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 Montgolfier sent up. There are
aeronauts, however, who prefer a journey in a Montgolfier 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 rechaud over the middle of the car, rendered this sort of
air travelling subject to many dangers and difficulties. Recently, M.
Eugene 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 Montgolfiere 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 nearest gas-works the quantity of gas necessary, and
to lead it, by means of a pipe or tube, from the gasometer to the mouth
or neck of the machine.

The balloon is made of long strips of silk, sewn together, and rendered
air-tight by means of a coating of caoutchouc. A valve is fitted to the
top, and by means of it the aeronaut can descend to the earth at will,
by allowing some quantity of the gas to escape. The car in which he
sits is suspended to the balloon by a network, which covers the whole
structure. Sacks of sand are carried in this car as ballast, so
that, when descending, if the aeronaut sees that he is likely to be
precipitated into the sea or into a lake, he throws