AERONAUTICS.
PRESSURE OF AIR IN BODIES—EARLY ATTEMPTS TO FLY IN THE AIR—DISCOVERY
OF HYDROGEN—THE MONTGOLFIER BALLOONS—FIRST EXPERIMENTS IN PARIS—NOTED
ASCENTS.
In the first part of this volume we entered into the circumstances of
air pressure, and in the Chemistry section we shall be told about the
atmosphere and its constituents. We know that the air around us is
composed principally of two gases, oxygen and nitrogen, with aqueous
vapour and some carbonic acid. An enormous quantity of carbonic acid
is produced every day, and were it not for the action of vegetation
the amount produced would speedily set all animal life at rest. But
our friends, the plants, decompose the carbonic acid by assimilating
the carbon and setting free the oxygen which animals consume. Thus our
atmosphere keeps its balance, so to speak. Nothing is lost in nature.
We have illustrated the pressure of the atmosphere by the Magdeburg
hemispheres, and we know that the higher we ascend the pressure is
lessened. The weight of the atmosphere is 15 lbs. to the square inch
at sea level. This we have seen in the barometer. Now pressure is
equal. Any body immersed in a liquid suffers pressure, and we remember
Archimedes and the crown. It displaced a certain amount of water when
immersed. A body in air displaces it just the same. Therefore when any
body is heavier than the air, it will fall just as a stone will fall in
water. If it be of equal weight, it will remain balanced in the air,
if lighter it will rise, till it attains a height where the weight of
the atmosphere and its own are equal; there it will remain till the
conditions are altered. Now we will readily understand why balloons
float in the air, and why clouds ascend and descend in the atmosphere.
In the following pages we propose to consider the question of
ballooning, and the possibility of flying. We all have been anxious
concerning the unfortunate balloonist who was lost in the Channel, so
some details concerning the science generally, with the experiences of
skilled aeronauts, will guide us in our selection of material. We will
first give a history of the efforts made by the ancients to fly, and
this ambition to soar above the earth has not yet died out.
From a very early period man appears to have been desirous to study the
art of flying. The old myths of Dædalus and Icarus show us this, and it
is not to be wondered at. When the graceful flight of birds is noticed,
we feel envious almost that we cannot rise from the earth and sail away
at our pleasure over land and sea. Any one who has watched the flight
of the storks around and above Strasburg will feel desirous to emulate
that long, swift-sailing flight without apparent motion of wing, and
envy the accuracy with which the bird hits the point aimed at on the
chimney, however small. It is small wonder that some heathens of old
time looked upon birds as deities.
The earliest flying machine that we can trace is that invented by
Archytas, of Tarentum, B.C. 400. The historian of the “Brazen
Age” tells us how the geometrician, Archytas, made a wooden pigeon
which was able to sustain itself in the air for a few minutes, but
it came down to the ground after a short time, notwithstanding the
mysterious “aura spirit” with which it was supposed to be endowed. The
capability of flying has for centuries been regarded as supernatural.
Putting angels aside, demons are depicted with wings like bats’ wings,
while witches, etc., possessed the faculty of flying up chimneys upon
broomsticks. We even read in childish lore of an old woman who “went up
in a basket” (perhaps a balloon-car), and attained a most astonishing
altitude-an elevation no less than “seventy times as high as the moon!”
But to descend to history. It is undoubtedly true that in the time of
Nero, Simon Magus attempted to fly from one house to another by means
of some mechanical contrivance, and failing, killed himself. Roger
Bacon, the “admirable doctor,” to whom the invention of gunpowder
is generally attributed, had distinct notions of flying by means of
machines, and “hollow globes,” and “liquid fire.” But he did not
succeed, nor did many successive attempts succeed any better in
subsequent years. Bishop Wilkins treated of the art of flying, but
most, if not all who discussed the subject appear to have been indebted
to Roger Bacon for the idea.
When the nature and pressure of the atmosphere by Torricelli’s
experiments became better known, Father Lana, a Jesuit priest,
constructed a flying machine or balloon of curious shape. He proposed
to fix four copper globes, very thin, and about twenty feet in
diameter, and to these he fastened a boat or car, looking very much
like a basin. His idea was to empty his great copper globes, and that
their buoyancy would then bear the weight of the traveller. But he
overlooked or was ignorant of the effect of the atmospheric pressure,
which would have speedily crushed the thin copper globes when empty.
