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