equal volume of atmospheric air; but as the gas is much lighter than the air, it is pushed up by a force equal to the difference of the density of air and hydrogen gas. The balloon then rises in the atmosphere to where it reaches layers of air of a density exactly equal to its own, and when it gets there it remains poised in its place. In order that it may descend, it is necessary to let out a portion of the hydrogen gas, and admit an equal quantity of atmospheric air; and the balloon does not come to the ground till all, or nearly all, the gas has been expelled and common air taken in.
Balloons inflated with hydrogen gas are almost the only ones in use at the present day. Scarcely ever is a Montgolfière sent up. There are aeronauts, however, who prefer a journey in a Montgolfière to one in a gas-balloon. The air voyager in this description of balloon had formerly many difficulties to contend with. The quantity of combustible material which he was bound to carry with him; the very little difference that there is between the density of heated and of cold air; the necessity of feeding the fire, and watching it without a moment's cessation, as it hangs in the réchaud over the middle of the car, rendered this sort of air travelling subject to many dangers and difficulties. Recently, M. Eugène Godard has obviated a portion of this difficulty by fitting a chimney, like that which is found of such incalculable service in the case of the Davy lamp. It is principally on account of this improvement that the Montgolfière has risen so highly in popular esteem.
Generally it is not pure hydrogen that is made use of in the inflation of balloons. Aeronauts content themselves with the gas which we burn in our streets and houses, and thus it suffices, in inflating the balloon, to obtain from the