Lana’s suggestion was made in 1670, the barometer had been discovered
in 1643.
There were some fairly successful experiments made in flying in 1678
and in 1709. The former attempt was made by Besmir, a locksmith of
Sable, who raised himself by means of wings up to the top of a house by
leaps, and then succeeded in passing from one house to another lower
down by supporting himself in the air for a time. He started from an
elevated position, and came down by degrees. Dante, a mathematician,
also tried to fly, but without great success. He broke his thigh on one
occasion. Laurence de Gusman claimed an invention for flying in 1709,
and petitioned for a “patent,” which was granted by the king’s letter.
The machine appears to have borne some resemblance to a bird.
It was not till 1782, however, that the true art of aerial navigation
was discovered. The knowledge of hydrogen gas possessed by Cavendish
in 1766 no doubt led up to it, and in the year following its discovery
Professor Black, lecturing in Edinburgh, stated that it was much
lighter than the atmosphere, and that any vessel filled with the
gas would rise in the air. We now come to the invention of the
BALLOON (so called from its shape being similar to a vessel
used in the laboratory) by the Brothers Montgolfier.
[Illustration: Fig. 293.—Montgolfier balloon.]
Stephen and James Montgolfier were paper-makers, and carried on their
business at Annonay, near Lyons, but it was partly by accident that
the great discovery was made. They had no knowledge of the buoyancy
of hydrogen gas. They took their idea of the balloon (inflated) from
noticing an ascending column of smoke. It occurred to Stephen that
if a paper bag were filled with smoke it would ascend into the air.
A large bag was made and some paper burnt beneath it in a room. When
the smoke had filled the bag it was released, and immediately ascended
to the ceiling. Here was the germ of the Montgolfier or heated air
balloon. The experiment was repeated in the open air with even greater
success, and a trial upon a larger scale was immediately determined
upon. A story is related of Mongolfier when prosecuting his researches,
that a widow whose husband had belonged to the printing firm with whom
Montgolfier was then connected in business, saw the smoke issuing from
the room in which the little balloon was being filled. She entered, and
was astonished to see the difficulty experienced by the experimenter
in filling the balloon. It swerved aside, and increased the trouble
he had to keep it above the chafing dish. Montgolfier was greatly
troubled, and seeing his disappointment, the widow said, “Why don’t you
fasten the balloon to the chafing dish?” This had not occurred to the
experimenter, and the idea was a valuable one. That was the secret of
success.
The Montgolfier Brothers determined to exhibit their successful
experiment, and accordingly on the 5th of June, 1783, a great concourse
assembled to see the wonderful sight. A large canvas or linen balloon
was made and suspended over a fire of chopped straw. The heated air
quickly filled the balloon, which rose high in the air, and descended
more than a mile away. This balloon contained 22,000 cubic feet of
heated air, which is lighter than cold air, and of course rising
carried the globe with it. As soon as the air began to cool the balloon
ceased to rise, and as it got colder descended.
Here was the actual discovery of the science of Aerostatics. The
intelligence of the success achieved soon spread from France to other
countries. Paris, however, was in advance, and the Brothers Robert
applied hydrogen gas to a balloon which was sent up from the Champ de
Mars in August 1783. There was some trouble experienced in filling
it, but when the balloon was at length released it realized all
expectations by remaining in the air nearly an hour. When at length it
fell it met with a worse fate than it deserved, for the ignorant and
superstitious peasantry at once destroyed it. After this Montgolfier
exhibited his experiment next time at Versailles in the presence of
the Court. The first aerial travellers appeared on this occasion—viz.,
a sheep, a cock, and a duck, which were secured in the car. They all
descended in safety, and this success encouraged M. Pilatre de Rozier
to make an attempt in a “fire balloon.” He went up first in a captive
balloon, and at length he and a friend, the Marquis d’Arlandes,
ascended from the Bois de Boulogne. The trip was a decided success, and
the possibility of navigating the air was fully demonstrated.
