CHAPTER III.
THE THEORY OF BALLOONS.
A certain proposition in physics, known as the "Principle of Archimedes," runs to the following effect:—"Every body plunged into a liquid loses a portion of its weight equal to the weight of the fluid which it displaces." Everybody has verified this principle, and knows that objects are much lighter in water than out of it; a body plunged into water being acted upon by two forces—its own weight, which tends to sink it, and resistance from below, which tends to bear it up. But this principle applies to gas as well as to liquids—to air as well as to water. When we weigh a body in the air, we do not find its absolute weight, but that weight minus the weight of the air which the body displaces. In order to know the exact weight of an object, it would be necessary to weigh it in a vacuum.
If an object thrown into the air is heavier than the air which it displaces, it descends, and falls upon the earth; if it is of equal weight, it floats without rising or falling; if it is lighter, it rises until it comes to a stratum of air of less weight or density than itself. We all know, of course, that the higher you rise from the earth the density of the air diminishes. The stratum of air that lies upon the surface of the earth is the heaviest, because it supports the pressure of all the other strata that lie above. Thus the lightest strata are the highest.
The principle of the construction of balloons is, therefore in perfect harmony with physical laws. Balloons are simply globes, made of a light, air-tight material, filled with hot air or hydrogen gas which rise in the air because they are lighter than the air they displace.
The application of this principle appeared so simple, that at the time when the news of the invention of the balloon was spread abroad the astronomer Lalande wrote—"At this news we all cry, 'This must be! Why did we not think of it before?'" It had been thought of before, as we have seen in the last chapter, but it is often long after an idea is conceived that it is practically realised.
The first balloon, Montgolfier's, was simply filled with hot air; and it was because Montgolfier exclusively made use of hot air that balloons so filled were named Montgolfières. Of course we see at a glance that hot air is lighter than cold air, because it has become expanded and occupies more space—that is to say, a volume of hot air contains actually less air than a volume of the same size of air that has not been heated. The difference between the weight of the hot air and the cold which it displaced was greater than the weight of the covering of the balloon. Therefore the balloon mounted.
And, seeing that air diminishes in density the higher we ascend, the balloon can rise only to that stratum of air of the same density as the air it contains. As the warm air cools it gently descends. Again, as the atmosphere is always moving in currents more or less strong, the balloon follows the direction of the current of the stratum of air in which it finds itself.
Thus we see how simply the ascent of Montgolfières, and their motions, are explained. It is the same with gas-balloons. A balloon, filled with hydrogen gas, displaces an equal volume of atmospheric air; but as the gas is much lighter than the air, it is pushed up by a force equal to the difference of the density of air and hydrogen gas. The balloon then rises in the atmosphere to where it reaches layers of air of a density exactly equal to its own, and when it gets there it remains poised in its place. In order that it may descend, it is necessary to let out a portion of the hydrogen gas, and admit an equal quantity of atmospheric air; and the balloon does not come to the ground till all, or nearly all, the gas has been expelled and common air taken in.
Balloons inflated with hydrogen gas are almost the only ones in use at the present day. Scarcely ever is a Montgolfière sent up. There are aeronauts, however, who prefer a journey in a Montgolfière to one in a gas-balloon. The air voyager in this description of balloon had formerly many difficulties to contend with. The quantity of combustible material which he was bound to carry with him; the very little difference that there is between the density of heated and of cold air; the necessity of feeding the fire, and watching it without a moment's cessation, as it hangs in the réchaud over the middle of the car, rendered this sort of air travelling subject to many dangers and difficulties. Recently, M. Eugène Godard has obviated a portion of this difficulty by fitting a chimney, like that which is found of such incalculable service in the case of the Davy lamp. It is principally on account of this improvement that the Montgolfière has risen so highly in popular esteem.
Generally it is not pure hydrogen that is made use of in the inflation of balloons. Aeronauts content themselves with the gas which we burn in our streets and houses, and thus it suffices, in inflating the balloon, to obtain from the nearest gas-works the quantity of gas necessary, and to lead it, by means of a pipe or tube, from the gasometer to the mouth or neck of the machine.
The balloon is made of long strips of silk, sewn together, and rendered air-tight by means of a coating of caoutchouc. A valve is fitted to the top, and by means of it the aeronaut can descend to the earth at will, by allowing some quantity of the gas to escape. The car in which he sits is suspended to the balloon by a network, which covers the whole structure. Sacks of sand are carried in this car as ballast, so that, when descending, if the aeronaut sees that he is likely to be precipitated into the sea or into a lake, he throws over the sand, and his air-carriage, being thus lightened, mounts again and travels away to a more desirable resting-place. The idea of the valve, as well as that of the sand ballast, is due to the physician Charles. They enable the aeronaut to ascend or descend with facility. When he wishes to mount, he throws over his ballast; when he wants to come down, he lets the gas escape by the valve at the roof of the balloon. This valve is worked by means of a spring, having a long rope attached to it, which hangs down through the neck to the car, where the aeronaut sits.
The operation of inflating a balloon with pure hydrogen is represented in the engraving on the next page.
