calm
prevailing in the car of the balloon and the great solar radiation,
the readings of the thermometer would be affected, Welsh enclosed the
thermometers in polished tubes through which air was forced by
bellows. This was the first aspirated thermometer, which alone gives
the true temperature of the air with the conditions prevailing in a
balloon. The instrument fell into oblivion until a few years ago, and
to this fact is due the fictitious temperatures generally obtained by
aeronauts. Welsh reached heights of from 12,500 to 23,000 feet, and
his observations showed that the temperature of the air decreased
uniformly with height until at a certain elevation, varying on
different days, the decrease is arrested, and for a space of 2000 or
3000 feet the temperature remains nearly constant, or even increases
slightly; the regular diminution being afterwards resumed and
generally maintained at a less rapid rate than in the lower air, and
commencing from a higher temperature than would have existed but for
the interruption. The variation of the decrease with the seasons was
also demonstrated. The humidity did not change much with height, and
it was nowhere very dry. Finally, the light of the clouds was proved
not to be polarized, and the permanent composition of the atmosphere
was confirmed.

In 1861 another grant of money was made by the British Association for
balloon experiments to be performed, under the direction of a
Committee, by Mr. James Glaisher, then engaged in geodetic and
meteorological work in England. Between 1862 and 1868 Glaisher,
accompanied by the aeronaut Coxwell, made thirty ascents. They
attained three times a height exceeding 23,000 feet, and once more
than 29,000 feet, when they believed that the balloon rose to 37,000
feet. The primary objects of Glaisher's experiments were as follows:
determination of the temperature of the air and its hygrometrical
conditions up to five miles, comparisons of an aneroid barometer with
a mercurial one, determination of the electrical state of the air and
of its oxygenic state by means of ozone papers, time of vibration of a
magnet at different distances from the earth. Secondary objects of
study were the composition of the air, the form and thickness of
clouds, the atmospheric currents, acoustical phenomena, etc. In order
to obtain many observations frequent ascents were necessary, as the
insular position of England precluded long voyages. During 1869
ascents in a captive balloon up to 1700 feet supplemented the
employment of the free balloon, which from its rapid rise and fall
made observations in it near the earth impossible. Glaisher was a good
observer; his instruments were excellent, and had been previously
tested, but their exposure in the basket of the balloon was bad, and
although the thermometer was provided with an aspirator similar to
Welsh's, Glaisher, noticing that the readings agreed with those of a
freely exposed thermometer, hastily concluded that the use of the
aspirator was unnecessary, and so discarded it.

Until quite recently Glaisher's results were accepted as representing
the conditions of the free air up to the greatest height which it was
possible to reach. These results showed that the temperature did not
fall uniformly with height, but that it fell most rapidly near the
earth and much less rapidly at great heights. In cloudy weather up to
the height of a mile the mean decrease of temperature in the day-time
differed little from the theory of 1° per 300 feet, but in clear or
partly clear weather the decrease was more rapid, commencing with 1°
for 160 feet near the ground and diminishing to 1° for 1000 feet at an
elevation exceeding six miles. The observations in the captive balloon
up to a third of a mile indicated a daily range in the vertical
decrease of temperature. The observations of relative humidity agreed
with Welsh's in showing a slight increase up to about half-a-mile,
then a decrease up to above five miles, where there seemed to be an
almost entire absence of water. The other observations were
inconclusive, except that the time of vibration of a magnet was found
to be somewhat longer than on the earth, which was contrary to
Gay-Lussac's experience. The most remarkable of Glaisher's ascents was
made from Wolverhampton on September 5, 1862, when in less than one
hour he had passed the altitude of five miles, exceeding the greatest
height hitherto reached. To quote from Glaisher's narrative: "Up to
this time I had taken observations with comfort and experienced no
difficulty in breathing, whilst Mr. Coxwell, in consequence of the
exertion he had to make, had breathed with difficulty for some time.
Having discharged sand, we ascended still higher; the aspirator became
troublesome to work, and I also found a difficulty in seeing
clearly.... About 1 hour 52 min., or later, I read the dry-bulb
thermometer as minus 5°; after this I could not see the column of
mercury in the wet-bulb thermometer, nor the hands of the watch, nor
the fine divisions of any instrument. I asked Mr. Coxwell to help me
to read the instruments. In consequence, however, of the rotatory
motion of the balloon, which had continued without ceasing since
leaving the earth, the valve-line had become entangled, and he had to
leave the car and mount into the ring to readjust it. I then looked at
the barometer, and found its reading to be 9-3/4 inches, still
decreasing fast, and implying a height exceeding 29,000 feet. Shortly
after, I laid my arm upon the table, possessed of its full vigour, but
on being desirous of using it, I found it powerless.... Trying to move
the other arm, I found it powerless also. Then I tried to shake myself
and succeeded, but I seemed to have no limbs.... I dimly saw Mr.
Coxwell, and endeavoured to speak, but could not. In an instant
intense darkness overcame me, so that the optic nerve lost power
suddenly, but I was still conscious, with as active a brain as at the
present moment whilst writing this. I thought I had been seized with
asphyxia, and believed I should experience nothing more, as death
would come unless we speedily descended; other thoughts were entering
my mind, when I suddenly became unconscious.... I cannot tell anything
of the sense of hearing, as no sound reaches the air to break the
perfect stillness and silence of the regions between six and seven
miles above the earth. My last observation was made at 1 hour 54 min.,
above 29,000 feet.... Whilst powerless I heard the words,
'temperature' and 'observation,' and I knew Mr. Coxwell was in the car
speaking to and endeavouring to rouse me.... I then heard him speak
more emphatically, but could not see, speak, or move. I heard him
again say, 'Do try; now do!' Then the instruments became dimly
visible, then Mr. Coxwell, and very shortly I saw clearly.... Mr.
Coxwell told me that while in the ring he felt it piercingly cold,
that hoarfrost was all round the neck of the balloon, and that on
attempting to leave the ring he found his hands frozen. He had,
therefore, to place his arms on the ring and drop down.... He wished
to approach me, but could not; and when he felt insensibility coming
over him too, he became anxious to open the valve. But in consequence
of having lost the use of his hands he could not do this; ultimately
he succeeded, by seizing the cord with his teeth, and dipping his head
two or three times, until the balloon took a decided turn downwards.
No inconvenience followed my insensibility; and when we dropped, it
was in a country where no conveyance of any kind could be obtained, so
I had to walk between seven and eight miles.... I have already said
that my last observation was made at a height of 29,000 feet; at this
time (1 hour 54 min.) we were ascending at the rate of 1000 feet per
minute; and when I resumed observations we were descending at the rate
of 2000 feet per minute. These two positions must be connected, taking
into account the interval of time between, viz. 13 minutes, and on
these considerations the balloon must have attained the altitude of
36,000 or 37,000 feet. Again, a very delicate minimum thermometer read
minus 11°.9, and this would give a height of 37,000 feet. Mr. Coxwell,
on coming from the ring, noticed that the centre of the aneroid
barometer, its blue hand, and a rope attached to the car were all in
the same straight line, and this gave a reading of seven inches and
leads to the same result. Therefore, these independent means all lead
to about the same elevation, viz. fully seven miles."

Mr. Glaisher's circumstantial evidence of the height he reached has
been assailed lately, partly from his assumption that the velocity of
the balloon while rising and falling during the thirteen minutes was
uniform, but principally from the supposition that men could have
survived in that region of death, without at least artificial means of
respiration. While it is certain that Berson's observations, which are
described later, were made at a greater height than Glaisher's, yet
all credit must be given to this Nestor of aeronautical and
meteorological science in Great Britain, who is still living at the
advanced age of ninety.

The example of Glaisher was not followed in England, but it stimulated
interest in the balloon again in France, where MM. Flammarion, de
Fonvielle, and Tissandier have made many ascents for scientific
purposes, and have presented the results in a popular form to the
public. Photography in a balloon is generally a failure on account of
the intense reflection from the upper cloud surfaces and the haze
which masks the earth. Consequently, for scenic effects we must rely
upon sketches, of which those in that interesting, but now rather rare
book, _Travels in the Air_, may be referred to. The high atmosphere is
often filled with fine ice crystals which, though invisible from
below, occasion curious optical phenomena, and some of these have
been sketched by M. Albert Tissandier, who has the advantage of being
an artist as well as an aeronaut.

Of the many narratives of balloon voyages, one of the most thrilling
is the tragedy of the _Zenith_. In 1875, through the co-operation of
the French Academy of Sciences and other scientific bodies, it was
arranged to make two voyages, one of long duration, the other to a
great height, in the balloon _Zenith_. The long voyage from Paris to
Bordeaux was successfully accomplished in twenty-four hours, and on
April 15 the _Zenith_ again rose from Paris, carrying MM. Gaston
Tissandier and Crocé-Spinelli, with Sivel as aeronaut. By the advice
of M. Paul Bert, the distinguished physiologist, three small balloons
of oxygen were provided to assist respiration. The scientific
apparatus was as follows: a pump was arranged to draw air through
tubes filled with potash in which to store the carbonic acid at
different heights in the atmosphere, in order that analysis might
determine if its proportion diminished at great heights; a
spectroscope was employed to examine the line of water-vapour in the
atmosphere, and two aneroid barometers were provided, one giving the
pressure corresponding to heights up to 13,000 feet, the other the
pressure between 13,000 and 30,000 feet. There were also two
barometric tubes registering the lowest pressure, as well as
thermometers and other scientific instruments. At 15,000 feet the
voyagers began to breathe oxygen, which had been used beneficially by
Sivel and Crocé-Spinelli in a high ascent the previous year. At 24,000
feet Tissandier wrote in his notes: "My hands are freezing. I am well.
We are all right. Haze on horizon with small rounded cirrus. We are
rising. Crocé pants. We breathe oxygen. Sivel shuts his eyes, Crocé
does the same." Five minutes later: "Sivel throws out ballast,
temperature -11° Cent., barometer 300 millimeters." After this,
Tissandier became so weak that he could not turn his head to look at
his companions. He tried to seize the oxygen tube, but was unable to
move his arms. His mind was clear, and he saw the barometer sink below
280 millimeters, indicating a height of 27,000 feet. Then he fainted.
After a half-hour of unconsciousness he revived and wrote: "We are
falling, temperature -8°, barometer 315 millimeters. I discharge
ballast. Crocé and Sivel unconscious in bottom of basket. We fall
rapidly." Again he fell into a stupor, from which he was roused by
Crocé shaking his arm, saying, "Throw out ballast!" which he did,
together with the pump, wraps, etc. What happened after this is
unknown, but probably the balloon, thus lightened and the gas in it
being warm, rose again nearly as high as before. When Tissandier came
to his senses the balloon was falling with frightful speed, and in the
bottom of the basket, which was oscillating violently from side to
side, were crouched his two companions with black faces and bloody
mouths. The shock of striking the ground was terrific, but the anchor
held, and the balloon soon emptied. From the barometric data it
appears probable that the _Zenith_ attained twice a height of about
28,000 feet, and that asphyxiation from the long deprivation of
sufficient oxygen killed the two companions of Tissandier and nearly
proved fatal to him.

This disaster discouraged further attempts to reach high altitudes,
and with the exception of the ascent to 23,000 feet in France by MM.
Jovis and Mallet, no more were made until the past decade. The results
of the meteorological observations were seen to be strangely
discordant; for example, the temperature of 40° below zero, observed
by Barral and Bixio at a height of 23,000 feet, and 80° above zero,
noted by the American aeronaut Wise, at 6000 feet. The prophecy "that
the balloon-basket would be the cradle of the young science of
meteorology" seemed unlikely to be realized, but, nevertheless,
observations in balloons continued to be made in France, Italy, and
Russia. In the United States a series of balloon ascents was
conducted by the Signal Service, which then included the Weather
Bureau, and the height of 15,500 feet reached by Professor Hazen in
1887 is probably the greatest at which observations in the free air
have been made in America.

The difficulty of obtaining the true temperature of the air from a
balloon is great, and without special precautions the observations
give the conditions of the free air even less well than do
observations on mountain summits. During a rapid ascent the air is
carried up in the balloon basket like water in a well-bucket, and
since the balloon drifts with the wind it is relatively in a calm, so
that there is no circulation of air; the thermometers, even when
screened from direct sunshine, are affected by radiation from the
heated gas-bag above, and moreover they are not sufficiently sensitive
to follow the changing temperature of the air strata so quickly
traversed by the balloon. The aneroid barometer, from which the height
of the balloon is calculated, cannot respond to rapid changes of
pressure; consequently there is a double source of error in
determining the height at which the temperature is measured.
Ordinarily, the temperature of the air may be obtained quite
accurately by slinging in a circle a thermometer attached to a cord,
even though this is done in sunshine. During two balloon ascents by
the writer, a sling thermometer was found in extreme cases to read 14°
lower than was recorded by automatic instruments, hung in their usual
position from the ring of the balloon. The sling thermometer, however,
is influenced by intense insolation, and moreover cannot be swung far
enough outside the basket of a balloon to insure good results. The
standard instrument for obtaining the temperature of the air under all
conditions, adopted for international use in 1898, is a modification
of that used by Welsh forty-five years before. This instrument, which
is the invention of Dr. Assmann of Berlin, is called the aspiration
thermometer, and is designed to prevent the casing surrounding the
thermometer from being heated by insolation or conduction, and to
insure a flow of air past the thermometer bulbs.

   [Illustration: FIG. 3.--German Balloon equipped for
   Meteorological Observations.]

