Wonderful Balloon Ascents; Or, The Conquest of the Skies

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.