fore 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 Montgolfières. 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 the 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 Montgolfières, and their motions, are explained. It is the same with gas-balloons. A balloon, filled with hydrogen gas, displaces an