The reorganization of balloon observations was accomplished by the
German Society for the Promotion of Aërial Navigation, which has been
assisted by the Prussian Meteorological Institute, and by officers of
the German Army Balloon Corps. The German Emperor takes a personal
interest in the work, and has aided it by the gift of a considerable
sum of money. The first voyage under the direction of the Society was
made in 1888, and many notable ones followed. In 1891, through the
courtesy of the president, Dr. Assmann, the writer made an ascent
from Berlin in a balloon equipped for accurate observations, with the
special purpose of comparing the sling with the aspiration
thermometer. The car of the balloon is shown in Fig. 3. A companion
was the now famous Dr. Berson, who then made his second ascent, but
who has now become an expert aeronaut by reason of more than fifty
ascensions, some of them to great heights. On December 4, 1894, he
ascended alone from Stassfurt, Prussia, in the _Phoenix_, to
probably the greatest height ever reached by man, at least in a
conscious state. By breathing oxygen he was able to keep his senses
and to read the barometer at 9·1 inches, indicating approximately an
altitude of 30,000 feet, and the aspirated thermometer at 54° below
zero. An ordinary thermometer read 11° below zero in the sun, showing
its heat was much diminished in consequence of the haze that prevailed
even at this enormous height. The cirriform clouds which surrounded
the balloon were found to have the structure of snow-flakes rather
than that of ice-crystals. The chief result of this record-breaking
ascent was the extraordinarily low temperatures recorded at great
heights, as compared with those observed by Glaisher, Tissandier, and
others. An inversion of temperature--that is an increase of
temperature with height--prevailed up to a mile, but above that the
temperature fell at a rapid and accelerated rate which approached the
adiabatic fall above 26,000 feet. The wind, which was almost calm at
the earth's surface, increased to a gale in the high atmosphere, and
carried the balloon along at an average speed of thirty-six miles an
hour. Wishing to demonstrate conclusively whether the insular position
of England influenced the temperature of the high atmosphere, as had
been suggested, Dr. Berson determined to execute a high ascension in
England during the prevalence of a barometric maximum in summer, when
the air column would be abnormally warmed and the upper isothermal
surfaces elevated. An opportunity was afforded Berson to follow in
Glaisher's footsteps on September 14, 1898, when abnormal heat
prevailed in Europe. Berson, with the aeronaut Spencer, in the balloon
_Excelsior_, rose from the Crystal Palace in London to the height of
27,300 feet, where he observed a temperature of -29°. The oxygen
inhaled prevented harmful physiological effects except for the
discomfort caused by the enormous reduction of temperature from 80° at
the ground only thirty-five minutes before. The temperature decreased
rapidly at first, then moderately up to three miles, and above that it
fell almost at the adiabatic rate. Even in this hot summer maximum of
pressure and notwithstanding the maritime climate and south-westerly
currents, a temperature about 29° below zero reigned at 27,000 feet,
being only a few degrees warmer than Berson had observed in winter at
the same height above Germany. Yet Glaisher, in all his ascents, two
of which exceeded 26,000 feet, never recorded a temperature of less
than 5° below zero. These relatively high temperatures, obtained also
by Welsh, Tissandier, and Gay-Lussac, must be attributed to the
insufficient protection of the thermometers against insolation, to the
proximity of the instruments to the heated basket and its occupants,
and lastly, to the sluggishness of the thermometers themselves, from
lack of ventilation, during the rapid passage through air-strata of
different temperatures. Plate VI. indicates the change of temperature
with height observed during the four highest balloon ascents in Europe
and in the United States. Dots indicate the observations while
ascending, and crosses the observations while descending; these are
connected by full and broken lines respectively, an inclination upward
to the left showing a decrease of temperature with height and _vice
versâ_. The adiabatic lines, representing a fall of temperature of 1°
Fahrenheit per 183 feet of ascent, serve for comparison.

   [Illustration: PLATE VI.--Temperatures observed in Four High
   Balloon Ascents.]

This account of notable balloon ascents should not be closed without
mentioning the most daring and unique of all, the voyage of Mr. S. A.
Andrée towards the north pole in 1897. Although his was a voyage of
geographical discovery, and not one for the exploration of the air,
yet meteorological and other observations were to be made, and Andrée
had familiarized himself with the instruments and the management of a
balloon during several voyages in Sweden. The success of the polar
voyage depended primarily upon the prevalence of southerly winds, and
the ability of the balloon to keep afloat long enough to profit by
them, even should they be light and variable at times. Therefore the
impermeability of the balloon to hydrogen gas was of vital
importance, and it was the conviction that the _Eagle_, of 140,000
cubic feet, was neither sufficiently large nor staunch to sustain
itself for thirty days, the time which might be required to reach
Behring Straits, that led Dr. Nils Ekholm, the meteorologist and
physicist, to withdraw from the expedition. Unfortunately, his fears
seem to have been well founded, and it is probable that we must now
abandon hope of the safety of the brave Andrée and his two companions.

A less perilous voyage northward across the Alps was attempted in 1898
by Professor Heim, the Swiss geologist, and two associates, conducted
by the Italian aeronaut, Spelterini. With an automatic photographic
camera, similar to one described in the next chapter, it was hoped to
get views of the high Alps from above, which would be alike valuable
for geologic and topographic study. Extensive meteorological
observations were made in connection with the sixth international
balloon ascent, but only the Jura was crossed, at an altitude of
13,000 feet, because the balloon travelled in a north-westerly
direction, instead of north-east as was expected.

Many years ago Wise and Donaldson, the American aeronauts, proposed to
cross the Atlantic Ocean in a balloon. The difficulties which present
themselves in such an undertaking are purely technical, and given a
balloon which loses its gas so slowly that its buoyancy can be
maintained for several days, there seems to be no reason why such a
balloon, at a height of four or five miles, could not pass from San
Francisco to New York, or from the United States to Europe, since the
motion of the upper clouds proves that the high atmosphere moves
almost constantly with great velocity from the west to the east. The
dirigible balloon has not been realized except in nearly calm weather,
but the aeronaut can often reverse his direction by ascending or
descending into a contrary wind to that in which he has been
travelling. Frequently no clouds separate these opposing currents,
which become apparent only when a balloon enters them.

It has been mentioned that in 1869 Glaisher made observations in a
captive balloon in England up to the height of 1700 feet in order to
study the conditions of the air within this distance of the earth,
which could not be done in a free and rapidly moving balloon. Although
captive balloons are frequently used in the European cities to lift
people who wish to enjoy the view from a height of 500 or 1000 feet,
they appear to have been little used by scientific observers since the
time of Glaisher. In 1890-91 the aeronautical society at Berlin
employed a captive balloon in connection with the observations in free
balloons which have been described. This captive balloon had a
capacity of only 5000 cubic feet, but it sufficed to lift an apparatus
weighing sixteen pounds, designed by Dr. Assmann to record atmospheric
pressure, as well as the temperature and relative humidity of the air.
The balloon, attached to a cable 2600 feet long, was drawn down by a
steam engine. It was possible in this way to have simultaneous
observations at three levels, viz. near the ground, in the free air at
a height of about half-a-mile, and at the highest level attained by a
free balloon. But the captive balloon is often at a disadvantage, for
the wind drives it down, and although the meteorograph mentioned had
ingenious devices to neutralize the violent shocks caused by this and
by the rebound of the balloon after the gust of wind, yet these
impaired the automatic record. The height to which the balloon rose
was so much diminished by the wind that instead of 2600 feet, which
the balloon attained in calm weather when the cable was vertical, the
average height of the twenty-four ascents was but half this, and in
very windy weather the balloon could not rise at all.

   [Illustration: FIG. 4.--German Kite-balloon.]

To obviate these difficulties, a few years ago there was invented by
two officers of the German army, Lieutenants von Siegsfeld and von
Parseval, a captive balloon capable of resisting strong winds, called,
from its action as a kite, the _Drachen-Ballon_ or kite-balloon, and
which at the present time is being successfully used in the German
Army and Navy for reconnoitring in all kinds of weather. A smaller
kite-balloon, of 7700 cubic feet capacity, filled either with
hydrogen or with illuminating gas, was first used to lift
meteorological instruments at Strassburg in 1898, where it remained at
a height of several hundred feet during twenty-four hours. As is seen
from Fig. 4, the balloon is cylindrical, with hemispherical ends, and
is attached to its cable like a kite, so that the wind acts to lift
and not to depress it. The cylinder is divided by a diaphragm near its
lower end into two chambers, the upper and larger one being filled
with gas, while the lower chamber, by means of a valve opening
inwards, receives the pressure of the wind which presses against the
diaphragm, and preserves the sausage-like form of the balloon in spite
of leakage of gas. Another wind-bag encircling the bottom of the
air-chamber serves as a rudder, and lateral fins or wings give
stability to the balloon about its longer axis. The instruments are
placed in a basket hung far below the balloon. In cases where there is
little or no wind at the ground, captive balloons can render valuable
service for meteorological observations, but in all other cases kites
are preferable. The reasons for this assertion will be given when we
consider kites.

From what has been said it will be perceived how much the Germans did
to advance scientific ballooning, yet their constant rivals, the
French, found a way to surpass them in the exploration of the
atmosphere. For several years the struggle for supremacy in the
attainment of the greatest heights was keen between the scientific men
of both countries, but a truce was declared at Paris in 1896, and
since then both nations have worked together harmoniously. The
friendly meeting of French and German physicists at Strassburg in 1898
to agree upon the details of co-operation, typified the union of
nations through science, and while it is true that the atmosphere has
no boundaries and cannot be pre-empted, let us hope that the common
aims of science will ultimately obliterate even political boundaries.



                               CHAPTER IV

   _BALLONS-SONDES_ FOR GREAT ALTITUDES--THE INTERNATIONAL ASCENTS


We have seen that the ascent of human beings to heights of six miles
is attended with difficulty and danger, and even with apparatus for
supplying the life-sustaining oxygen, man can hardly hope to reach
much greater altitudes. Consequently, to obtain information about the
atmospheric strata lying above six miles, that is to say, those facts
which require to be ascertained in the medium itself, we must employ
the so-called _ballons-sondes_, carrying self-recording instruments
but no observers. This method, which was proposed in Copenhagen as
long ago as 1809, was first put into execution by the French
aeronauts, Hermite and Besançon, who, it may be remarked, suggested
attempting to reach the North Pole by balloon some time before Andrée
announced his scheme.

A balloon is the best of anemometers, since it takes the direction
and speed of the currents in which it floats, and hence it is
customary, before a manned balloon starts, to dispatch several small
pilot-balloons in order to judge of the direction and strength of the
upper winds. Even if we do not know the height of the currents in
which they float, though this can be ascertained by measuring the
height of the balloon trigonometrically or micrometrically, we still
obtain a general knowledge of the direction and speed of the currents.
With this idea, M. Bonvallet in 1891 dispatched from Amiens, France,
ninety-seven paper balloons, each provided with a postal card asking
for the time and place of descent. Sixty of these cards were returned,
almost all the balloons having been carried east by the upper current,
ten going beyond one hundred and thirty miles, and one travelling at a
speed of almost one hundred miles an hour.

The next year the experiment was continued by MM. Hermite and Besançon
with balloons of thirty-five cubic feet contents, and about half of
those dispatched from Paris were recovered within a radius of one
hundred miles. The height to which the balloons could rise is
determined by the following considerations: to ascend 18,000 feet,
where the atmospheric pressure is one-half that at the earth (see
Plate I.), the balloon when half full of gas must lift itself from the
ground; to rise 35,000 feet, where the pressure is reduced to
one-quarter, it must be able to start upward when one-quarter filled,
and so on. In practice the ascensional force usually diminishes at
first from various causes, such as the escape of gas, its cooling, and
the deposit of moisture on the outside of the balloon. To penetrate
the clouds, therefore, it is necessary to have a considerable excess
of ascensional force, but above the clouds, since the heating effect
of the sun increases greatly with altitude, the gas in the balloon is
warmed much above the surrounding air, and so the theoretical
altitudes are exceeded.

