PNEUMATICS, is that branch of na tural philosophy which treats of the weight, pressure, and elasticity of the air, with the effects arising from them.
Galileo, whose name is presented as of itself, whenever the enquiry relates to the first researches concerning gravity, had verified that of the air, which was denied almost universally before him, though it had been discovered by some few philosophers of antiquity. This cele brated philosopher having injected air into a glass vessel, so that it there mained compressed, found that the•vessel weighed more than when the contained air was in its natural state. He inquired also, by another experiment, into the heaviness of this fluid compared with that of water ; but he found it only in the ratio of 1 to 400, wide!' is much too small, as we shall soon see. The pneumatic machine, or air-pump, was not then known. It is to Otto Guericke, a burgo master of Magdeburg, that we are in debted for the invention of this elegant machine, which is not, like many others, confined to one part of experimental phi losophy, for almost all branches derive aid from it. This machine, which will be presently described, when reduced to its greatest simplicity, is composed of a vertical cylinder of brass, in which a pis ton is moved ; its upper base carries a cock, above which is soldered a circular brass plate situated horizontally. On this plate the receiver is placed, from which we would exhaust the air, which is exe cuted by making the piston descend and ascend alternately. By the use of this instrument, the gravity ofthe air has been verified, by first weighing a ball or blad der full of air, and then weighing it, after the hall or bladder has been exhausted of the air ; a sensible diminution will be perceived in the weight of the ball. Phi losophers have attempted likewise to de termine, with precision, the specific gra vity of the air.
According to the results of Deluc, the ratio between the weight of common air and distilled water, at the temperature of thawing ice, and under a medium pres sure of 29.9 English inches of mercury, is that of 1 to 760 ; and from the experi ments of Lavoisier, it follows that a cubic inch of air, taken at 10 degrees of Reau mur, weighs 0.46005 grains, and that the weight of a cubic foot of the same fluid is one ounce, three drams, and three grains. but by some very accurate experiments of Mr. Cavendish, it was ascertained that the weight of water is to that cf air as 800 to 1 : this was the case when the barometer stood at 291 inches, and the thermometer at 50°. Sir George Slut :k bnrgh found it to be as 836 to 1, when the barometer was 29.37, and the thermo meter at 51°. The medium of many ex periments by the gentlemen already mentioned, and by Mr. Hauksbee, Dr. Halley, Mr. Cotes, and other philosophers eqnally zealous in the improvements of natural science, is about 832 to 1, when the barometer is 30°, and the thermome thr this ratio must vary in propor tion to the changes in the height of the barometer, and it varies also part for every degree of the thermometer above or below temperature : hence the cubic foot of air, of water, and of quick silver, may be taken as 11 ounce, 1000 ounces, and 13,600 ounces.
The gravity of the air being once known, it should seem that it could not be difficult to infer that the ascent of water, in the body of a pump, must be occasioned by the pressure of that fluid. This, however, was not the case : Galileo had no notion of it.
Some Italian conduit-makers being ask ed if they would construct sucking-pumps, whose tubes should be more than 33 feet in height, remarked, with surprise, that the water refused to rise above that limit. They requested of Galileo the explication of this singular fact ; and it is affirmed that the philosopher,being taken unawares, replied, that nature did not entertain the horror of a vacuum beyond 33 feet, Torricelli, a disciple of Galileo, having meditated upon this phenomenon, conjectured that water is elevated in pumps by the pressure of the exterior air ; and that this pressure has only the degree of force necessary to MID terbalance the weight of a column of water of 33 feet. He verified this con
jecture by an experiment, for which na tural philosophy owes him a double obli gation, since it serves to render evident an important discovery, while it has pro cured us the barometer. Torricelli saw the mercury stand 29 or 30 inches in a glass tube, sealed at its tipper part, and situated vertically ; and the height thus under consideration being to that of 33 feet in the inverse ratio of the densities of water and of mercury ; he concluded that the phenomenon belonged to statics, and that it was really, as he had conjectured, the pressure of the air which caused wa ter or mercury to rise until an equilibri um was produced : this occurred in 1643. The year following, the new of Torri celli's experiment was disseminated in France by a letter written from Italy to Father Mersenne. The experiment was performed again in 1646, by Mersenne and Pascal ; and the latter devised, in 1647, a method of rendering it still more decisive, by making it at different alti tudes. He invited, in consequence, his friend Perrier to repeat the experiment upon the mountain Puy-de-Dome, and to observe whether the column of mercury would descend in the tube in proportion as it became more elevated. We may see from the letter of Pascal to Perrier, where he seems to avoid the name of Torricelli, that he had not yet entirely renounced the chimera of the horror at a vacuum which was attributed to nature, and that by admitting that this horror was net invincible, he was not bold enough to assert that it never obtained. The suc cess of the experiment completely re moved the delusion. Yet this experi ment was only a confirmation of that by Torricelli, and therefore yielded an addi tional ray to the stream of light which issued from it. The pressure of the at mosphere, upon a given surface, being nearly the same as would be exerted up on that surface by a column of water of 33 feet high ; from this datum has been computed the effect of the pressure un der consideration, with respect to a man of medium magnitude, and it has been found that it is equivalent to a weight of about 33,600 pounds. Considerable as this weight is, its pressure is exerted un known to us, because it is continually balanced by the re-action of the elastic fluids comprised in the interior cavities of our bodies ; and though the air is subject to continual variations, which augment or diminish its density, in consequence of changes of temperature, and of the ac tion of different natural causes, yet as these variations are generally confined within narrow limits, and succeed each other with comparative tardiness, they do not affect us commonly, except in a manner scarcely perceptible. But if there happen a sudden change, as when a man is raised to great heights, the rupture of the equilibrium which ensues, has a very marked influence upon the animal economy. He then experiences an extreme fatigue, and absolute ty to continue his progress ; a drowsiness under which he sinks in spite of himself; the respiration becomes thick and difli. cult ; the pulsations take an accelerated motion. To explain these effects, it must be considered that the state of well-being, in all that depends upon re spiration, requires that a determinate quantity of air should pass through the lungs in a given time. If, therefore, the air that we respire becomes much more rare, the inspirations must of ne cessity be proportionally more frequent ; which will render the respiration more difficult, and will occasion the various symptoms to which we have referred. With regard to the inconveniences that, would result from an air too condensed, man is not exposed to them by the ac tion of natural causes ; and it appears that, in general, they are less than those which are caused by the rarefaction of the air.