"
"Hydrogen gas, used in the inflation of balloons, is forty times
lighter than air. If a balloon is made large enough, the weight of the
car and all that it contains, added to that of the gas, will fall
considerably short of the weight of the air displaced by the machine."
"I suppose it rises in the air just as an empty bottle well corked
rises in the water?"
"Very nearly. Air is lighter than water; consequently, any vessel
filled with the one will rise to the surface of the other. So in the
case of balloons. The gas, in the first place, must be inclosed in an
envelope through which it cannot escape. Silk prepared with
India-rubber is the material usually employed. As the balloon rises,
the gas in the interior distends, because the air becomes lighter the
less it is condensed by its superincumbent masses; hence it is
requisite to leave a margin for this increase in the volume of the
gas, otherwise the balloon would burst in the air."
"If a balloon were allowed to ascend without hindrance where would it
stop?"
"It would continue ascending till it reached a layer of air as light
as the gas; beyond that point it could not go."
"And if the voyagers do not wish to go quite so far?"
"Then there is a valve by which the gas may be allowed to escape, till
the weight of the machine and its volume of air are equal, when it
ceases to ascend.
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