Soon after this,—viz., in December 1783,—an Italian Count, named
Zambeccari, made an ascent in London, and came down safely at Petworth.
MM. Charles and Robert ascended from Paris in December, and in February
a balloon crossed the English Channel. We must pass over some time
and come to the ascents of Lunardi, which caused great excitement in
London. His balloon was a very large one, and was inflated, or rather
partially so, at the Artillery ground. Some delay occurred, and fearing
a riot, M. Lunardi proposed to go up alone with the partially-filled
balloon. A Mr. Biggin who had intended to ascend was left behind. The
Prince of Wales was present, with thousands of spectators. Lunardi
cast off and ascended rapidly, causing great admiration from the whole
metropolis. Judge and jury, sovereign and ministers, all turned out
to gaze at the balloon; a guilty prisoner was acquitted hurriedly, so
that no time was lost in discussion, and one lady died of excitement.
Lunardi was regarded as a hero, and made many other ascents. He died in
1806.
In those earlier days one or two fatal accidents happened. Count
Zambeccari and a companion were in a balloon which caught fire, and
both occupants of the car leaped from it as they were descending. The
Count was killed on the spot, and his companion was much injured.
Pilatre de Rozier made an attempt to cross the channel to England in
1785; he had reached three thousand feet when the balloon caught fire,
and the unfortunate traveller was precipitated to the ground. His
associate only survived him a few minutes.
[Illustration: Fig. 294.—MM. Charles’ and Roberts’ balloon.]
[Illustration: Fig. 295.—Blanchard’s balloon.]
It is to the celebrated English aeronaut, Mr. Green, that the
substitution of carburetted hydrogen or street gas for hydrogen is due,
and since his ascent in 1821 no other means of inflation have been
used. A great many quite successful and a few unsuccessful ascents
have been made for pleasure or profit. Mr. Green, in the _Nassau_
balloon, passed over to Nassau, a distance of five hundred miles, in
eighteen hours. This exploit was the cause of the name being bestowed
upon the balloon. The _Giant_ of M. Nadar was exhibited in England,
and it was an enormous one, being an hundred feet high, and nearly as
wide in the widest part. But even this machine was outdone by the
Godard “Montgolfier” balloon, which was one hundred and seventeen feet
high, and carried a stove. We give illustrations of these celebrated
balloons, and will now pass on to the more scientific portion of the
subject and the ascents of Mr. Glaisher and other aeronauts for the
purpose of making meteorological observations, and the use of balloons
for purposes of observation in war.
It appears that the first ascent for scientific investigation was made
in the year 1803. The aeronauts were Messrs. Robertson and Lhoest.
They ascended from Hamburg and came down at Hanover, and made meantime
several experiments with reference to the electrical condition of the
atmosphere, its influence upon a magnetic needle, and some experiments
with regard to acoustics and heat. The report was presented to the
St. Petersburg Academy, and contains the result of their interesting
observations. The travellers ascertained that at the elevation to which
they attained,—viz., 25,500 feet,—the temperature was on that July
day fifty degrees colder, falling to 19·6°, while on the earth the
thermometer had shown 68°. They ascertained that glass and wax did not
become electric when rubbed, that the Voltaic battery lost much of its
power, that the oscillation of a “dipping needle” increased as they
mounted into the air, while sound was certainly less easily transmitted
at that elevation, and struck them as less powerful in tone. The heat
experiment was not a success, owing to the breaking of the thermometer.
They wished to find the temperature of boiling water at that elevation,
but when the experiment was about to be made Robertson accidentally
plunged the instrument _into the fire_ instead of into the water. So
the question was not settled.
The effect upon the aeronauts was a sensation of sleepiness, and two
birds died. The muscular powers of the voyagers also appear to have
been much affected, and similar sensations may be experienced by
travellers on high mountains who find their breath very short and a
disinclination to exertion oppress them.
