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.