necessary to weigh it in a vacuum.

If an object thrown into the air is heavier than the air which it
displaces, it descends, and falls upon the earth; if it is of equal
weight, it floats without rising or falling; if it is lighter, it
rises until it comes to a stratum of air of less weight or density than
itself. We all know, of course, that the higher you rise from the earth
the density of the air diminishes. The stratum of air that lies upon the
surface of the earth is the heaviest, because it supports the pressure
of all the other strata that lie above. Thus the lightest strata are the
highest.

The principle of the construction of balloons is, therefore, in perfect
harmony with physical laws. Balloons are simply globes, made of a light,
air-tight material, filled with hot air or hydrogen gas which rise in
the air because (they are lighter than the air they displace).

The application of this principle appeared so simple, that at the time
when the news of the invention of the balloon was spread abroad the
astronomer Lalande wrote--"At this news we all cry, 'This must be! Why
did we not think of it before?'" It had been thought of before, as we
have seen in the last chapter, but it is often long after an idea is
conceived that it is practically realised.

The first balloon, Montgolfier's, was simply filled with hot air; and it
was because Montgolfier exclusively made use of hot air that balloons
so filled were named Montgolfiers. Of course we see at a glance that
hot air is lighter than cold air, because it has become expanded and
occupies more space--that is to say, a volume of hot air contains
actually less air than a volume of the same size of air that has not
been heated. The difference between the weight of the hot air and the
cold which it displaced was greater than the weight of tire covering of
the balloon. Therefore the balloon mounted.

And, seeing that air diminishes in density the higher we ascend, the
balloon can rise only to that stratum of air of the same density as the
air it contains. As the warm air cools it gently descends. Again, as the
atmosphere is always moving in currents more or less strong, the balloon
follows the direction of the current of the stratum of air in which it
finds itself.

Thus we see how simply the ascent of Montgolfiers, and their motions,
are explained. It is the same with gas-balloons. A balloon, filled with
hydrogen gas, displaces an equal volume of atmospheric air;