MM. Biot and Gay-Lussac made a very interesting ascent in 1804. We will
detail their experiences at some length, for the coolness displayed
and the value of the observations made are remarkable in the history
of scientific ballooning. They started, at 10 o’clock a.m. on the 23rd
of August, and when the balloon had carried them up to an altitude of
8,600 feet they commenced their experiments. They had some animals in
the car with them, a bee amongst the number, and the insect was let go
first. It flew away swiftly, not at all inconvenienced apparently. The
sun was very hot at 56° Fahr. Their pulses were beating very fast, but
no inconvenience was felt.
When 11,000 feet had been reached a linnet was permitted to go at
large, but after a little time the bird returned to the balloon. It
remained perched for a few minutes, and then dashed downwards at a
tremendous pace. A pigeon was then liberated. It also appeared very
uncertain, and wheeled around in circles for a time. At last it gained
confidence, and descended, and disappeared in the clouds beneath.
They made other experiments, but descended without having obtained as
accurate results as had been anticipated.
[Illustration: Fig. 296.—The Nassau balloon.]
[Illustration: Fig. 297.—The “Giant” balloon of M. Nadar.]
On the next occasion, however, every care was taken, and on the 15th
of September the important ascent was made by Gay-Lussac alone. He
fixed hanging ropes to the balloon with the view to check the rotating
movements, and having provided himself with all necessary apparatus
and two vacuum flasks to bring down some of the upper air, the young
man started. The barometer marked 30·66", the thermometer 82° (Fahr.).
At an elevation of 12,680 feet Lussac perceived that the variation of
the compass was the same as on land. Two hundred feet higher up he
ascertained that a key held in the magnetic direction repelled with
the lower, and attracted with its upper extremity the north pole of
a needle. This experiment was repeated with the same result at an
elevation of 20,000 feet, which shows how the earth exercises its
magnetic influence. The temperature of the air was found to decrease
in proportion as the ascent up to 12,000 feet, where the reading was
47·3°. It then increased up to 14,000 feet by 6°, and then regularly
diminished again as the balloon rose, till at the greatest elevation
reached, 23,000 feet, there was a difference of 67° in the temperature
on the earth, for at the maximum height attained the thermometer stood
at 14·9°.
But the most important fact ascertained, and one which set many
theories at rest, was the composition of the atmosphere in those high
altitudes. We mentioned that Gay-Lussac took up two empty flasks from
which the air had been taken. The vacuum was almost perfect. When
the aeronaut had reached 21,460 feet he opened one flask, and it was
quickly filled; he secured it carefully; and when at his highest
point,—four miles and a half above the sea-level,—he opened the
other flask. The barometer stood at 12.95 inches, and the cold was
very great. The voyager felt benumbed, and experienced difficulty of
breathing; his throat was parched and dry. So Lussac determined to
return, he could go no higher. He dropped gently near Rouen, and soon
reached Paris. As soon as possible the air in the flasks was submitted
to very delicate tests, and to the satisfaction of the scientists
engaged it was found to be in exactly the same proportions as that
collected near the earth—two hundred and fifteen parts of oxygen to
every thousand of atmospheric air.
Messrs. Banal and Bixio, in 1850, also made some observations, and
found the temperature very variable. At 23,000 feet they found the
thermometer at _minus_ 38·2° Fahr., which was much below the cold
experienced by Gay-Lussac. We may still conclude that the various
currents of the atmosphere cause considerable variation, and that it is
impossible to lay down anything respecting the degrees of heat and cold
likely to be found at certain elevations. We quote Arago’s observations
upon this ascent:—
“This discovery” (the ice particles found in the air) “explains how
these minute crystals may become the nucleus of large hailstones,
for they may condense round them the aqueous vapour contained in the
portion of the atmosphere where they exist. They go far to prove the
truth of Mariotte’s theory, according to which these crystals of ice
suspended in the air are the cause of parahelia—or mock-suns and
mock-moons. Moreover, the great extent of so cold a cloud explains very
satisfactorily the sudden changes of temperature which occur in our
climates.”
M. Flammarion gives in his “Voyages” some very interesting and amusing
particulars, as well as many valuable scientific observations. During
one ascent he remarked that the shadow of the balloon was _white_, and
at another time dark. When white the surface upon which it fell looked
more luminous than any other part of the country! The phenomenon was
an _anthelion_. The absolute silence impressed the voyager very much.
