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- AIR. Aristotle , who flourished about 340 B. C., observedthat a bladder inflated with air weighed more than when itwas empty ; therefore, that the air had the property of gra-vity was known to the ancients. Its elasticity was also noticedby Ctesibius , of Alexandria, 120 B. C. Galileo , about theyear 1630, proportioned the weight of air to water, and hisscholar Torricelli improved on the discovery by substitutingquicksilver.

AIR BALLOON . The Romances of almost every nationhave recorded instances of persons being carried through theair, both by the agency of spirits and by mechanical inven-tions 3 but no rational principle appears to have been thoughtof, by which this might be accomplished, till near the end ofthe thirteenth century, when Friar Bacon informs us heknew how to construct a machine, capable of transporting aperson through the air like a bird, and that the experimenthad been successfully tried, by means of two large thin shellsor hollow globes of copper, which being exhausted of air, andthus made lighter than the common atmosphere, were foundcapable of supporting the weight of a person sitting on achair. No further experiments on the subject are noticed tillthe year 1709, when Friar Guzman, a Portuguese projector,applied to his sovereign to encourage his invention of a flyingmachine, which did not then prove successful 5 but in 1736,he constructed a wicker basket of about seven feet diame-ter, covered with paper, which on being deprived of air, roseto about two hundred feet from the ground, and the effectwas, even in those days, attributed to magic.

In 1766, Mr. Henry Cavendish , among other discoveries,ascertained the weight and other properties of inflammableair to be about seven times lighter than that of common air,and thus revived the subject of rerostation, and most probablysuggested to Stephen and John Montgolfier, natives of An-