“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 else.”

“The fellow that made the first voyage must have been very daring.”

“The first ascent was made by Montgolfier in 1782, and he was followed by Rosiers and d’Arlandes.”

“With your permission, father,” said Ernest, “I will claim priority in aerial travelling for Icarus, Doedalus, and Phaeton.”

“Certainly; you are justified in doing so.  Gay-Lussac, a philosophic Frenchman, rose, in 1804, to the height of seven thousand yards.”

“He must have felt a little giddy,” remarked Jack.

“Most of the functions of the body were affected, more or less, by the extreme rarity of the air at that height.  Its dryness caused wet parchment to crisp.  He observed that the action of the magnetic needle diminished as he ascended, sounds gradually ceased to reach his ear, and the wind itself ceased to be felt.”

“That, of course,” remarked Ernest, “was when he was travelling in the same direction and at the same speed.”

“Well,” said Jack, “we can find materials here for a balloon; the ladies have silk dresses, there is plenty of India-rubber—­we used to make boots and shoes of it; hydrogen gas can be obtained from a variety of substances.  What, then, is to prevent us paying a visit to some of Ernest’s friends in the skies?”

“Unfortunately for your project, Jack, no one has discovered the art of guiding a balloon; consequently, instead of finding yourself at Cassiope, you might land at Sirius, where your reception would be somewhat cool.”