He adds, “The silence was so oppressive that we cannot help asking
ourselves are we still alive! We appear to appertain no longer to the
world below.” M. Flammarion’s observations on the colour of what we
term the sky are worth quoting—not because they are novel, but because
they put so very clearly before us the appearance we call the “blue
vault.” He says,—speaking of the non-existence of the “celestial
vault,”—“The air reflects the blue rays of the solar spectrum from
every side. The white light of the sun contains every colour, and the
air allows all tints to pass through it except the blue. This causes
us to suppose the atmosphere is blue. But the air has no such colour,
and the tint in question is merely owing to the reflection of light.
Planetary space is absolutely black; the higher we rise the thinner the
layer of atmosphere that separates us from it, and the darker the sky
appears.”
[Illustration: Fig. 298.—The “Eagle” of M. Godard.]
Some beautiful effects may be witnessed at night from a balloon, and
considering the few accidents there have been in proportion to the
number of ascents, we do not wonder at balloon voyages being undertaken
for mere pleasure. When science has to be advanced there can be no
objection made, for then experience goes hand-in-hand with caution.
It is only the ignorant who are rash; the student of Nature learns to
respect her, and to attend to her admonitions and warnings in time.
The details of the ascents of famous aeronauts give us a great deal
of pleasant and profitable reading. The phenomena of the sky and
clouds, and of the heavens, are all studied with great advantage from
a balloon, or “aerostat,” as it is the fashion to call it. The grand
phenomena of “Ulloa’s circles,” or _anthelia_, which represent the
balloon in air, and surrounded by a kind of glory, or aureola, like
those represented behind saintly heads, appear, as the name denotes,
opposite to the sun.
The various experiments made to ascertain the intensity of sounds have
resulted in the conclusion that they can be heard at great distances.
For instance, the steam whistle is distinctly audible 10,000 feet up in
the air, and human voices are heard at an altitude of 5,000 feet. A
man’s voice alone will penetrate more than 3,000 feet into the air; and
at that elevation the croaking of frogs is quite distinguishable. This
shows that sound ascends with ease, but it meets with great resistance
in its downward course, for the aeronaut cannot make himself audible
to a listener on the earth at a greater distance than 300 or 400 feet,
though the latter can be distinctly heard at an elevation of 1,600
feet. The diminution of temperature noted by M. Flammarion is stated
to be 1° Fahr. for every 345 feet on a fine day. On a cloudy day the
mean decrease was 1° for every 354 feet of altitude. The temperature
of clouds is higher than the air surrounding them, and the decrease is
more rapid near the surface, less rapid as the balloon ascends. We may
add that at high elevations the cork from a water-bottle will pop out
as if from a champagne flask.
We have hitherto referred more to M. Flammarion and other French
aeronauts, but we must not be considered in any way oblivious of
our countrymen, Messrs. Glaisher, Green, and Coxwell, nor of the
American,—one of the most experienced of aerial voyagers,—Mr. Wise. The
scientific observations made by the French voyagers confirmed generally
Mr. Glaisher’s experiments. This noted air-traveller made twenty-eight
ascents in the cause of science, and his experiences related in
“Travels in the Air,” and in the “Reports” of the British Association,
are both useful and entertaining. For “Sensational ballooning” one
wishes to go no farther than his account of his experience with Mr.
Coxwell, when (on the 5th of September, 1862) he attained the greatest
elevation ever reached, viz., seven miles, or thirty-seven thousand
feet.
We condense this exciting narrative for the benefit of those who have
not seen it already.
The ascent was made from Wolverhampton. At 1.39 p.m., the balloon was
four miles high, the temperature was 8°, and by the time the fifth
mile had been reached the mercury was below zero, and up to this time
observations had been made without discomfort, though Mr. Coxwell,
having exerted himself as aeronaut, found some difficulty in breathing.
About 2 o’clock, the balloon still ascending, Mr. Glaisher could not
see the mercury in the thermometer, and Mr. Coxwell had just then
ascended into the ring above the car to release the valve line which
had become twisted. Mr. Glaisher was able to note the barometer,
however, and found it marked 10 inches, and was rapidly decreasing. It
fell to 9¾ inches, and this indicated an elevation of 29,000 feet!