Shavings of iron and zinc, water, and sulphuric acid, occupy a number of casks, which communicate, by means of tubes, with a central cask, which is open at the bottom, and is plunged in a copper full of water. The gas is produced by the action of the water and the sulphuric acid upon the zinc and the iron this is hydrogen mixed with sulphuric acid. In passing through the central copper, or vat, full of water, the gas throws off all impurities, and comes, unalloyed with any other matter, into the balloon by a long tube, leading from the central vats. In order to facilitate the entrance of the gas into the balloon two long poles are erected. These are furnished with pulleys, through which a rope, attached also to a ring at the top of the
Inflating Balloon with Hydrogen.
balloon, passes. By means of this contrivance the balloon can be at once lightly raised from the ground, and the gas tubes easily joined to it. When it is half full it is no longer necessary to suspend the balloon; on the contrary, it has to be secured, lest it should fly off. A number of men hold it back by ropes; but as the force of ascension is every moment increasing, the work of restraining the balloon is most difficult and exciting. At length, all preparations being complete, the car is suspended, the aeronaut takes his seat, the words "Let go all!" are shouted, and away goes the silken globe into space.
The balloon is never entirely filled, for the atmospheric pressure diminishing as it ascends, allows the hydrogen gas to dilate, in virtue of its expansive force, and, unless there is space for this expansion, the balloon is sure to explode in the air.
An ordinary balloon, with a lifting power sufficient to carry up three persons, with necessary ballast and matériel, is about fifty feet high, thirty-five feet in diameter' and 2,250 cubic feet in capacity. Of such a balloon, the accessories—the skin, the network, the car—would weigh about 335 lbs.
To find out the height at which he has arrived, the aeronaut consults his barometer. We know that it is the pressure of the air upon the cup of the barometer that raises the mercury in the tube. The heavier the air is, the higher is the barometer. At the level of the sea the column of mercury stands at 32 inches; at 3,250 feet—the air being at this elevation lighter—the mercury stands at 28 inches; at 6,500 feet above sea level it stands at 25 inches; at 10,000 feet it falls to 22 inches; at 20,000 feet to 15 inches. These, however, are merely the theoretic results, and are subject to some slight variation, according to locality, &c.
Sometimes the aeronaut makes his descent by means of the parachute, a separate and distinct contrivance. If, from any cause, it appears impracticable to effect a descent from the balloon itself, the parachute may be of the greatest service to the voyager at the present day it is chiefly used to astonish the public, by showing them the spectacle of a man who, from a great
The Parachute.
elevation in the air, precipitates himself into space, not to escape dangers which threaten him in his balloon, but simply to exhibit his courage and skill. Nevertheless, parachutes are often of great actual use, and aeronauts frequently attach them to their balloons as a precautionary measure before setting out on an aerial excursion.
The shape of a parachute, shown on the previous page, very much resembles that of the well-known all serviceable umbrella. The strips of silk of which it is formed are sewn together, and are bound at the top around a circular piece of wood. A number of cords, stretching away from this piece of wood, support the car in which the aeronaut is carried. At the summit is contrived an opening, which permits the air compressed by the rapidity of the descent to escape without causing damage to the parachute from the stress to which it is subjected.
The rapidity of the descent is arrested by the large surface which the parachute presents to the air. When the aeronaut wishes to descend by the parachute, all that is required is, after he has slipped down from the car of the balloon to that of the parachute, to loosen the rope which binds the latter to the former, which is done by means of a pulley. In an instant the aeronaut is launched into space with a rapidity in comparison with which the wild flights of the balloon are but gentle oscillations. But in a few moments, the air rushing into the folds of the parachute, forces them open like an umbrella, and immediately, owing to the wide surface which this contrivance presents to the atmosphere, the violence of the descent is arrested, and the aeronaut falls gently to the ground, without receiving too rude a shock.
The virtues of the parachute were first tried upon animals. Thus, Blanchard allowed his dog to fall in one from a height of 6,500 feet. A gust of wind caught the falling parachute, and swept it away up above the clouds. Afterwards, the aeronaut in his balloon fell in with the dog in the parachute, both of them high up in the cloudy reaches of the sky, and the poor animal manifested by his barking his joy at seeing his master. A new current separated the aerial voyagers, but the parachute, with its canine passenger, reached the ground safely a short time after Blanchard had landed from his balloon.
Experience has proved that, in the case of a descending parachute, if the rapidity of the descent is doubled the
Garnerin's descent in a Parachute.
resistance of the air is quadrupled; if the rapidity is triple the resistance is increased ninefold; or, to speak in language of science, the resistance of the air is increased by the square of the swiftness of the body in motion. This resistance increases in proportion as the parachute spreads, and thus the uniformity of its fall is established a minute after it has been disengaged from the balloon. We can, therefore, check the descent of a body by giving it a surface capable of distension by the action of the air.
Garnerin, in the year 1802, conceived the bold design of letting himself fall from a height of 1,200 feet, and he accomplished the exploit before the Parisians. When he had reached the height he had fixed beforehand, he cut the rope which connected the parachute with the balloon. At first the fall was terribly rapid; but as soon as the parachute spread out the rapidity was considerably diminished. The machine made, however, enormous oscillations. The air, gathering end compressed under it, would sometimes escape by one side sometimes by the other, thus shaking and whirling the parachute about with a violence which, however great, had happily no unfortunate effect.
The origin of the parachute is more remote than is generally supposed, as there was a figure of one which appeared among a collection of machines at Venice, in 1617.
Another species of parachute, less perfect, to be sure; than that of Garnerin, but still a practical machine, was described 189 years before the great aeronaut's feat at Paris. We read in the narrative of the ambassador of Louis XIV at Siam, at the end of the seventeenth century, the following passage:—"A mountebank at the court of the King of Siam climbed to the top of a high bamboo-tree, and threw himself into the air without any other support than two parasols. Thus equipped, he abandoned himself to the winds, which carried him, as by chance, sometimes to the earth, sometimes on trees or houses, and sometimes into the river, without any harm happening to him."
Is not this the idea of our parachutes?