Having determined that balloons inflated with one hundred and fifty
cubic feet of coal-gas would rise to great heights, simple and light
registering instruments, as well as the postal cards, were attached to
them. As the pressure diminished, an aneroid barometer traced a line
on a smoked glass, and after the descent was placed under the receiver
of an air-pump, and the pressure required to reproduce the trace was
measured by a manometer. From this the height could be computed
approximately. The maximum and minimum thermometer was of the
well-known U-form, and instructions appended asked that it be read as
soon as found. A slow-match was arranged to detach postal cards
successively, so that if they were found and mailed, the track of the
balloon could be determined. These balloons at first were called
_ballons perdus_, or lost balloons, but when it was known that most of
the fourteen balloons liberated from Paris were recovered, the name
_ballons explorateurs_ was given, which was afterwards changed to
_ballons-sondes_, or sounding balloons. The Germans call them,
_Registrir-Ballons_, and in English they have been designated unmanned
balloons also. One of these paper balloons having reached a height of
nearly 30,000 feet, MM. Hermite and Besançon proceeded to construct a
balloon of gold-beater's skin, having a capacity of 3960 cubic feet,
in order to carry a better instrumental equipment. The self-recording
instruments made by the French firm of Richard Brothers were well
adapted for this purpose, and a combined barometer and thermometer,
registering in ink on an upright drum that is turned by clockwork
inside, is shown in Fig. 5. The exhausted pair of boxes B of the
barometer actuates the lower pen, while the curved tube C, which is
filled with alcohol, by its change of shape moves the upper pen and
records the temperature. From the indications of the barometer and the
temperature of the mass of air, it is possible by Laplace's formula to
calculate the height at any hour of the registration. The balloon
mentioned was the first of the so-called _Aérophiles_, and when
inflated with coal-gas it could lift seventy-seven pounds besides its
own weight of forty pounds. It carried two of the baro-thermographs
described, and a package of information cards arranged to be detached
by a slow-match. To mitigate the shock of striking the ground one of
the instruments was hung by rubber cords inside a wicker basket that
in the first ascent was not screened from the sun. It was decided to
liberate these balloons entirely filled with gas (instead of partly
full, to allow for its expansion), and to utilize all possible
ascensional force at first rather than to weight the balloon with an
automatic discharger of ballast (Fig. 6). The trial of the _Aérophile_
occurred March 21, 1893, and the next day one of the cards was
returned announcing its fall in the department of the Yonne, where the
balloon and the instruments were recovered injured. From the blurred
traces of the latter it was computed that at an altitude of about
49,000 feet a temperature of -60° Fahrenheit had been met with, both
pressure and temperature being the lowest measured in a balloon up to
that time. Although the data secured by this ascent were somewhat
doubtful, yet the feasibility of exploring the atmosphere by
_ballons-sondes_ was proved. It was seen that the enormous velocity of
ascent overcame the wind and permitted the path of the balloon to the
summit of its trajectory to be followed, the balloon appearing like a
meteor visible in daylight, and so its height could be calculated by
trigonometrical measurements; while the descent, caused by the escape
and cooling of the gas, was gentle and regular, permitting the
delicate instruments to be recovered uninjured.

   [Illustration: FIG. 5.--Baro-thermograph of Richard.]

   [Illustration: FIG. 6.--The _Aérophile_ rising. The left-hand
   picture shows the deformation caused by the resistance of the
   air to its rapid ascent, and the right-hand one the violent
   oscillations when first liberated.]

The second ascent of this _Aérophile_ was its last, for, after falling
in the Black Forest, it was burned by children. However, M. Besançon,
not discouraged, constructed the _Aérophile II._ of 6300 cubic feet,
and improved the instruments as experience suggested. The records had
often been interrupted by freezing of the ink, so the pen was replaced
by a needle marking with less friction on smoked paper surrounding the
record drum. To avoid heating of the thermometers by the sun, they
were placed in a wicker cylinder open at both ends and covered with
bright metallic paper. This was hung below the balloon with its axis
vertical, in order that the draught through the cylinder when the
balloon was rising or falling should counteract the insolation, and in
the next ascent, at about the same altitude, a temperature lower by
36° Fahrenheit indicated the effect of the protection. To secure an
independent record of the lowest temperature an ingenious device was
used, consisting of a thermometer tube filled with alcohol and having
black divisions. The lowest point to which the alcohol sank was
recorded on photographic paper placed behind the tube, the whole being
enclosed in a metallic box that was automatically closed on striking
the ground, and so was preserved against the meddling of curious
persons. Up to the middle of 1898 ten voyages had been made by the
_Aérophiles_, which were now constructed of varnished silk to hold
16,000 cubic feet of gas.

One of the objects sought was the collection of samples of air at
great heights, but this was not accomplished until recently. In the
first apparatus for this purpose, an aneroid barometer at a
predetermined pressure turned the cock communicating with an exhausted
receiver that filled with air and was then closed. The cock leaked, so
next the ingenious device of generating heat chemically to seal the
glass tube was tried. This, too, failed, but finally, an apparatus of
M. Cailletet solved the problem. It is advisable to control the height
deduced from the barometric records by direct observations so long as
the balloon remains visible, and for this purpose micrometric measures
were made with a telescope as soon as the balloon left the ground.
There was also used a species of registering theodolite which, when
kept pointed at the balloon, automatically traced on paper its azimuth
and angular altitude. These records, when combined with the barometric
height at a known hour, permitted the horizontal distance traversed,
and hence the velocity, to be calculated, or, with two such
instruments at ends of a base line, the height of the balloon could be
found.

The first experiments with _ballons-sondes_ in France were soon
repeated in Germany, where a balloon of rubber-fabric holding 8700
cubic feet was obtained by the German Society for the Promotion of
Aërial Navigation. When inflated with coal-gas it had a lifting force
of about two hundred and ninety pounds, in excess of its envelope,
etc., weighing ninety-three pounds, and the meteorological apparatus
weighing six pounds. The _Cirrus_, as it was called, burst on its
first trial, but in July 1894 it made a remarkable voyage from Berlin
to the boundary of Bosnia, a distance of seven hundred miles, at an
average speed of sixty-two miles an hour. A maximum height of 54,000
feet and a minimum temperature of -63° Fahrenheit were recorded. The
_Cirrus_ on its third voyage was accompanied by manned balloons in
order to have simultaneous observations at different levels, and this
time it travelled eighty-three miles an hour and rose 61,000 feet. The
lowest temperature of -88° Fahrenheit was supposed to be too high, for
the reason that whereas the ventilation of the thermometers in a
rapidly ascending or descending balloon might be sufficient to
counteract solar radiation, this would not be the case when the
balloon was approaching its culminating point with a diminishing
speed. Therefore, Dr. Assmann, under whose supervision the German
experiments were conducted, employed the thermometer, which in the
captive balloon was aspirated electrically, but now was driven by a
weight, and later, because the ink froze, the registration was made
photographic. The efficacy of the aspirator was seen in the ascent
referred to, for, when its action stopped, a higher temperature was
recorded though the balloon continued to ascend. In April 1895 the
_Cirrus_ rose to the extraordinary height of 72,000 feet, or more than
thirteen and a quarter miles, where the barometric pressure was
reduced to one and a half inches of mercury. (In Plate I. this extreme
and possibly excessive height is not shown as the height of the
_ballon-sonde_, but the average of the three highest ascents of the
_Cirrus_ is indicated.) The comparative warmth (-50° Fahrenheit)
recorded has led Dr. Assmann himself to doubt the accuracy of the
usual methods of registering temperature at such extremely low
pressures. Plate VII. shows the heights in metres, and the
temperatures in degrees Centigrade, during eight voyages from Berlin
prior to June 1897.

Notwithstanding the rivalry and difference of opinion between the
Germans and French as to the methods of exploring the high atmosphere,
there was also a sincere desire to co-operate, and the International
Meteorological Conference which was held at Paris in September 1896
furnished an opportunity to make the arrangements. Resolutions were
adopted favouring ascents with manned balloons, as well as
simultaneous ascents of _ballons-sondes_ in the different countries.
The successful use of kites at Blue Hill to lift self-recording
instruments more than a mile into the air led to the wish that similar
experiments should be tried elsewhere. An International Committee was
appointed to carry out these resolutions, of which Professor Hergesell
of Strassburg is president, and the veteran Parisian aeronaut and
journalist, Wilfrid de Fonvielle, is secretary.

   [Illustration: PLATE VII.--HEIGHTS AND TEMPERATURES RECORDED IN
   EIGHT ASCENTS OF THE _CIRRUS_.]

It was agreed to make a night ascent and to use identical instruments,
in order that the observations might be made everywhere under the same
conditions. Accordingly, on the early morning of November 14, 1896,
five balloons manned by observers, and three _ballons-sondes_ with
recording instruments, were liberated in France, Germany, and Russia.
By means of the automatic diagrams from the _ballons-sondes_, and the
direct observations in the manned balloons, it was sought to determine
the decrease of temperature with height in vertical sections of the
atmosphere connecting the various centres from which the balloons
started. Seven such sections were available by connecting Paris and
Strassburg, Berlin and St. Petersburg, Warsaw and Munich, etc., but,
unfortunately, observations in the highest strata were generally
lacking.

Three more international ascents were made during the year 1897, which
were participated in less extensively. At this time it was necessary
to decide questions that had arisen, and to make plans for the future,
consequently a meeting of the International Committee was held at
Strassburg in 1898. Many technical questions were settled, but the
chief result accomplished was the dissipation of misunderstandings and
prejudices, not only between French and Germans, but between the
German representatives themselves, for no doubt personal intercourse
is the greatest good of such conferences. Although it was not a
surprise, nevertheless it was regretted that no one came from Great
Britain, where, since Glaisher's epoch-making balloon ascensions,
little has been done to explore the air. The beneficial results of the
Conference were apparent at the fifth international ascent, which
occurred in the early morning of June 8, 1898. Austria and Belgium
joined Germany, France, and Russia, and the field of atmospheric
survey was extended over a good part of Europe. A veritable aeronautic
fleet was launched from Paris, Brussels, Berlin, Warsaw, St.
Petersburg, Strassburg, Munich, and Vienna, consisting of twenty-one
balloons, of which thirteen carried observers, who all used the
aspiration thermometers, and eight were equipped only with
self-recording instruments. Some of the latter balloons reached
altitudes of 50,000 feet, and the former attained extreme heights of
one-third this. On the day selected the atmosphere was in a state of
repose, with light variable winds, except high up, where they blew, as
is usual, from the west or south-west. These observations were
sufficiently numerous to form a synoptic chart at a considerable
height above Europe for comparison with the usual chart drawn from the
surface observations.

Besides the general work of the International Committee, special
investigations have been undertaken by the French, who formed an
Aerostatic Commission in Paris. The services of the eminent
physicists, MM. Cailletet and Violle, have been enlisted, while a
generous patron has been found in Prince Roland Bonaparte. The
apparatus of M. Cailletet to bring down samples of air from the high
regions may now be described. When the balloon has reached its
greatest height a cock of special construction, turned by clockwork,
opens and allows the air to enter a reservoir in which a vacuum
exists, and then the cock is automatically and hermetically closed. As
it is known that the balloon reaches its extreme height in about an
hour and a quarter, the time of opening the cock is so regulated, the
closing taking place a little later by its further rotation. In order
to protect the moving parts from the extreme cold, a receptacle filled
with fused acetate of soda is placed in the box containing the motor,
so that, notwithstanding the intense cold of the high regions, this
salt in assuming a crystalline state gives out enough heat during
several hours at least. During the ascent of an _Aérophile_ air was
collected at 50,000 feet, which when analyzed by M. Müntz showed what
was supposed, viz. that at this altitude the composition of the air
does not differ much from that of the lower air. The slight excess of
carbonic acid found in the upper air might be due to the oxidation of
the grease used on the cock, and the smaller quantity of oxygen, as
compared with normal air, might be caused by the absorption of this
gas by the grease or even the absorption by the tinned sides of the
copper reservoir. By eliminating all possible sources of error in
future ascensions, M. Müntz thinks that it can be proved whether there
are real differences in the air at different heights, for the methods
of analysis are to-day accurate enough to show such differences if
they exist. But since it is probable that in the regions which can be
explored by the _ballons-sondes_, the air undergoes the same mixing
that renders the lower air nearly uniform, only the smallest
variations in its composition would naturally be found, requiring
minute precautions against errors. This is no doubt why previous
measures agreed in showing the invariable composition of the air at
lower altitudes.

Another important contribution of M. Cailletet is an apparatus for
measuring the height of the balloon by photography in order to verify
Laplace's formula connecting the barometric pressure with the
altitude. The idea was to replace the observers on the ground, who
sometimes made the trigonometrical measurements of the balloon
described, by a photographic apparatus carried by the balloon itself,
and which at frequent intervals should photograph automatically the
ground over which it passed, at the same time that an aneroid
barometer was photographed on the same sheet. The apparatus is hung
vertically below the balloon; in the lower portion of the box is an
objective which photographs the ground, and in the upper portion is a
second objective which photographs the face of an aneroid barometer
placed at the proper distance above. A clock-movement makes exposures
every two minutes, and a sensitive film unrolled between the
objectives receives the images on each side. If there are known, the
focal length of the objective, the distance of two points on the
ground, and the distance of two points on the photograph, a simple
proportion permits the height of the balloon to be determined at that
time, and consequently, from the barometric record, the law connecting
the pressure with height can be deduced. The apparatus was
successfully used in the voyage of a large balloon with observers, and
the accuracy of the determination of height was found to be within
1/250. If the apparatus is to be used at great heights it would be
necessary to protect the barometer and the camera from the very low
temperatures. Besides the use for which it was designed, this
apparatus may serve to trace the route of a balloon and to determine
the horizontal velocity at the different points of its path.

The exploration of the high atmosphere by _ballons-sondes_, which can
aid so many investigations, has been utilized by M. Violle to obtain
actinometric measures, that is, to determine the amount of heat given
by the sun, or what is called the "solar constant." This has been done
on mountains with varying results, due to the changing amount of
atmospheric absorption. In regions traversed by the balloon where the
pressure of the air is reduced to a few inches of mercury, where there
is a complete absence of water-vapour, and at heights to which
terrestrial dust does not extend, the measure of the quantity of heat
sent by the sun towards the earth is freed from almost all the errors
which we encounter on its surface. The actinometer of M. Violle is, in
principle, a sphere of copper, blackened externally, and having inside
a thermometric apparatus which registers some distance away. Under the
action of the solar rays the sphere is heated, and assumes equilibrium
when the loss by radiation and by contact with the air compensates for
the gain by the absorption of the direct heat. While at low levels the
atmosphere also contributes to heat the sphere, at great heights the
sun shines from an almost black sky and alone heats the sphere. Since
the balloon follows the wind the apparatus is protected from air
currents which would otherwise introduce errors. Each twenty minutes a
screen cuts off the solar rays from the sphere so that it cools to
the temperature of the air, which is also recorded. On account of its
weight this apparatus has not yet been carried by a _ballon-sonde_,
but it has operated successfully in a balloon with observers.