But the idea was to ascend as high as possible, so the upward direction
was maintained. “Shortly afterwards,” writes Mr. Glaisher, “I laid my
arm upon the table possessed of its full vigour, and on being desirous
of using it I found it powerless,—it must have lost power momentarily.
I tried to move the other arm, and found it powerless also. I then
tried to shake myself, and succeeded in shaking my body. I seemed to
have no limbs. I then looked at the barometer, and whilst doing so my
head fell on my left shoulder.”
Mr. Glaisher subsequently quite lost consciousness, and “black
darkness” came. While powerless he heard Mr. Coxwell speaking, and then
the words, “Do try, now do.” Then sight slowly returned, and rousing
himself, Mr. Glaisher said, “I have been insensible.” Mr. Coxwell
replied, “You have, and I, too, very nearly.” Mr. Coxwell’s hands were
black, and his companion had to pour brandy upon them. Mr. Coxwell’s
situation was a perilous one. He had lost the use of his hands,
which were frozen, and had to hang by his arms to the ring and drop
into the car. He then perceived his friend was insensible, and found
insensibility coming on himself. There was only one course to pursue—to
pull the valve line and let the gas escape, so as to descend. But his
hands were powerless! As a last resource he gripped the line with his
teeth, and bending down his head, after many attempts succeeded in
opening the valve and letting the gas escape. The descent was easily
made, and accomplished in safety.
[Illustration: Fig. 299.—A descending balloon.]
Some pigeons were taken up on this occasion, and were set free at
different altitudes. The first, at three miles, “dropped as a piece of
paper”; the second, at four miles, “flew vigorously round and round,
apparently taking a dip each time”; a third, a little later, “fell like
a stone.” On descending a fourth was thrown out at four miles, and
after flying in a circle, “alighted on the top of the balloon.” Of the
remaining pair one was dead when the ground was gained, and the other
recovered.
The observations noted are too numerous to be included here. Some,
we have seen, were confirmed by subsequent aeronauts, and as we have
mentioned them in former pages we need not repeat them. The results
differed very much under different conditions, and it is almost
impossible to decide upon any law. The direction of the wind in the
higher and lower regions sometimes differed, sometimes was the same,
and so on. The “Reports” of the British Association (1862-1866) will
furnish full particulars of all Mr. Glaisher’s experiments.
We have scarcely space left to mention the parachutes or umbrella-like
balloons which have occasionally been used. Its invention is traced
to very early times; but Garnerin was the first who descended in a
parachute, in 1797, and continued to do so in safety on many subsequent
occasions. The parachute was suspended to a balloon, and at a certain
elevation the voyager let go and came down in safety. He ascended once
from London, and let go when 8,000 feet up. The parachute did not
expand as usual, and fell at a tremendous rate. At length it opened
out, and the occupier of the car came down forcibly, it is true, but
safely. The form of the parachute is not unlike an umbrella opened,
with cords attaching the car to the extremities of the “ribs,” the top
of the basket car being fastened to the “stick” of the umbrella.
Mr. Robert Cocking invented a novel kind of parachute, but when he
attempted to descend by it from Mr. Green’s balloon it collapsed, and
the unfortunate voyager was dashed to pieces. His remains were found
near Lee, in Kent. Mr. Hampton did better on Garnerin’s principle, and
made several descents in safety and without injury.
The problem of flying in the air has attracted the notice of the
Aeronautical Society, established in 1873, but so far without leading
to practical results, though many daring and ingenious suggestions have
been put forth in the “Reports.”
It is not within our province to do more than refer to the uses of
the balloon for scientific purposes, but we may mention the services
it was employed upon during the French war, 1870-71. The investment
of Paris by the German army necessitated aerial communication, for no
other means were available. Balloon manufactories were established,
and a great number were made, and carried millions of letters to the
provinces. Carrier-pigeons were used to carry the return messages to
the city, and photography was applied to bring the correspondence into
the smallest legible compass. The many adventures of the aeronauts are
within the recollection of all. A few of the balloons never reappeared;
some were carried into Norway, and beyond the French frontier in other
directions. The average capacity of these balloons was 70,000 cubic
feet.
