PNEUMATICS, HISTORY or. Although the ancients did not investigate the properties of air with the same mina teness as the moderns have done, yet the subject evidently engaged the attention of Aristotle and ether Grecian philosophers. Aristotle was aware of the gravity of the air, and observed that a bladder idled with air would weigh more than the same bladder when empty ; and Erapedocles awribed the cause of resptration to the weight of the air, which, by its pressure, insinuates itself into the lungs. Its elasticity also, as well as its gravity, must have been known and made the subject of experiments, if it be true, as is related by historians, that Hero of Alexandria, and Cu-rabbis, his contemporary, invented air-guns. The 'subjects-as, however, for a long time neglected, or treated superfi cially ; for the effects which are now known to arise from the gravity and elasticity of the air were for a long time amibuted to the imagina ry principle of a Fnga Vacui, or N'ature's ab horrence of a vacuum, a principle which Ga lileo did not altogether discard, although he was fully aware of the gravity of the air, and pointed oat two methods of demonstrating it by weighing the air in bottles • but the pres sure tithe atmosphere was first discovered by Torricelli, and clearly demonstrated by an ex periment, which terminated in the invention of the barometer in 1643. It occurred to him, on observing that a column of water 34 ,et high is sustained above its level in the tube of a common pump, that the same force, what ever that was, would sustain a column of any other fluid which weighed -as much as that column of water on the same base ; and hence he concluded that quicksilver, being about 14 times as heavy as water, would. not be sus tained at a greater height than 29 or 30 inches. Accordingly he took a glass tube of several feet in length, and having sealed it hermeti cally at one end, he filled it with quicksilver ; then inverting it, he held it upright, and keeping his finger against the open or lower orifice, he immersed that end in a vessel of quicksilver, and on withdrawing his finger, he found that the quicksilver, according to hie expectation, descended till the column of it was about 30 inches above that in the open vessel ; whence he was led to infer that it could be no other than the weight of the atmosphere in cumbent on the external surface of the quick silver, which counterbalanced the fluid in the tube, and thus introduced the principle, that the air had not only weight, but that its weight was the. cause of the suspension of water in
pumps, and of the quicksilver in the tube. This principle, after repeated experiments from the philosophers in France, England, and Germany, became established, and that of the vacuum was finally exploded.
From this time they proceeded to be more minute in their investigations, and Father Mersenne, who was the first philosopher in France that was apprized of Torricelli's ex periment, set about determining the specific gravity of air, which he made to be as 1 to 400; but Mr. Boyle, by a more accurate ex periment, came to a nearer result, and made it to be to that of water in the proportion of 1 to 938 ; and Mr. Hawksbee succeeded, after repeated experiments, in obtaining the propor tion of 1 to 885. Experiments were made by others in the winter as well as the summer, and the medium of all is about 1 to 832.
As to the elasticity of air, which also be came the subject of experiment, it was shown that, with moderate pressure, it is always proportional to the. density, and that the den sity is always as the compressing force ; whence also the elasticity of air is as the force by which it is compressed.
In consequence of these investigations into the inechranical properties of the air, not only was the barometer invented, and the air-gun revived, but other pneumatic machines were contrived, as the air-pump, air-balloon, ther mometer, dm •, of which a more particular account will be found under their respective heads.