MECHANICS. 63 
than air. It derives its name from Professor Charles of Paris, who also, in 
1783, employed this method of filling, and with one companion ventured on 
the first aerial voyage in a car attached to the balloon. Balloons of this 
latter construction are decidedly preferable, as being less exposed to the 
danger of catching fire than the other; and secondly, on account of the 
greater lightness of hydrogen, they may be made smaller, or when of equal 
size, they will sustain a much greater weight, and will ascend higher in the 
atmosphere. Hence, when an ascent is to be made by individuals, the 
Charliére balloon is almost always employed. The descent of this kind of 
balloon is effected by the escape of gas through a valve attached to the 
upper part, and regulated by a cord; and the higher ascent, by the 
discharge of sand bags taken along as ballast. The ascent of a balloon 
must of course cease as soon as it attains to a stratum of air of so slight 
density that the air displaced is no heavier than the balloon with its 
load. 
Pl. 17, fig. 39, exhibits the construction of the valve for the escape of 
the gas used in the so-called Hampton Balloon. The balloon itself consists 
of forty-one strips of oiled silk, each of which is sixty-seven feet long and 
three feet broad; its circumference amounts to one hundred and twenty- 
three feet, its diameter to forty-one feet. The first constructed valves con- 
sisted of a simple door opened by a cord, in which case the aeronaut could 
pot see how much gas escaped, and consequently sometimes let out more 
than he wished. The present valve consists of a hoop, A, four and a half 
feet in circumference, and six inches deep. At dd are spiral springs at- 
tached inside and inclosing the axis cc. The whole resembles the upper 
part of adrum. To the valve proper which turns about the axis cc, the 
draw cords, bb, are attached, of which the right opens the valve and the 
left closes it. The spiral springs dd would of themselves close the valve, 
the cord being attached merely by way of precaution. Over the straight 
part of the springs pass two rings which spring off when the valve is 
opened to a certain point. This latter then remains open and the gas 
entirely escapes. This takes place when the balloon is on the ground, 
otherwise the aperture may be regulated to 4, of an inch. Thecords used 
in this balloon are of cocoa fibres, as being stronger and lighter than 
common. 
An appendage very frequently attached to the balloon, for the sake of 
descending from a considerable height, is the Parachute, A, fig. 40. Its 
principle depends on the resistance of the atmosphere, which diminishes 
the velocity of descent of every falling body, and this the more, as the 
surface of the body is greater in proportion to its weight, and as the ve- 
locity already attained is greater. The parachute, at the ascent of the 
balloon, is placed between it and the car, C, to which latter it is fastened : 
on breaking the connexion between balloon and car, the latter immediately 
falls with increasing velocity, the parachute being at first folded up, but 
expanding more and more until at length it sweeps over the car in the 
form of a great umbrella from 25 to 30 feet in diameter. The velocity then 
decreases to a less dangerous amount, which it retains until the ground is 
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