Of course it will be understood how balloons are enabled to navigate
the air. The envelope being partly filled with coal-gas-heated air and
hydrogen, is much lighter than the surrounding atmosphere, and rises
to a height according as the density of the air strata diminishes. The
density is less as we ascend, and the buoyant force also is lessened
in proportion. So when the weight of the balloon and its occupants is
the same as the power of buoyancy, it will come to a stand, and go no
higher. It can also be understood that as the pressure of the outside
becomes less, the expansive force of the gas within becomes greater.
We know that gas is very compressible, and yet a little gas will fill
a large space. Therefore, as the balloon rises, it retains its rounded
form, and appears full even at great altitudes; but if the upper
part were quite filled before it left the ground, the balloon would
inevitably burst at a certain elevation when the external pressure of
the air would be removed, unless an escape were provided. This escape
is arranged for by a valve at the top of the balloon, and the lower
part above the car is also left open very often, so that the gas can
escape of itself. When a rapid descent is necessary, the top valve is
opened by means of a rope, and the balloon sinks by its own weight. Mr.
Glaisher advises for great ascensions a balloon of a capacity of 90,000
cubic feet, and only filled one-third of that capacity with gas. Six
hundred pounds of ballast should be taken.
[Illustration: Fig. 300.—Filling a balloon.]
A very small quantity of ballast thrown away will make a great
difference; a handful will raise the balloon many feet, and a chicken
bone cast out occasions a rise of thirty yards. The ballast is carried
in small bags, and consists of dry sand, which speedily dissipates in
the air as it falls. By throwing out ballast the aeronaut can ascend
to a great height—in fact, as high as he can go, the limit apparently
for human existence being about seven miles, when cold and rarefied air
will speedily put an end to existence.
It is a curious fact, that however rapidly the balloon may be
travelling through the air, the occupants are not sensible of the
motion. This, in part, arises from the impossibility of comparing it
with other objects. We pass nothing stationary which would indicate
the pace at which we travel. But the absence of oscillation is also
remarkable; even a glass of water may be filled brim-full, and to
such a level that the water is above the rim of the glass, and yet
not a drop will fall. This experiment was made by M. Flammarion. When
the aeronaut has ascended some distance the earth loses its flat
appearance, and appears as concave as the firmament above. Guide ropes
are usually attached to balloons, and as they rest upon the ground
they relieve the balloon of the amount of weight the length trailing
would cause. They thus act as a kind of substitute for ballast as the
balloon is descending. Most of the danger of aerial travelling lies in
the descent; and though in fine weather the aeronaut can calculate to a
nicety where he will descend, on a windy day, he must cast a grapnel,
which catches with an ugly jerk, and the balloon bounds and strains at
her moorings.
Although many attempts have been made to guide balloons through the
air, no successful apparatus has ever been completed for use. Paddles,
sails, fans, and screws have all been tried, but have failed to achieve
the desired end. Whether man will ever be able to fly we cannot of
course say. In the present advancing state of science it may not be
impossible ere long to supply human beings with an apparatus worked
by electricity, perhaps, which will enable them to mount into the air
and sustain themselves. But even the bird cannot always fly without
previous momentum. A rook will run before it rises, and many other
birds have to “get up steam,” as it were, before they can soar in the
atmosphere. Eagles and such heavy birds find it very difficult to rise
from the ground. We know that vultures when gorged cannot move at all,
or certainly cannot fly away; and eagles take up their positions on
high rocks, so that they may launch down on their prey, and avoid the
difficulty of rising from the ground. They swoop down with tremendous
momentum and carry off their booty, but often lose their lives from
the initial difficulty of soaring immediately. We fear man’s weight
will militate against his ever becoming a flying animal. When we obtain
a knowledge of the atmospheric currents we shall no doubt be able to
navigate our balloons; but until then—and the information is as yet
very limited, and the currents themselves very variable—we must be
content to rise and fall in the air, and travel at the will of the wind
in the upper regions of the atmosphere.