M. Teisserenc de Bort, who is actively engaged in exploring the air
from a meteorological standpoint, has constructed a very sensitive
thermometer made of a blade of German silver set in a frame of
nickel-steel that does not expand with heat. This may be ventilated by
a fan, and, while extremely sensitive to changes of temperature, it is
not affected by shocks, and consequently is well adapted for use in
_ballons-sondes_ that pass rapidly through air-strata of varying
temperature.

From this review of the development of the _ballons-sondes_ it is
evident that they offer possibilities of obtaining data in the high
atmosphere, perhaps up to fifteen miles or more, which, though subject
to inaccuracies, are of great interest to the physicist and
astronomer. The meteorologist is chiefly concerned with that portion
of the atmosphere which lies within two or three miles of the earth,
and he requires, moreover, accurate measurements for his conclusions.
The new and most satisfactory way of obtaining these data is by kites,
and the remaining chapters will treat of this method of exploring the
atmosphere and the results.



                               CHAPTER V

   KITES--HISTORY AND APPLICATION TO METEOROLOGICAL PURPOSES AT
   BLUE HILL AND ELSEWHERE


Kites are supposed to have been invented four hundred years before the
Christian era by Archytas, and at Smyrna the flying of kites remains a
national sport to this day. We are told that two hundred years later,
a Chinese general, Han Sin, employed kites as a means of communication
with the garrison of a besieged town, and there is a legend about
their use in Japan to dislodge and carry away a golden ornament from a
tower. Whatever may be the truth of these stories, we know that
kite-flying in the Malay Archipelago, in China, and in Japan, has been
a pastime for all classes during centuries, and that the Asiatic
people have always been the expert kite-fliers of the world.

Kites with tails seem to have been introduced into England about two
hundred and fifty years ago, and Isaac Newton when a school-boy made
some improvements in them. Notwithstanding the fact that generations
of boys have flown kites and so eminent a mathematician as Euler
investigated their theory, until recently kites remained toys unsuited
for practical purposes. Since the tailless kite has become a familiar
object, it has been said facetiously that kites lost their tails by
the same process of evolution which deprived man of his caudal
appendage; but as kites without tails have been flown in Asia for
centuries, the truth is that the tailed kites were the ones first
brought to Europe as playthings. To-day in Holland we see boys flying
the English bow-kite and the common kite with crossed sticks, both of
which require tails, and by the side of them tailless kites imported
from the Dutch colonies in Java. Fig. 7 represents a kite from the
east coast of Java, drawn from a model in a museum at Amsterdam, and
also a drawing of a Chinese bird-kite in the National Museum at
Washington. Like most of the oriental kites, they are made flat, but
when exposed to the wind the extremities of the wings, which have a
frame of split bamboo, bend backward, securing in this way the
stability which in our common flat kite is gained by the action of the
tail in lowering the centre of gravity and in maintaining the
inclination to the wind.

   [Illustration: FIG. 7.--Oriental tailless Kites.]

From historical researches that have been stimulated by the recent
practical applications of kites, it appears that their first use for
scientific purposes was in 1749, when Dr. Alexander Wilson of Glasgow,
and his pupil, Thomas Melvill, used kites to lift thermometers. Their
kites, from four to seven feet in height, and covered with paper, were
fastened behind one another, each kite taking up as much line as could
be supported, thereby allowing its companion to soar to an elevation
proportionally higher. It is related that "the uppermost one ascended
to an amazing height, disappearing at times among the white summer
clouds, whilst all the rest, in a series, formed with it in the air
below such a lofty scale, and that too affected by such regular and
conspiring motions, as at once changed a boyish pastime into a
spectacle which greatly interested every beholder.... To obtain the
information they wanted they contrived that thermometers, properly
secured, and having bushy tassels of paper tied to them, should be let
fall at stated periods from some of the higher kites, which was
accomplished by the gradual singeing of a match-line." How the
thermometers were prevented from changing their readings while falling
to the ground is not explained. The account concludes: "When engaged
in these experiments, though now and then they communicated
immediately with the clouds, yet, as this happened always in fine
weather, no symptoms whatever of an electrical nature came under their
observation. The sublime analysis of the thunderbolt, and of the
electricity of the atmosphere, lay yet entirely undiscovered, and was
reserved two years longer for the sagacity of the celebrated Dr.
Franklin." Hence it seems that Franklin's famous experiment of
collecting the electricity of a thunder-cloud by means of a kite,
performed at Philadelphia in 1752, was not its first scientific
application, and therefore America can claim only the later and most
remarkable development of this means of exploring the air.

About 1837 there existed in Philadelphia an organization called the
Franklin Kite Club that flew kites for recreation. Espy, the eminent
meteorologist, was a member, and he states "that on those days when
columnar clouds form rapidly and numerously the kite was frequently
carried upward nearly perpendicularly by columns of ascending air," a
phenomenon which is often observed to-day. Espy calculated the height
at which clouds should form by the cooling of the air to its
dew-point, and then employed kites to verify his calculations of the
heights of the clouds. It will be remembered that both these methods
are utilized in the measurements of cloud-heights at Blue Hill. Kites
were employed to get temperatures a hundred or more feet above the
Arctic ocean early in the present century, and in 1847 W. R. Birt flew
a kite at Kew Observatory, with which he hoped to obtain measures of
temperature, humidity, wind velocity, etc. This kite, hexagonal in
shape, required three divergent strings attached to the ground to keep
it steady, and the instruments were to be hoisted up to the kite by a
pulley.

Perhaps the first person to soar aloft on a kite was a lady, who, more
than fifty years ago, was lifted some hundred feet by a great kite
constructed by George Pocock, an Englishman, to serve as an aërial
observatory in warfare, and also to drag carriages along the ground.
It was proposed afterwards to make use of kites in shipwrecks to take
persons or life-lines ashore, and in 1865 Sir George Nares invented a
storm-kite, so called, with a tail made up of hollow cones. This form
of tail, subsequently used for both kites and balloons, is very
efficient, since it offers increasing resistance as the wind becomes
stronger.

In 1882 Mr. Douglas Archibald in England revived the use of kites for
meteorological observations, and outlined a comprehensive scheme of
exploring the air with kites which included almost all that has been
done since, but his actual work, performed during the next three
years, was limited to ascertaining the increase of wind velocity with
height. To do this, he attached registering anemometers at four
different points on the kite-wire, but since the total wind movements
only were registered from the time the anemometers left the ground
until they returned, it was impossible to obtain simultaneous records
near the ground and at the kite, as is done to-day. Still, Archibald
got differential measurements of the velocity of the wind up to the
height of 1200 feet. The kites he employed were diamond-shaped,
covered with silk, and were flown tandem, with the hollow cones,
already mentioned, attached to the tails. Although copper and iron
wire had been used for flying kites many years before, yet Archibald
was the first to substitute steel pianoforte wire for the string,
thereby increasing the strength while diminishing the weight, size,
and cost of the line. Mr. Archibald in 1887 took the first photograph
from a kite, a method which MM. Batut and Wenz developed in France,
and Messrs. Eddy and Woglom in the United States.

The subsequent progress of kite-flying for meteorological purposes has
taken place in the last-named country, and may be chronologically
stated as follows: in 1885 Mr. Alexander McAdie (later of the U. S.
Weather Bureau) repeated Franklin's kite experiment on Blue Hill, with
the addition of an electrometer; in 1891, and again in 1892, he
measured simultaneously the electric potential at the base of Blue
Hill, on the hill, and with kites as collectors several hundred feet
above the hill-top, about the same time that Dr. Weber, in Breslau,
Germany, was making a more extensive use of kites for the same
purpose. It was no doubt William A. Eddy of Bayonne, N. J., who turned
the attention of American scientific men to kite-flying, and created
the widespread interest in kites which exists to-day. About 1890 Mr.
Eddy lifted thermometers with an ordinary kite, but soon afterwards
devised a tailless kite, resembling the Java kite except that the
horizontal cross-piece is nearer the top of the vertical stick, and
its ends are bent backward in a bow and connected by a cord. This kite
starts upward on being held in the wind at the end of a taut line, and
continues to rise until the increasing wind-pressure on the portion
above the cross-stick balances the pressure on the larger lower
portion. The kite is kept from falling to one side by the looseness of
the covering on either side of the backbone, and if there is more
material on one side than on the other, or if the covering is too
tight to form pockets in the wind, the kite requires a tail.[1]

   [1] A tail will prevent any kite from turning over, or "diving,"
       because its weight keeps the lower end down while the
       pressure of the wind on the tail also pulls the lower end
       backward and maintains the necessary angle of the kite to the
       wind, the most efficient angle being about 22 degrees. Bending
       back the ends of the cross-stick gives stability to a kite
       because, when, on account of the eddies in the wind, a stronger
       pressure is exerted on one side of the kite, this side is
       driven backward, thereby presenting less effective surface to
       the wind, while as the other side comes forward more nearly at
       right angles to the wind, it receives greater pressure than
       before. In this way the equilibrium about the central stick is
       automatically maintained, the required inclination to the wind
       being secured by the greater surface presented to the wind
       below the point of attachment of the bridle.

In 1891 Mr. Eddy lifted a minimum thermometer by several of these
kites flown tandem, and proposed to obtain in this way data to
forecast the weather. In the _Proceedings of the Aeronautical
Conference_, held in connection with the Chicago Exposition, Prof. M.
W. Harrington, then Chief of the U. S. Weather Bureau, quoted Mr.
Eddy's estimate of the cost of exploring the air by means of kites
flown in series, and advocated their use.

Up to this time it does not appear that self-recording
instruments--that is to say, those which make continuous graphic
records--had been raised by kites. In the days of the early
experimenters such instruments were too heavy and cumbersome to be
lifted by the more or less unmanageable kites, but within the past few
years M. Richard of Paris has made the simple and light recording
instruments described in connection with balloons, which can be
attached to kites. In this way it is possible to obtain simultaneous
records at the kite and at a station on the ground, and from them to
study the differences of temperature and humidity, and this seems to
have been done first at Blue Hill Observatory. In August 1894 Mr. Eddy
brought his kites to Blue Hill and with them lifted a Richard
thermograph, which had been partly reconstructed of aluminium by Mr.
Fergusson so that it weighed but 1-1/4 lbs., to the height of 1500
feet, and so the earliest automatic record of temperature was obtained
by a kite. During the next summer, Mr. Eddy assisted again in the
experiments at Blue Hill, and secured photographs of the Observatory
and the hill by a camera carried between his kites to the height of a
hundred feet or more.

Now that the possibility of lifting self-recording meteorological
instruments to considerable heights had been demonstrated, an
investigation of the thermal and hygrometric conditions of the free
air was undertaken by the staff of the Blue Hill Observatory, who had
already made an investigation of the movements of the clouds by the
methods described in the second chapter.

The development of the kite and its accessory apparatus, and the
acquisition of the knowledge how to use them, required much time, and
resulted in the damage or loss of many kites. Two meteorographs, as
the combination of two or more self-recording instruments is called,
were dropped from a great height and no trace of them was found. When,
however, by the breaking of the line both kites and instrument are
carried away, the kites act as a parachute and bear the instrument
gently to the ground, where both are usually recovered uninjured; to
facilitate their return should they fall at a distance, the name and
address are marked on each. It would be tedious to relate the ups and
downs of scientific kite-flying at Blue Hill before the wind was
successfully harnessed to the service of science, and the kites were
prevented from kicking over the traces, or from breaking away, so only
a brief account of the progress of the work will be given, and then
the methods at present used will be described. At first the Eddy, or
Malay kites, as they are also called, covered with paper or with
varnished cloth, were coupled tandem to secure greater safety and
lifting power. The principle of attaching kites at several points on
the line was early adopted at Blue Hill, for although it can be
demonstrated theoretically that a greater height is possible by
concentrating all the pull at the end of the line, yet in the case of
a line which is not infinitely strong the best results are got by
distributing the pull, and in this way, too, kites can be added as the
wind conditions aloft warrant. To obviate the frequent breaking of the
bowed cross-piece, Mr. Fergusson made it in two pieces, each being
held in a metal socket on the central stick, the two pieces forming a
dihedral angle towards the wind. It had the advantage also of being
readily taken apart for transportation. This kite, shown in Fig. 8,
flew at a high angle above the horizon and through a considerable
range of wind velocity, but it could not be kept permanently in
balance, or made to adjust itself to great variations in wind
velocity, and therefore it was discarded.

The first meteorograph, a combined recording thermometer and barometer
(from which the height can be calculated), was constructed by Mr.
Fergusson in August 1895, and three months later he united a recording
anemometer to the thermometer, which was probably the first apparatus
of this kind to be attached to kites. A meteorograph, recording the
atmospheric pressure, air temperature, and relative humidity, was
ordered from M. Richard of Paris in 1895, like one already carried by
French aeronauts, except that, since for kites lightness is
all-essential, M. Richard constructed this triple-recorder for the
first time of aluminium, and hereby reduced its weight to 2-4/5 lbs.

   [Illustration: FIG. 8.--Eddy tailless Kite.]

   [Illustration: FIG. 9.--Hargrave Kite.]

