over the sand, and
his air-carriage, being thus lightened, mounts again and travels away to
a more desirable resting-place. The idea of the valve, as well as that
of the sand ballast, is due to the physician Charles. They enable the
aeronaut to ascend or descend with facility. When he wishes to mount,
he throws over his ballast; when he wants to come down, he lets the gas
escape by the valve at the roof of the balloon. This valve is worked by
means of a spring, having a long rope attached to it, which hangs down
through the neck to the car, where the aeronaut sits.

The operation of inflating a balloon with pure hydrogen is represented
in the engraving on the next page.

Shavings of iron and zinc, water, and sulphuric acid, occupy a number of
casks, which communicate, by means of tubes, with a central cask, which
is open at the bottom, and is plunged in a copper full of water. The gas
is produced by the action of the water and the sulphuric acid upon the
zinc and the iron this is hydrogen mixed with sulphuric acid. In passing
through the central copper, or vat, full of water, the gas throws off
all impurities, and comes, unalloyed with any other matter, into the
balloon by a long tube, leading from the central vats. In order to
facilitate the entrance of the gas into the balloon two long poles
are erected. These are furnished with pulleys, through which a rope,
attached also to a ring at the top of the balloon, passes. By means
of this contrivance the balloon can be at once lightly raised from the
ground, and the gas tubes easily joined to it. When it is half full it
is no longer necessary to suspend the balloon; on the contrary, it has
to be secured, lest it should fly off. A number of men hold it back by
ropes; but as the force of ascension is every moment increasing, the
work of restraining the balloon is most difficult and exciting. At
length, all preparations being complete, the car is suspended, the
aeronaut takes his seat, the words "Let go all!" are shouted, and away
goes the silken globe into space.

The balloon is never entirely filled, for the atmospheric pressure
diminishing as it ascends, allows the hydrogen gas to dilate, in virtue
of its expansive force, and, unless there is space for this expansion,
the balloon is sure to explode in the air.

An ordinary balloon, with a lifting power sufficient to carry up three
persons, with necessary ballast and materiel, is about fifty feet high,
thirty-five feet