_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 BesanÁon, who, it may be remarked, suggested
attempting to reach the North Pole by balloon some time before AndrÈe
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 BesanÁon
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 BesanÁon 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 _AÈrophiles_, 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 _AÈrophile_
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 _AÈrophile_ 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 _AÈrophile_ was its last, for, after falling
in the Black Forest, it was burned by children. However, M. BesanÁon,
not discouraged, constructed the _AÈrophile 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
_AÈrophiles_, 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
AÎrial 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 _AÈrophile_ air was
collected at 50,000 feet, which when analyzed by M. M¸ntz 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. M¸ntz 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.