One of these meteorographs was hung to a ring at the point of
attachment of the two kite-lines to the main line, a method which was
used until recently. In August 1895, besides the Eddy kites, there was
first used the cellular or box kite, invented by Lawrence Hargrave of
Sydney, Australia, which bears no resemblance to the conventional
forms of kites and which it would not be supposed could fly. As seen
from Fig. 9 its appearance is that of two light boxes without tops or
bottoms, fastened some distance above each other. The wind exerts its
lifting force chiefly upon the front and rear sides of the top box,
the lower box, which inclines to the rear, and so receives less
pressure, preserving the balance. The ends of the boxes, being in line
with the wind, keep the kite steady and serve the purpose of the
dihedral angle in the Malay kite. The Japanese are said to fly a
single box, which is the prototype of the Hargrave double cell.

At the present time some form of the Hargrave kite is generally
employed for scientific purposes. On account of the weight of the
large cord necessary to control these kites, and the surface which it
presented to the wind, a height of 2000 feet could not be reached, so,
during the winter of 1895-6, following Archibald's example and the
methods of deep-sea sounding employed by Captain Sigsbee, U. S. N.,
steel pianoforte wire was substituted for the cord. This wire is less
than half as heavy, and less than one-fourth the size of cord having
the same strength, and, moreover, its surface is polished, which
reduces the friction of the wind blowing past it. With the wire the
height of a mile was reached in July, and a mile and two-thirds above
Blue Hill in October 1896.

Up to this time a reel turned by two men sufficed to draw down the
kites, but the increasing pull and length of wire made recourse to
steam-power necessary. In January 1897 a grant of money was allotted
from the Hodgkins Fund of the Smithsonian Institution for the purpose
of obtaining meteorological records at heights exceeding ten thousand
feet, and no doubt the first application of steam to kite-flying was
the winch built by Mr. Fergusson with ingenious devices for
distributing, oiling, and measuring the length of wire. The cumulative
pressure of the successive coils of wire finally crushed the drum, and
the next apparatus applied the principle of Sir William Thomson's
deep-sea sounding apparatus, in which there is no accumulation of
pressure. In October 1897 records were brought down from eleven
thousand feet, or a thousand feet above the prescribed height.

The kites and apparatus at present employed at Blue Hill will now be
described.

The kites are all of the multiplane type, and mostly of Hargrave's
construction with two rectangular cells. These cells are covered with
cloth or silk, except at their tops and bottoms, and one is secured
above the other by four or more sticks. The wooden frames are as light
as possible, but are made rigid by guys of steel wire that bind them
in all directions. The average weight is about two ounces a square
foot of lifting surface, which is about the same weight a square foot
as the Eddy kites when all the surface is included in the estimate.
The largest of the Hargrave kites stands nine feet high, weighs eleven
pounds, and contains ninety square feet of lifting surface, which in
the recent kites is arched, resembling the curvature of a bird's
wings, a construction that was proposed many years ago by Phillips
(Fig. 10). These curved surfaces increase the lift, or upward pull,
more than the drift, or motion to leeward, and so the angular
elevation is augmented without materially adding to the total pull on
the wire, which should not exceed one-half its breaking strength.

  [Illustration: FIG. 10.--Modified Hargrave Kite at
   Blue Hill.]

Perhaps the most important factor in the success of the Blue Hill work
was the invention by Mr. Clayton of the regulating bridle which is
applied to every kite. An elastic cord is inserted in the lower part
of the bridle, to which the flying-line is attached, and when the
wind-pressure increases this cord stretches, and causes the kite to
diminish its angle of incidence to the wind until the gust subsides. A
kite can be set to pull only a fixed amount in the strongest wind,
when the kite will fly nearly horizontal. We are therefore able to
calculate the greatest pull which can be exerted on the wire by all
the kites. With this device the kites have flown through gales of
fifty or sixty miles an hour without breaking loose or injuring
themselves. Another efficient kite which has been used at Blue Hill is
the so-called "aero-curve kite" made by Mr. C. H. Lamson of Portland,
Maine. As is seen from Fig. 11, this kite resembles a soaring bird,
and it can be taken apart and folded up for storage or transportation.

In general, the angle of the flying lines of the Blue Hill kites is
50° or 60° above the horizon, and in winds of twenty miles an hour the
pull on the line is about one pound for each square foot of lifting
surface in the kite. Kites can be raised in a wind that blows more
than twelve miles an hour at the ground, and as the average velocity
of the wind for the year on Blue Hill is eighteen miles an hour, the
days are few when kites will not fly there.

   [Illustration: FIG. 11.--Lamson's Aero-curve Kite.]

The wire to which the kites are attached is steel music-wire, 32/1000
of an inch in diameter, weighing fifteen pounds a mile, and capable of
withstanding a pull of three hundred pounds. The wire is spliced in
lengths of more than a mile with the greatest care, special pains
being taken that no sharp bends or rust-spots occur which would cause
it to break. To lift the increasing weight of wire, kites are
attached at intervals of a few thousand feet, so that the angle may be
maintained as high as is consistent with a safe pull, and this is done
by screwing on the wire aluminium clamps, to which the kite-lines are
fastened. On account of the greater stability and strength of the new
kites, the meteorograph is suspended directly from the top kite. The
Richard meteorograph, contained in an aluminium cage of about a foot
cube, weighs less than three pounds, and it is only necessary to
screen the thermometer from the sun's rays to obtain the true
temperature of the air, since the wind insures a circulation of air
around the thermometer. Another meteorograph, constructed by Mr.
Fergusson, records the velocity of the wind in addition to the three
other elements, and it weighs no more than the French instrument.

The reeling apparatus is an example of how the same apparatus may
serve diametrically opposite purposes. In sounding the deep sea the
wire must be pulled upwards, whereas in sounding the heights of the
atmosphere the wire must be pulled in the reverse direction. Therefore
the deep-sea sounding apparatus has been altered by Mr. Fergusson to
pull obliquely downwards, the wire passing over a swivelling pulley
which follows its direction and registers on a dial the exact length
unreeled. Next the wire bears against a pulley carried by a strong
spiral spring, by which the pull upon it at all times is recorded on a
paper-covered drum turned by clockwork. The wire passes now several
times around a strain-pulley, and finally is coiled under slight
tension upon a large storage-drum. When the kites are to be pulled
down, the strain-pulley is connected with a two-horse-power
steam-engine, and the wire is drawn in at a speed of from three to six
miles an hour; but when the kites are rising the belt is removed, and
the pull of the kites unreels the wire.

  [Illustration: FIG. 12.--Meteorograph lifted by Kites
   at Blue Hill.]

The method of making a kite-flight for meteorological purposes at Blue
Hill is as follows: a kite, fastened by a long wire to the ring in the
main wire, being in the air, and the meteorograph suspended, another
kite is attached to the ring by a shorter cord (Fig. 12). They are
then allowed to rise, and to unreel the wire, until its angle with the
horizon becomes low, when, by means of the clamps described, other
kites are added, the number depending on the size of the kites and the
strength of the wind. After a pause at the highest attainable
altitude, the winch is connected with the steam-engine and the kites
are drawn down. The pauses at the highest point, and when kites are
attached or detached, are necessary to allow the recording instruments
to acquire the conditions of the surrounding air; and because at
these times the meteorograph is nearly stationary, measurements of its
angular elevation are made with a surveyor's transit, while
observations of azimuth give the direction of the wind at the
different heights. The time of making each angular measurement is
noted, so that the corresponding point on the trace of the
meteorograph may be found. From the length of the wire and its
vertical angle, the height of the meteorograph can be calculated, it
having been found that the sag of the wire, or its deviation either in
a vertical or a horizontal plane from the straight line joining the
kite and the reel, does not cause an error exceeding three per cent.
in the height so computed. When the meteorograph is hidden by clouds,
the height above the last point trigonometrically determined is
computed from the barometer record by Laplace's formula. At night
there is only the barometer from which to determine the height; for
although an attempt was made to use a lantern to sight upon, yet it
soon became invisible, or, when seen, was confounded with the stars.
Before and after the flight the meteorograph is hung upon a tripod in
the free air, in order that its thermometer and hygrometer may be
compared with the standards.


HEIGHTS ABOVE SEA-LEVEL OF KITE-FLIGHTS.

(_Blue Hill is 630 feet above the sea_)

 ----+----+----------------++-------------------------------------------
     | No.|Heights in Feet ||        Percentages of Records above
 Year| of +-------+--------++-------+--------+--------+--------+--------
     |Rec-|Mean of|Absolute|| 500 m.| 1000 m.| 1500 m.| 2000 m.| 3000 m.
     |ords|Maximum|Maximum || (1640 |  (3280 |  (4920 |  (6560 |  (9840
     |    |       |        ||  ft.) |   ft.) |   ft.) |   ft.) |   ft.)
 ----+----+-------+--------++-------+--------+--------+--------+--------
 1894|  2 | 1,860 |  2,070 ||   50  |    0   |     0  |     0  |    0
 1895| 28 | 1,673 |  2,490 ||   59  |    0   |     0  |     0  |    0
 1896| 86 | 2,772 |  9,327 ||   78  |   28   |     9  |     4  |    0
 1897| 38 | 4,557 | 11,716 ||   95  |   68   |    45  |    21  |    5
 1898| 35 | 7,350 | 12,070 ||  100  |   92   |    80  |    66  |   20
 ----+----+-------+--------++-------+--------+--------+--------+--------


Since the use of wire and more efficient kites, the heights have been
greatly increased. Thus the average height above the hill attained by
the meteorograph in thirty-five flights made during 1898 was more than
a mile and a quarter, whereas the average height of all the ascents
prior to 1897 was about a quarter of a mile (see Table). The average
height of the meteorograph above the hill, in all the flights during
August 1898, was nearly a mile and a half, and on August 26 the
meteorograph was raised 360 feet higher than ever before, its
altitude, determined trigonometrically, being 11,440 feet above Blue
Hill, or 12,070 feet above the neighbouring ocean. The meteorograph
was suspended from the topmost kite, one of the Lamson pattern, having
71 square feet of lifting surface, and this was increased to a total
of 149 square feet by four kites of the modified Hargrave type, that
were attached at intervals to the wire. The five miles of wire in the
air weighed 75 lbs., and the total weight including kites and
apparatus was 112 lbs. The meteorograph left the ground at 10:40 a.m.,
attained its greatest height at 4:15 p.m., and returned to the ground
at 8:40 p.m., a feat which it would be difficult for a man to equal on
a mountain. The cumulus clouds were traversed three-quarters of a mile
from the earth, and above them the air was found to be very dry. On
the hill the air temperature was 72°, when it was 38° in the free air
11,440 feet above, and the wind velocity increased from twenty-two to
forty miles an hour. These figures give an idea of the change of
atmospheric conditions which occurs, but the conclusions deduced from
the Blue Hill kite-flights will be discussed in the next chapter.
However, the phenomena of atmospheric electricity, which have become
noticeable since the use of wire, may be described here. Generally,
whenever the kites rise above seventeen hundred feet, the wire becomes
strongly charged with electricity, and when the great heights are
reached the electricity is discharged in long and brilliant sparks at
the reel, often to the inconvenience of the attendants. Usually, the
electrical potential increases with altitude, and it is greatest
during snow-storms or when the conditions favour thunder-storms.
Notwithstanding its intensity, the quantity of electricity in the
atmosphere is probably insufficient to make its collection and
storage for practical purposes worth while.

It must not be imagined that kite-flying for meteorological purposes
is a sinecure. At Blue Hill about two hundred flights have been made
in all seasons and in all weathers, with temperatures varying from
-5° to +90°, in gales, in rain, and in snow-storms, though not in
thunder-storms. Sometimes the kites are invisible from almost the time
they leave the earth until their return, but when the upper kites are
visible it is necessary to observe them with theodolites every few
minutes. Remembering that a high flight occupies ten or twelve hours,
and frequently terminates late at night, or even continues until
morning, it will be obvious that the work requires skill, energy, and
perseverance, which have been shown by my assistants at the Blue Hill
Observatory who have conducted the flights.

Occasionally, for lack of wind or from breakage of the line, the kites
fall to the ground, usually intact. If they were visible,
trigonometrical measurements on the hill enable the place of descent
to be located, and then the kites and meteorograph are sent for and
the wire is reeled up. But at night, or when clouds hide the kites,
the direction in which they fall is not known, because the azimuth of
the wire at the reel often differs from that of the kites; so last
autumn several hundred miles of road, path, wood, and swamp were
traversed before the aërial apparatus, which had been lost during a
flight at night, was found comparatively close at hand.

From what has been said it will be evident that a former toy has been
proved to be of the greatest importance for meteorological
investigation at the Blue Hill Observatory. On account of the success
there attained it is coming into use elsewhere for meteorological
observations. In 1898 the United States Weather Bureau created
seventeen kite stations, chiefly in the Mississippi Valley, with the
intention of obtaining data every day, at the height of a mile or
more, with which to plot a synoptic weather map similar to the map
that is now drawn from the data at the ground. From a knowledge of the
weather conditions prevailing simultaneously in the upper and lower
air, it was expected that the weather forecasts could be improved, but
unfortunately, on account of the light winds during the summer, it was
impossible to make enough simultaneous kite-flights to construct the
upper-air map, and therefore the scheme was abandoned. However, the
data obtained will no doubt furnish valuable information about the
vertical temperature gradient, etc., in various conditions of weather.
The chief meteorological bureaus of Germany and Russia are equipping
stations with kites and balloons, and M. Teisserenc de Bort, who has
provided his private observatory near Paris with kite apparatus of the
Blue Hill type, has already reached high altitudes. In Scotland too,
which was the birthplace of scientific kite-flying, experiments have
been resumed by a Scotchman and an American--a happy union of forces.

