Practice os Aerostation. 213

markable peculiarity, which is its beingincomparably lighter than common air;whereas the other elastic fluids are verylittle heavier or lighter than that element,the difference being very trifling indeed.

Now, if a bag be filled with inflammableair, of such a size as that the excess of theweight of an equal bulk of common air, overthe enclosed inflammable air, be greaterthan the weight of the bag, then the bag willascend into the atmosphere, for the reasonsmentioned above: and this is the other fortof aerostatic machines, namely, the inflam-mable-air balloons.

The air which forms the atmosphereround the earth, being elastic, is of differentdensities at different heights, because the airnext to the surface of the earth, being pressedby all the rest of the air above in a perpendi-cular direction, must be much more com-pressed and more heavy than the air at a mileabove the earth, which has a mile less ofair to compress it; and so on, the higheryou ascend, the lighter is the air.

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