a doubtful scheme afloat, he was sure to
take shares in it. Nothing delighted him more than to go up in a
balloon; he would have gladly swung himself on the car outside if the
proprietor had allowed him."

"I have often seen balloons in the air," remarked Willis, "but I could
never make out their dead reckoning."

"A balloon," replied Ernest, "is nothing more than an artificial
cloud, and its power of ascension depends upon the volume of air it
displaces.

"Very good, Master Ernest, so far as the balloon itself is concerned;
but then there is the weight of the car, passengers, provisions, and
apparatus to account for."

"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. If a little more is permitted to escape, the balloon
descends."

"And should it land on the roof of a house or the top of a tree, the
voyagers have their necks broken."

"That can only happen to bunglers; there is not the least necessity
for landing where danger is to be apprehended. When the aeronaut is
near the ground, and sees that the spot is unfavorable for
debarkation, he drops a little ballast, the balloon mounts, and he
comes down again somewhere e