From these preparations it appears that the resolution of the
International Aeronautical Conference, recommending that all central
observatories should employ this method of investigation as being of
prime importance for the advancement of meteorological knowledge, is
being carried out, and seems likely to produce important results.



                               CHAPTER VI

   RESULTS OF THE KITE-FLIGHTS AT BLUE HILL--FUTURE WORK


Kites possess several advantages over other methods of exploring the
air up to heights of at least 12,000 feet whenever there is wind, but
their chief merit is, that with them the true conditions of the air
may be ascertained. The disadvantages of other methods of exploring
the air, as compared with kites, are these:

1. =Mountains= not only affect by contact the adjacent air, but by
deflecting the air-currents cause mixture and ascent, which give
conditions differing widely from those of the free air.

2. =Free Balloons= are more or less surrounded by heated or stagnant
air, because they drift with the wind, and on account of the
sluggishness of the thermometers, the temperatures observed at a given
height in a balloon are generally higher during the ascent, _i.e._
when passing from warm to cold air, than during the descent, when the
conditions are reversed. Again, it is not possible to study the
progressive changes in the atmospheric conditions at one place,
because observations in a drifting balloon are not comparable with
simultaneous ones made at a station on the ground below. With kites,
however, the possibility of making frequent and nearly vertical
ascents and descents permits observations to be obtained almost
simultaneously in superincumbent strata of air. The height of the kite
can usually be determined with an accuracy not attainable by the
barometer in a balloon.

3. =Captive Balloons=, although constructed so as not to be driven
down by wind, cannot rise nearly so high as kites on account of the
weight and resistance of the cable necessary to control them, and even
the German kite-balloon, on account of its large surface, would hardly
withstand the strong winds in which kites can fly.

4. =The Cost= of installing and operating either mountain stations or
balloons is much greater than for kites.

The exploration of the lower two miles of air with kites flown from
Blue Hill is no doubt the most complete ever made at one place. Nearly
two hundred records have been obtained in all kinds of weather
conditions, and the progressive attainment of greater and greater
heights is shown in the table in the preceding chapter. The records
from the flights have been discussed by Mr. Clayton; those until
February 1897, with the Blue Hill Observations, in Vol. xlii., Part I.,
of the _Annals of the Astronomical Observatory of Harvard College_,
and later records in two _Bulletins_ of the Blue Hill Observatory, in
which the changes of temperature and humidity with height, and their
relation to the positions of cyclones and anti-cyclones, are
investigated. The use of kites for weather predicting, as was said,
has been tried by the United States Weather Bureau, but it is certain
that further studies, such as have been made on Blue Hill, are
necessary before the sequence of the conditions at the earth's surface
to the phenomena observed in the upper air is definitely known, so that
the latter can be utilized in forecasting.

Some of the deductions from the observations with kites at Blue Hill
follow. Plate VIII. is a facsimile of the record of the
baro-thermo-hygrograph during two flights on October 8, 1896, when for
the first time the height of a mile and a half was attained. The
record-sheet, it may be said, is wrapped around a cylinder that turns
on its axis in twelve hours, and the curved lines in each of the three
horizontal sections divide them into quarter hours. The lower section
contains the trace of the barometer, the horizontal lines being the
heights in metres and feet that correspond to the barometric
pressure with a temperature of 32° Fahrenheit; in the middle section
is the trace of the hygrometer on a scale of relative humidity in
percentages, and in the upper section is the trace of the thermometer
on a scale of temperatures in Fahrenheit and Centigrade degrees. It
will be observed that the record of the barometer is reversed, _i.e._
the trace rises for falling pressure, and in the second flight when
the unexpected height of 8697 feet above Blue Hill was reached, the
limit of the altitude scale was exceeded.

   [Illustration: PLATE VIII.--METEOROGRAM FROM THE KITE-FLIGHT OF
   OCT. 8, 1896, AT BLUE HILL.]


In order to study the changes of these elements with height during the
higher flight, in Plate IX., Figs. 4 and 5, the temperature and
humidity of the automatic record are plotted as abscissæ, with the
heights above sea-level in metres as ordinates. For those not familiar
with this unit of length, it may be said that 100 metres are about 330
feet, and that 1600 metres equal one mile approximately. When the
meteorograph was ascending, dots indicate the recorded temperatures
and humidities, which are each connected respectively by continuous
lines; when the meteorograph was descending, crosses indicate the
observations, which are connected by broken lines. Lines inclining
upwards to the left indicate decreasing temperature and humidity with
increase of height, and lines inclining to the right increasing
temperature and humidity with height. The straight dotted lines show
the adiabatic decrease of temperature for ascending dry air. The
ascent was made during the warmest part of the day, and the descent
for the most part after sunset. The two branches of the
temperature-lines typify the temperature change with height which
usually occurs in fair weather during the day and the night
respectively. The continuous line, representing the day observations,
shows a uniform fall of temperature at the adiabatic rate to the cloud
level. During the night, the lower part of the broken line bends
decidedly to the left, showing a body of relatively cold air near the
ground, caused by radiation. There is a rise of temperature with
increasing altitude above the ground up to a certain height, and
afterwards a comparatively uniform fall as high as the clouds, if they
exist; but the rate of fall with increasing altitude, shown by the
upper part of the diagram, is slower at night than during the day. It
appears that the diurnal change of temperature is very small at great
altitudes, compared with the change near the earth's surface. The
relative humidity (Fig. 5) up to 2000 metres varies inversely with the
temperature, and in the present case there was only a slight change in
the direction of the wind (Fig. 6).

   [Illustration: PLATE IX.--MEAN CHANGES WITH HEIGHT, AND CHANGES
   DURING THE KITE-FLIGHT OF OCT. 8, 1896.]

=Diurnal Changes of Temperature at Different Altitudes.=--The curve
representing the diurnal change in the air at some distance above the
ground is probably similar to one representing the change near the
ground, except that its amplitude is less. If this be true, then the
diurnal rate of fall for a given time at any two levels will be
proportional to the daily ranges of temperature at the two levels. It
is impossible in practice to keep a kite at exactly the same level for
twenty-four hours; hence the daily ranges for the different levels
must be found by comparing the rates of rise or fall of temperature
for given times with the rates found from records near the ground,
made simultaneously with those above. In Plate IX., Fig. 1, the
results for six stations, _i.e._ the kite at 1000 and 500 metres, the
Eiffel Tower in Paris (300 metres), the summit of Blue Hill, its base,
and the valley (200, 50, and 15 metres respectively), are connected,
and a smooth curve is drawn through them. The curve passes
approximately through every one of the observed and the computed
ranges, except the one at the summit of Blue Hill, which is too great.
This evidently is because insolation and radiation, acting through the
soil of the hill, heat and cool the air to a greater extent than the
free air is heated and cooled at the same altitude, and this must be
true at every mountain station. The smoothed curve passes also very
slightly to the left of the data for the Eiffel Tower, indicating
that the range there is about 1° greater than the true range on
account of the heating and cooling of the Tower. From this it appears
that the diurnal range of temperature diminishes rapidly with
increasing altitude in the free air, and almost disappears in the
average at a height of 1000 metres.

The records of the anemometer show that, as a rule, the wind increases
steadily as the kites rise, but the increase is greatest between
Boston and the top of Blue Hill, due probably to the retarding of the
lower winds by contact with the ground. The results are plotted in
Plate IX., Fig. 3, together with the mean wind velocity on Blue Hill
(209 metres), and the velocity on a tower in Boston (60 metres).
Single records of the kite-anemometer differ much, for sometimes the
wind velocity diminished with altitude, and at other times it
increased so rapidly that the kites were unable to rise higher. On
several occasions when the kites passed from one current into another,
having a different direction and a different temperature, the wind
suddenly increased, and was stronger between the two currents than
above or below that plane.

=Diurnal Changes of Humidity at Different Altitudes.=--It is found
that as night approaches the humidity at the altitude of 1000 metres
diminishes, while at the earth it increases. This agrees with the
evidence furnished by the cumulus clouds that form during the day
between 1000 and 2000 metres, and disappear at night, thus visibly
indicating an increase of humidity by day and a decrease by night. If
the trend of the humidity-curve at a height of 1000 metres is assumed
to be the reverse of its trend at the ground, then the results from
the kite-meteorograph show the minimum humidity to be at the coldest
and the maximum humidity at the warmest part of the day. The mean
daily ranges for different altitudes are plotted in Plate IX., Fig. 2.
The part of the curve at the left of the zero line shows the range at
different altitudes, with the minimum humidity near the warmest time
of day, while the part at the right of the zero shows the ranges at
different altitudes, with the minimum humidity at the coldest time of
day.

=Types of Change of Temperature with Altitude.=--When the records of
temperature and humidity made aloft by the kite-meteorograph and at
the stations near the ground are plotted in relation to altitude, they
are found to be easily divisible into a few types. In Plate X., Type 1
represents the decrease of temperature on most fair days from the
ground to altitudes of a mile or more, when no clouds are met. The
continuous line, plotted from the records of the ascent, represents
the day conditions, and the broken line, plotted from the records of
the descent, represents the night conditions. This curve shows that
with increasing altitude the temperature falls uniformly during the
day and approximately at the adiabatic rate represented by the dotted
lines. The fall of temperature with increasing altitude during the
night is slower than during the day, and in fact, from the earth's
surface to an altitude of a few hundred metres, there is often a rise
of temperature with height, so that the air at altitudes of from 300
to 500 metres may be considerably warmer than it is at the ground.
This was shown in the descent on October 8, 1896, and is found in Type
3.

   [Illustration: PLATE X.--CHANGES WITH HEIGHT RECORDED BY KITES
   AT BLUE HILL.]

When clouds are traversed during the flight, the temperature curve
assumes the form of Type 2. The continuous curve is plotted from the
records of an ascent; the broken curve from the records of the
descent, both occurring in the day-time. The temperature falls at the
adiabatic rate in unsaturated air till the base of the cumulus cloud
is reached. It falls at a slower rate in the cloud, the rate probably
being that computed by physicists as the adiabatic rate for air in
which condensation is taking place. Above the clouds, the fall of
temperature appears to be very slow.

Type 3 is a condition which persists throughout the day and night, and
it resembles the night form of Type 1. The temperature rises very
rapidly for a short distance above the ground and then falls, with
increase of height, somewhat slower than the adiabatic rate. The rise
of temperature near the ground with increasing height is more marked
after sunset than during the day-time.

Type 4 was illustrated by the ascent of October 8. This distribution
of temperature is caused by a warmer current overflowing colder air,
which is very commonly found at low altitudes in the atmosphere and
probably exists usually at some altitude, great or small. Recent
observations indicate that this type represents the normal condition
of the atmosphere in all sorts of weather. Frequently there are two or
more sudden rises of temperature at different heights, so that the
plotted data resemble inverted stair-steps. During the day there is a
decrease of temperature at the adiabatic rate (1°·8 in 100 metres)
from the ground to the height of several hundred metres, then a sudden
rise of temperature in the next one or two hundred metres, and above
this a slow fall of temperature with increasing altitude, usually much
less than the adiabatic rate. Generally, clouds are found near the
plane of meeting of the warm and cold current.

The reverse of Type 4, that is, a sudden fall of temperature, due to a
colder current overlying a warmer one, is probably impossible, because
the colder air, on account of its greater weight, would immediately
begin to sink and the warmer air would rise. This should cause a fall
of temperature at the adiabatic rate from the ground to the top of the
colder current, and is probably the origin of the "cold wave" shown in
Type 5. Both the continuous and broken curves (representing an ascent
and a descent) show a fall of temperature at the adiabatic rate of
unsaturated air, from about 500 metres to the highest point reached.
Up to 500 metres the decrease of temperature is more rapid than the
adiabatic rate, due to the rapid moving in of colder air above,
whereby air rising from the ground is cooled by contact as well as by
its expansion, and also because the air is heated more than usual by
contact with the ground, which under these conditions is abnormally
warmer. This is the special characteristic of the "cold wave" type of
curve during the day hours. The night form of Type 5, notwithstanding
the excessive radiation from the ground through the dry air, shows a
rapid decrease of temperature with increase of altitude from the
ground upward.

Type 6 shows a less common, but an interesting form, of vertical
distribution of temperature, in which the temperature is about the
same from 400 metres to 1400 metres or more. Up to 400 metres there is
a fall of temperature with increasing altitude during the day, and a
rise with increasing altitude at night. These last conditions can be
readily traced to the effects of insolation and radiation near the
ground. In the morning, if the temperature of the air be the same from
the ground up to 1000 metres or more, the heating of the ground by the
sun will cause ascending currents, until the warmest part of the day.
This air, cooling by expansion at the adiabatic rate, will rise to
about 440 metres before it assumes the mean temperature of the upper
air column. At night cooling takes place next the ground by radiation
and is gradually transferred upward a few hundred metres by
conduction, thus producing an increasing temperature with increasing
altitude, until sunrise. As a result of the conditions described, it
is evident that on certain days the diurnal range of temperature is
but little felt above 500 metres.

=Types of Change of Relative Humidity with Altitude.=--As in the
temperature types, the continuous lines represent the records of the
ascent, and the broken lines the records of the descent, generally
under changing conditions. Lines inclining upward to the left show a
decreasing humidity, and to the right an increasing humidity.

