The principle of Archimedes being thus true for bodies in air, all that has been said about bodies immersed in liquids applies to them, that is, that when a body is heavier than air it will sink, owing to the excess of its weight over the buoyancy. If it is as heavy as air, its weight will exactly counterbalance the buoyancy, and the body will float in the atmosphere. If the body is lighter than air, the buoyancy of the air will prevail, and the body will rise in the atmosphere until it reaches a layer of the same density as its own. The force of the ascent is equal to the excess of the buoyancy over the weight of the body. This is the reason why smoke, vapours, clouds, and air balloons rise in the air.

150. Air balloons.-Air balloons are hollow spheres made of some light impermeable material, which, when filled with heated air, with hydrogen gas, or with coal gas, rise in the air in virtue of their relative lightness.

They were invented by the brothers Montgolfier, of Annonay, and the first experiment was made at that place in June 1783. Their balloon was a sphere of 40 yards in circumference, and weighed 500 pounds. At the lower part there was an aperture, and a sort of boat was suspended, in which was burnt paper and straw. The heated air thus produced gradually inflated the balloon, and when it was full of expanded air, lighter than the external air, the weight of the balloon and its hot air being less than that of the air which it displaced, it soon rose to a height of more than 2,000 yards, to the great astonishment of the assembled spectators. It rapidly descended, however, the hot air it contained soon becoming cooled in the higher regions of the atmosphere.

The experiment at Annonay excited great interest all over France, and pending the repetition on a larger scale at the expense of the government, Charles, a professor of physics, constructed a smaller balloon, about 13 feet in diameter, which was filled with hydrogen instead of heated air. The use of hydrogen is very advantageous, for as it is almost 14 times less dense than air, its ascensional force is far greater than that of hot air, and it is also less dangerous, for in heating the air there is a great risk of setting fire to the balloon. Charles made an ascent in 1783 in a balloon inflated by hydrogen.

Since then, the art of ballooning has been greatly extended, and many ascents have been made. That which Gay-Lussac made in 1804 was the most remarkable for the facts with which it has enriched science, and for the height which he attained-23,000 eet

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above the sea level. At this height the barometer descended to 12.6 inches, and the thermometer, which was 31° C. on the ground, was 9 degrees below zero.

In these high regions, the dryness was such on the day of GayLussac's ascent, that hygrometric substances, such as paper, parchment, &c., became dried and crumpled as if they had been placed near the fire. The respiration and circulation of the blood were accelerated in consequence of the great rarefaction of the air. GayLussac's pulse made 120 pulsations in a minute, instead of 66, the normal number. At this great height the sky had a very dark blue tint, and an absolute silence prevailed.

One of the most remarkable recent ascents was made by Mr. Glaisher and Mr. Coxwell, in a large balloon belonging to the latter. This was filled with 90,000 cubic feet of coal gas (sp. gr. 0'37 to 0.33); the weight of the load was 600 pounds. The ascent took place at 1 P.M. on September 5, 1861; at 1° 28′ they had reached a height of 15,750 feet, and in eleven minutes after a height of 21,000 feet, the temperature being — 104°; at 1° 50' they were at 26,200 feet with the thermometer at -15.2°. At 1° 52′ the height attained was 39,000 feet, and the temperature 160 C. At this height the rarefaction of the air was so great and the cold so intense that Mr. Glaisher fainted, and could no longer observe. According to an approximate estimation the lowest barometric height they attained was 7 inches, which would correspond to a height of 36,000 to 37,000 feet.

151. Construction and management of balloons.-A balloon (fig. 133) is made of long bands of silk sewed together and covered with caoutchouc varnish, which renders it air-tight. At the top there is a safety-valve closed by a spring, which the aëronaut can open at pleasure by means of a cord. A light wicker-work boat is suspended by means of cords to a net-work, which entirely covers the balloon.

A balloon of the ordinary dimensions, which can carry three persons, is about 16 yards high, 12 yards in diameter, and its volume when it is quite full is about 680 cubic yards. The balloon itself weighs 200 pounds; the accessories, such as rope and boat, 100 pounds.

The balloon is filled either with hydrogen or with coal gas. Although the latter is heavier than the former, it is generally preferred, because it is cheaper and more easily obtained. It is passed into the balloon from the gas reservoir by means of a flexible pipe

(fig. 133). It is important not to fill the balloon quite full, for the atmospheric pressure diminishes as it rises, and the gas inside expanding in consequence of its elastic force, tends to burst it. It is sufficient for the ascent if the weight of the displaced air exceeds that of the balloon by 8 or 10 pounds. And this force

remains constant so long as the balloon is not quite distended by the dilatation of the air in the interior. If the atmospheric pressure, for example, has diminished to one-half, the gas in the balloon, according to Boyle and Mariotte's law, has doubled its volume. The volume of the air displaced is therefore twice as great; but since its density has become only one-half, the weight, and consequently

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the upward buoyancy, are the same. When once the balloon is completely dilated, if it continue to rise, the force of the ascent decreases, for the volume of the displaced air remains the same, but its density diminishes, and

a time arrives at which the buoyancy is only equal to the weight of the balloon. The balloon can now only take a horizontal direction, carried by the currents of air which prevail in the atmosphere. The aëronaut knows by the barometer whether he is ascending or descending; and by the same means he determines the height which he has reached. A long flag fixed to the boat would indicate, by the position it takes either above or below, whether the balloon is descending or ascending.

When the aëronaut wishes to descend, he opens the valve at the top of the balloon by means of the cord, which allows gas to escape, and the balloon sinks. If he wants to descend more slowly, or to rise again, he empties out bags of sand, of which there is an ample supply in the car. The descent is facilitated by means of a grappling iron fixed to the boat. When once this is fixed to any obstacle, the balloon is lowered by pulling the cord.

The only practical applications which air balloons have hitherto

Fig. 134.

had, have been in military reconnoitring. At the battle of Fleurus in 1794, a captive balloon, that is, one held by a cord, was used, in which there was an observer who reported the movements of the enemy by means of signals. At the battle of Solferino the move

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ments and dispositions of the Austrian troops were watched by a captive balloon; and in the war in America balloons were frequently used; while the part which they played in the siege of Paris is still fresh in all memories. Many ascents have recently been made by Mr. Glaisher for the purpose of making meteorological observations in the higher regions of the atmosphere. Air balloons can only be truly useful when they can be guided, and as yet all attempts made with this view have completely failed. There is no other course at present than to rise in the air, until there is a current which has more or less the desired direction.

152. Parachute.-The object of the parachute is to allow an aëronaut to leave the balloon, by giving him the means of lessening

the rapidity of his descent. It consists of a large circular piece of cloth (fig. 135) about 16 feet in diameter, and which by the resistance of the air spreads out like a gigantic umbrella. In the centre there is an aperture, through which the air, compressed by