Type 1 may be called a normal type of curve when there are clouds. A
variation of this type was met with in the ascent on October 8, 1896,
and it differed from that now illustrated in indicating in its upper
part a fall of humidity rather than a rise. These two types can be
taken as the normal change of humidity with change of altitude in
cloudy or partly cloudy weather. The humidity increases steadily to
the base of the cloud, then there is complete saturation in the cloud,
and above it is a sudden fall of humidity, on entering the dry air
above the cloud, into which the ascending currents from the ground
have not penetrated.

Type 3 is a clear-weather form of curve in which the humidity
increases until a certain altitude is reached, probably at the upper
limits of the currents rising from the ground. Above this altitude the
humidity decreases rapidly.

Type 5 is also a clear-weather form and accompanies the "cold wave"
type of temperature, also numbered 5. The very dry descending air
mingles with air rendered damp by ascent, and the result is a nearly
uniform relative humidity at different altitudes, although the
absolute humidity diminishes on account of decreasing pressure and
temperature. In Type 6 both the relative and the absolute humidity
decrease rapidly, this type coinciding with the temperature, Type 6.

During the week of September 5 to 11, 1897, kite-flights were made
daily on Blue Hill. Twice the kites were maintained in the air, and
continuous records were obtained during most of twenty-four hours.
These records furnish an example of the small diurnal changes of
temperature in the free air at short distances above the ground, which
were deduced from the average changes at different hours and at
different heights. From 2 p.m. of the fifth to 2 p.m. of the sixth,
the altitude of the self-recording instruments varied between 500 and
1000 metres above sea-level, averaging about 700 metres and varying
little from this height during much of the night. The times when the
kite-meteorograph crossed the 700-metre level in ascending and
descending were determined from its barograph trace, and the
synchronous temperatures and humidities were read from the records of
its thermograph and hygrograph. The results have been plotted in Plate
XI., Figs. 1 and 2, together with the temperatures recorded
simultaneously at the summit and valley stations of the Observatory
and the humidities at the summit. Fig. 1 shows that the diurnal
variation of temperature, well marked at the lower levels, is very
slight or has entirely disappeared at 700 metres. Fig. 2 shows that
the course of the relative humidity at 700 metres is exactly opposite
in phase to that recorded at lower levels, for at 700 metres the
minimum humidity was recorded at night and the maximum during the day,
while the opposite conditions prevailed on the hill. Repeated
kite-flights indicate that these are the normal conditions at the two
levels.

In Plate XI., Fig. 3, is plotted a curve from the hourly readings of
the thermograph at the Blue Hill valley station (fifteen metres)
during the week, and also a curve connecting temperatures recorded by
the kite-meteorograph once or twice each day during the same week at a
level of 500 metres, obtained in the way described or computed from
the adiabatic change. All the night records show that it was decidedly
warmer at the height of 500 metres during the night than it was at the
ground, except during the cool wave on the seventh and eighth.
Furthermore, the curves in Fig. 3 indicate a control of the surface
temperatures during the day by those above. For instance, on the
seventh there was a distinct flattening of the day curve, evidently
because, as the temperature on the ground rose 10° above that at 500
metres, the air was in unstable equilibrium, and colder air descended
to take the place of the surface air so that its temperature could
rise no higher. On the tenth, the temperature at 500 metres was
considerably greater than the mean of the day at the ground, and the
air at the ground did not acquire the unstable condition in any volume
until the warmest part of the day, so that the diurnal curve at the
lower station forms a sharp peak.

   [Illustration: PLATE XI.--KITE OBSERVATIONS AT BLUE HILL,
   SEPT. 5-11, 1897.]

Since there appears to be no appreciable diurnal period in the
temperature at and above 500 metres, a better comparison of the
relative changes aloft and below during the passage of warm and cold
waves is obtained by smoothing out the diurnal period below. This has
been done in Plate XI., Fig. 4, with the data given by the kites at
500 and 1000 metres plotted in curves, which it was necessary to
complete by extrapolation. It is seen that there is a much greater
range in the temperature from the crest of a warm to the crest of a
cold wave at a height of 500 metres than at the ground. At 1000 metres
the range appears to be slightly greater than at 500 metres, and the
crests of the warm and cold waves occur successively earlier than they
do at the ground. On the approach, and until the passage of the crest
of the cold wave the air is colder aloft than at the ground, the
difference being apparently that of the adiabatic cooling of ascending
air. After the passage of the crest of the cold wave, the temperature
aloft rises much more rapidly than at the ground, and at the crest of
the warm wave the air at 500 metres is some 10° warmer than the mean
daily temperature at the ground. In many kite-flights the difference
was found to be even greater than this. Taking the mean temperature of
twenty-four-hours, it is seen that the average temperature at the
ground during a week or more is about the same as it is at 500
metres. Fig. 5 shows the change in the vertical distribution of
temperature during the oncoming of the warm wave on the eighth and
early morning of the ninth, as determined by four ascents, culminating
at 11 a.m., 9 p.m., 11 p.m., and 4 a.m. The lines of 59°, 62°, 65°,
and 68° show that there was a gradual rise of temperature aloft, which
extended downwards to 200 metres, or to the top of Blue Hill. Clouds
formed at the level of lowest temperature, and these sank also until
they covered the top of the hill.

Plate XII. is a facsimile of the meteorogram during the kite-flight of
October 15, 1897, the lower part showing the trace of the barometer on
a scale of heights in metres, the middle section the trace of the
hygrometer, and the upper one the trace of the thermometer on a scale
of Centigrade degrees. The temperature followed the normal change,
which is as follows: during the day, up to a certain height, which
varies under different conditions, there is a decrease nearly at the
adiabatic rate of 1°·8 F. per hundred metres. Above that height the
air suddenly becomes warmer, and then cools with ascent at a rate
somewhat less than the adiabatic rate. During the night there is a
marked inversion of temperature between the ground and 200 or 300
metres.

   [Illustration: PLATE XII.--AUTOMATIC RECORDS DURING A HIGH
   KITE-FLIGHT AT BLUE HILL.]

Higher than this, the temperature decreases at a fairly uniform rate,
but more slowly than the adiabatic rate. Although no clouds were
visible, yet the relative humidity increased greatly, both during the
ascent and descent, near 1500 and 2700 metres, these being about the
heights at which cumulus and alto-cumulus clouds usually form.

During September 1898 four kite-flights were made on four successive
days when an anti-cyclone and a cyclone passed nearly over Blue Hill.
This is a rare occurrence, and the mechanism of these phenomena was
accordingly studied by Mr. Clayton, some of whose deductions will now
be given, illustrated by Plate XIII. Figs. 1 and 2 give the
temperature plotted according to height on September 21 in the
anti-cyclone, and on September 22, when the barometric pressure was
falling, the full lines, as in previous diagrams, indicating
observations during the ascents, and the broken lines observations
during the descents. It is seen that from the ground the lines all
incline upward to the left, indicating a fall of temperature, to a
certain height when the lines bend to the right sharply, showing a
sudden rise of temperature. Above this, the temperature again falls,
but more slowly than at lower levels. The general prevalence of this
phenomenon was noted by Welsh in his balloon ascents in England in
1854, and the high kite-flights at Blue Hill show it to be very
frequent below 2000 metres. The plane of increased temperature usually
determines the height of the tops of cumulus and strato-cumulus
clouds. Above 2000 metres other sudden rises of temperature are found
during the highest kite-flights.

   [Illustration: PLATE XIII.--RESULTS OF KITE-FLIGHTS AT BLUE
   HILL DURING AN ANTI-CYCLONE AND A CYCLONE.]

Figs. 3 to 6 show the changes in the various elements during the four
days at some of the following levels, viz. near sea-level, 200, 1000,
2000, and 3000 metres. Fig. 3 shows the changes in the barometer at
the four levels, from which it is evident that the fall of pressure
was greatest near sea-level.

Fig. 4 shows temperature changes at the different levels, and
indicates that the changes were of the same nature up to 3000 metres.
The greatest non-diurnal range of temperature is seen to be at 1000
metres, and it diminishes both at higher and at lower levels.

Fig. 5 shows changes in relative humidity at 200, 1000, and 2000
metres. The curves show that the greatest range of humidity was at
2000 metres. There the relative humidity rose from almost zero, in the
anti-cyclone on the twenty-first, to saturation at the same level in
the cyclone. At 200 metres the change is similar to that at 2000, but
is less in amount. At 1000 metres the relative humidity fell until the
twenty-second, but then rose rapidly, showing the very dry air at
2000 metres on the twenty-first had descended as low as 1000 metres on
the twenty-second.

Fig. 6 gives the change in wind velocity at the different levels.
There was an increase of wind at all the levels from the time of the
passage of the anti-cyclone to the passage of the cyclone. The minimum
of wind at 200 metres was in the anti-cyclone, with a secondary
minimum during the passage of the centre of the cyclone.

Figs. 7 to 10 show the changes in height from day to day of the equal
conditions at the different levels. Fig. 7 shows the change in level
of the isobars, which, although very small, is largest at the lower
levels. The light broken lines in Fig. 7 and subsequent figures
indicate the axes of the anti-cyclone and cyclone. That the axis of
the cyclone was inclined backward, and that the high pressure occurred
later at high than at low levels, was confirmed by the wind
observations on the twenty-first.

Fig. 8 shows the heights at which the same temperatures were found on
successive days. Since the isotherms rose until the twenty-third, the
temperature of the air up to 3000 metres was higher on the day of the
cyclone than on the day of the anti-cyclone. Previous high flights
indicate that this is the normal condition in the moving cyclones and
anti-cyclones of the eastern United States. As the light broken lines
represent the axes of the anti-cyclone and cyclone up to 3000 metres,
it is seen that at this level the temperature at the place of maximum
pressure is probably higher than at the place of minimum pressure,
although this is not true for a vertical column of air above the
earth.

Fig. 9 gives the positions of equal humidities on successive days,
saturated and cloudy areas being indicated by crossed shading, and
less humidity by single ruling. From the laws of thermo-dynamics the
unshaded curves should represent descending currents, and the shaded
portions ascending ones. In the first case, increased warmth and a
lower relative humidity are produced in the descent to a lower
altitude; in the last case, cooling, increasing relative humidity, and
condensation are produced by expansion in the ascent to a higher
altitude. Consequently, two regions of descending air are indicated,
one in the centre of the anti-cyclone, the other in the centre of the
cyclone.

Fig. 10 shows the change in height of the lines of equal wind
velocity. With ascending currents and precipitation, high wind
velocities were found at low levels, because of increased barometric
gradient, while with the descending currents in the anti-cyclone and
centre of the cyclone, the high velocities were found only at great
altitudes. The study of these data indicate that the cyclonic and
anti-cyclonic circulations observed in this latitude do not embrace
any air-movements at greater altitudes than 2000 metres, except in
front of the cyclone, when the air appears to be carried upward to a
great height. Above 2000 metres there are probably other weak cyclones
and anti-cyclones, or secondary ones, with their centres at different
places from those at the earth's surface and producing a different
circulation of wind. The observations of the cirrus clouds at Blue
Hill indicate that at their level exists a cyclonic circulation above
the anti-cyclone apparent at the earth's surface. The shallowness of
our anti-cyclones would be inferred from the great differences in
speed of the general atmospheric drift, for since the velocity of the
general drift from the west is more than thirty times greater at
10,000 metres than it is at 200 metres, a circulation of great depth
could not endure long. Cyclones and anti-cyclones appear to be but
secondary phenomena in the great waves of warm and cold air which
sweep across the United States from periodic causes.

The origin of cyclones and anti-cyclones is perhaps the most important
problem remaining for meteorological study. The theory that they are
produced by differences of temperature in adjacent masses of air, or,
as it is called, the convectional theory of the American
meteorologists, Espy and Ferrel, is opposed by the observations on
mountains in Europe which were collected by Dr. Hann of Vienna. If the
question can be solved by the use of kites, as seems to be
foreshadowed by the results just stated, another foundation-stone will
be laid in the science of meteorology and the status of the kite
established as an instrument of research. The kite fails when there is
little or no wind at the ground, but it seems possible in such cases
to lift the kite into the upper air, where there usually is wind, by
attaching it to a small balloon that, after the kite can support
itself, shall be detached automatically. While the height to which
kites can rise is limited, and the limit is probably being approached,
judging from the less gain of altitude in recent flights, yet it seems
reasonable to expect that, with favourable conditions, a height of at
least three miles will be reached.

Besides lifting the meteorological instruments described, kites can
carry apparatus for other investigations in the free air, such as the
measurement of atmospheric electricity, and the collection of samples
of air, to be examined for cosmic dust and bacteria. Cameras have been
lifted by kites, as already said, and for the purpose of photographing
the upper surfaces of clouds there is being constructed for the Blue
Hill Observatory a very light automatic camera, similar in principle
to M. Cailletet's apparatus for photographing the ground from a
balloon.

The use of the kite as an aeroplane can only be alluded to in this
book, and it may be sufficient to say that if a motor attached to a
kite can, by wings or screws, propel it against the wind, the
sustaining string is unnecessary, and we shall have the flying machine
which Professor Langley tells us will soon be realized. The surface of
our globe has been tolerably well explored; the exploration of the
atmosphere by balloons and kites will continue to make great progress
during the last year of the century, and at the end of the twentieth
century we may confidently expect that as the seas now are a medium
for transportation, so the ocean of air will have been brought
likewise into man's domain.



INDEX


 A

 Abercromby (R.), classification of clouds, 42
 Academy of Sciences, French, 18, 72-3
 Academy of Sciences, Russian, balloon ascent, 72
 Accademia del Cimento, 14
 Actinometer, Viollé's, 115
 Adiabatic rate of change of temperature, 29
 Aeronautical Conference at Chicago, 125
 ---- Conference at Strassburg, 97, 110
 ---- Committee; International, 108
 _Aérophile_ balloons, 102, 104
 Aerostatic Commission, French, 111
 Air, collection and analysis of, 70, 73, 75, 82, 112
 ---- weight of, 16
 Aitken (J.), dust particles, 39
 Alhazen (B. A.), height of atmosphere, 12
 Altitudes, comparative, 20
 Andrée (S. A.), balloon voyage to North Pole, 90
 Anti-cyclones, 59, 170
 Aratus, _Diosemeia_, 11
 Archibald (D.), kites for meteorological observations, 122
 Archytas, supposed inventor of kite, 117
 Aristotle, 10, 11, 15
 Assmann (R.), 86, 94, 108
 Atmosphere, composition of, 24
 ---- energy of upper portion, 58
 ---- extent of, 28
 ---- methods of exploring same, 35 _et seq._, 145
 ---- moisture of, 34
 ---- origin of, 23
 ---- phenomena showing height of, 26
 ---- Pliny on, 9
 ---- temperature of, 28
 Atmospheric circulation in cyclones and anti-cyclones, 60
 ---- electricity, 70, 76, 121, 141


 B

 Balloon ascents, international, 108 _et seq._
 ---- crossing the Atlantic by, 92
 ---- invention of hot-air, 19
 ---- kite, 94
 Balloons, 19, 20, 21, 37, 68, _et seq._
 ---- captive, 76, 93, 146
 ---- changes of temperature observed in, 71, 73, 75, 77, 84, 88, 90
 ---- changing the direction of, 93
 _Ballons-sondes_, 98 _et seq._
 Barometer, 15, 16, 85, 113
 Baro-thermograph of Richard, 102
 Barral, balloon ascent, 73, 84
 Batavia, Java, international cloud measurements, 65
 Batut (A.), photography from kites, 123
 Berson (A.), balloon ascents, 81, 87 _et seq._
 Bert (P.), respiration of oxygen, 82
 Besançon (G.), 98, 99, 101, 104
 Bezold (W. von), wave-cloud, 40
 Biot (J. B.), balloon ascent, 73
 Birt (W. R.), kite at Kew Observatory, 122
 Bixio, balloon ascent, 73, 84
 Blanc, Mont, 20, 21
 Blanchard, balloon ascent with Jeffries, 70, 71
 Blue Hill Observatory, 47, 51, 53, 64, 108, 126 _et seq._
 Bonaparte (Prince Roland), patron of aeronautics, 112
 Bonpland (A.), ascent in Andes, 20
 Bonvallet (L.), exploring balloons, 99
 Bouguer (P.), height of freezing-point, 18
 Boyle (R.), 16


 C

 Cailletet (L.), 112, 113
 Cambridge, Mass., clouds measured at, 53
 Castelli (B.), invented rain-gauge, 13
 Cavallo (T.), showed lightness of hydrogen, 69
 Celsius (A.), thermometer, 14
 Charles (J. A. C.), ascent in hydrogen balloon, 19, 68
 Cimento, Accademia del, 14
 _Cirrus_ balloon, 106
 Clayton (H. H.), 45, 47, 53, 62, 133, 147, 167
 Cloud, amount of, 47 _et seq._
 ---- atlases, 42, 43, 44
 ---- Committee, International, 44, 65
 ---- -year, international, 65
 Clouds, classification of, 41 _et seq._
 ---- definitions of, 45
 ---- formation of, 39
 ---- observations of direction and relative velocity, 51, 65
 ---- measurements of height and velocity, 52, 53 _et seq._, 65, 121
 ---- on Jupiter, 51
 ---- relation to forecasting, 63-4
 Cotte (L.), on clouds, 38
 Coxwell (H.), aeronaut for Glaisher, 75 _et seq._
 Crocé-Spinelli (J.), ascent in _Zenith_, 82
 Cyclones, 59, 170


 D

 Dalton (J.), water-vapour in the air, 38
 Daniell (J. F.), mountains a registering thermometer, 13
 Davis (W. M.), cloud measurements, 53
 Deluc (J. A.), theory of clouds, 42
 De Saussure (H. B.), 20, 42, 73
 ---- (H. B.), ascent of Mont Blanc, 20
 Deutsche-Seewarte, Hamburg, 43
 Donaldson (W. H.), proposed crossing Atlantic in a balloon, 92


 E

 Eddy (W. A.), 123, 124 _et seq._
 Eiffel Tower, Paris, 23, 152
 Ekholm (N.), 53, 92
 Electricity, atmospheric, 70, 76, 121, 141
 Espy (J. P.), kites to verify calculated height of clouds, 121
 Etna, ascended by ancients, 12
 Euler (L.), theory of kites, 118
 Exploring the atmosphere, methods of, 35 _et seq._, 145-6


 F

 Fahrenheit (D. G.), thermometer, 14
 Ferdinand II. (Grand Duke), distributed meteorological instruments, 17
 Fergusson (S. P.), 35, 53, 126, 128, 131, 136
 Ferrel (W.), theory of cyclones, 173
 Flammarion (C.), balloon ascents, 81
 Flying machines, future, 59, 174
 Fonvielle (W. de), 81, 108
 Förster (W.), hypothesis of _Himmelsluft_, 28
 Forecasting by kites, 143, 147
 Franklin (B.), experiment with kites, 121, 123
 Franklin Kite Club, 121


 G

 Galileo (G.), 14, 15
 Gay-Lussac (J. L.), balloon ascent, 20, 73
 German Emperor (William II.), patron of aeronautics, 86
 ---- Society for Promotion of Aërial Navigation, 86, 93, 106
 Glaisher (J.), balloon ascents, 75 _et seq._, 93
 Green (C.), aeronaut for Welsh, 74
 Grimaldi (F. M.), first measured clouds trigonometrically, 52
 Guericke (O. von), experiment of Magdeburg hemispheres, 16


 H

 Hagström (K.), measured clouds, 53
 Halley (E.), measured heights by barometer, 17
 Hann (J.), 36, 173
 Han Sin, employed kites in warfare, 117
 Hargrave (L.), invented cellular kite, 129
 Harrington (M. W.), advocated exploring air with kites, 125
 Harvard College Observatory, 35, 48
 Hazen (H. A.), highest balloon ascent in America, 85
 Height of balloon, Cailletet's apparatus for obtaining, 113
 Heights of kite-flights at Blue Hill, 21, 140
 ---- how measured by barometer, 17, 101
 Heim (A.), voyage across the Alps, 92
 Hellmann (G.), historical researches, 12
 Helmholtz (H. von), wave-cloud, 40
 Hergesell (H.), President of Aeronautical Committee, 108
 Hermite (G.), 98, 99, 101
 Hildebrandsson (H. H.), 42, 65
 Hodgkins' Fund of Smithsonian Institution, grant from, 131
 Howard (L.), cloud nomenclature, 41
 Humboldt (A. von), 18, 20, 35
 Humidity, changes with altitudes, 34, 71, 73, 77, 151, 169, 171
 ---- diurnal changes at different altitudes, 153
 ---- types of change with altitudes, 159 _et seq._
 Hutton (J.), cause of precipitation, 38
 Hygrometer, invention of, 13


 J

 Jeffries (J.), first scientific balloon ascent, 69
 ---- first to cross the English Channel, 71
 Jourdanet (D.), hypothesis of descent of man, 24
 Jovis, balloon ascent, 84
 Jupiter, analogy between cloudiness on earth and on, 51


 K

 Kepler (J.), height of atmosphere, 12
 Kew Observatory, 53, 74, 122
 Köppen (W.), cloud atlas, 43
 Kirwan (R.), temperature at different latitudes, 18
 Kite, antiquity of the, 117
 ---- Eddy or Malay, 124, 129
 ---- flights at Blue Hill, 21, 137, 140, 142
 ---- Hargrave, 129, 132
 ---- Lamson's "aero-curve," 134
 ---- photography, 123, 126, 173
 ---- theory of, 118, 124, 130
 Kites, first scientific use of, 120
 ---- first self-recording instruments raised by, 126
 ---- oriental tailless, 118
 ---- scientific uses of, 173
 Kite-winch at Blue Hill, steam, 131, 136
 Krakatoa, eruption of volcano, 27


 L

 Lamarck (J. B.), first to classify cloud forms, 41
 Lamson (C. H.), aero-curve kite, 134
 Langley (S. P.), 28, 174
 Laplace (P. S. de), 17, 23, 101
 Lavoisier (A. L.), 19, 68
 Ley (W. C.), classification of clouds, 42
 Lunardi (V.), balloon ascent, 69


 M

 M'Adie (A.), 53, 123
 Magnetism, variation with height, 73, 77
 Mallet (M.), balloon ascent, 84
 Manila, Philippine Islands, cloud measurements at, 65-6
 Mariotte (E.), law of gases, 16
 Melvill (T.), aided in first scientific use of kites, 120
 Merle (W.), oldest weather chronicles, 12
 Meteorograph for kites, 128, 136
 Meteorological conferences, international, 42, 44, 108
 Meteorology, first treated by Aristotle, 10
 ---- origin of, 10
 Misti, El, highest station, 35
 Montgolfier brothers, invented hot-air balloon, 19
 Müntz (A.), analysis of air, 112-13


 N

 Nares (Sir G.), storm-kite, 122
 Nebular hypothesis of Laplace, 23
 Neumayer (G.), cloud-atlas, 42
 Newton (I.), improved kites, 118


 O

 Olympus, mountain ascended by ancients, 11
 Oxford, oldest weather chronicles, 12


 P

 Parseval (A. von), kite-balloon, 94
 Pascal (B.), experiment with barometer, 15, 16
 Perier (F.), _idem_, 15
 Photography from balloons, 81, 113
 ---- from kites, 123, 126, 173
 Pickering (E. C.), pole-star recorder, 48
 Pike's Peak, meteorological station, 35
 Pilâtre de Rozier (J. F.), first to ascend in balloon, 19
 Pliny, the atmosphere, 9
 Pocock (G.), great kite, 122
 Poëy (A.), classification of clouds, 42
 Priestley (J.), oxygen in the air, 18


 R

 Rain-gauge, invention of, 13
 Réaumur (R. A. F. de), thermometer, 14
 Rey (J.), first thermometer filled with liquid, 14
 Riccioli (G. B.), first to measure clouds trigonometrically, 52
 Richard (Abbé), clouds, 38
 ---- (J.), self-recording instruments, 101, 125, 128, 136
 Robertson (E. G.), 72, 73
 ---- balloon ascent, 72
 Rotch (A. L.), balloon ascents, 85, 86


 S

 Sacharoff, balloon ascent, 72
 Siegsfeld (H. B. von), kite-balloon, 94
 Sigsbee (C. D.), 44, 131
 Sivel (T.), ascent in _Zenith_, 82
 Spelterini (E.), balloon voyage with Heim, 92
 Spencer (S.), aeronaut for Berson, 89
 Sweetland (A. E.), prognostics from clouds, 63
 Symons (G. J.), meteorologist and bibliophile, 12


 T

 Teisserenc de Bort (L.), 51, 116, 144
 Temperature, change with height, 18, 29, 71, 73, 75, 77, 89, 90, 104,
   107, 120, 121, 126, 141, 151, 167
 ---- diurnal changes at different altitudes, 152, 161-2
 ---- types of change with altitude, 154 _et seq._
 Theodolite, registering, 106
 Theophrastus, weather prognostics, 11
 Thermometer, aspiration, 74, 86
 ---- metallic, 116
 ---- sling, 85
 Thermometers, early, 14
 Tissandier (A.), sketches of optical phenomena, 81
 ---- (G.), ascent in _Zenith_, 81, 82
 Toronto, Canada, international cloud measurements, 65
 Torricelli (E.), invented barometer, 15
 Tycho Brahe, height of atmosphere, 12


 U

 United States hydrographic office, 44
 ---- weather bureau, 44, 65, 143
 Upsala, Sweden, clouds measured at, 53, 58, 66


 V

 Violle (J.), 112, 115


 W

 Washington (Mount), meteorological station, 35
 Weather chronicles, first, 12
 ---- forecasting by means of clouds, 63
 ---- prognostics of Aristotle and Theophrastus, 11
 ---- vane, oldest meteorological instrument, 13
 Weber (L.), measured electric potential with kites, 124
 Welsh (J.), balloon ascents, 74
 Wenz (E.), photography from kites, 123
 Wilson (A.), first scientific use of kites, 120
 Wind at different heights, 57 _et seq._, 98, 153, 170, 171
 Wire for kite-lines, 123, 131, 134
 Wise (J.), 84, 92
 Woglom (J. T.), photography from kites, 123


 Y

 Young (C. A.), limit of atmosphere, 28


 Z

 _Zenith_, catastrophe of balloon, 82
 Zero, absolute, 33



_Richard Clay & Sons, Limited, London & Bungay._



Transcriber's Notes


The text presented here is essentially that in the original printed
volume. The list of corrections (CORRIGENDA) which accompanied the
original has been applied. One additional  typo (see below) has also
been made. There may have been some minor corrections (missing period,
commas, etc. added) which are not detailed here. In the original
publication, several figures and plates were placed in the middle of
paragrahs. Here most were moved between paragraphs. The List of
Illustrations and any other page references still indicate the page
number of the original location. On page 88, the "oe" ligature was
replaced with the individual letters.

Typographical Correction

  Page   Correction
  =====  ==================================
  128    all-assential => all-essential





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