PNEUMATICS. This name is given to that part of physics in which are contemplated the mechanical properties of elastic fluids, principally atmospheric air. The word is derived from 7134 pa, breath or spirit,' and pneumatology is a term which was once applied to the division of science now called metaphysics.
The mechanical properties of air appear to have been, to a certain extent, known to the ancients. Aristotle must have been aware of its materiality, and of its tendency to descend towards the earth, like other heavy bodies, for he observes that if a bladder be filled with air, it weighs more than when empty : the observation proves at least that lie had the idea, but it may be doubted whether or not he ever made the experiment, since it is not probable that he had the means of weighing the full bladder in ramie, or that ho could keep it dis tended when the air was extracted from it. The invention of a species of forcing-pump for raising water, and of instruments for producing sounds by the passage of air through orifices in tubes, both of which are ascribed by Vitruvius (lib. ix., x.) to Ctesibius, who lived about 130 years B.C., are proofs that the elastic force of compressed air must have been then apprehended and made subservient to purposes con nected with practical ntility.
The ascent of water in pumps, on raising the piston, must have been for ages observed, without a suspicion being entertained of the cause ; and the principle that the existence of a vacuum was impos sible, was held to afford a sufficient explanation of the phenomenon. Even Galileo, when made aware, by the formation of a vacuum between the piston and the water when the latter was at its greatest height in a pump, that the principle was unfounded, had recourse to another which was equally remote from the truth ; for he is said to have ascribed the ascent of the water to the attraction of the piston, and to have accounted for the height of the column never exceeding a certain quantity (about 34 feet) by a supposed equilibrium between the weight of the column and the attractive power of the piston. The steps by which Torricelli arrived at the discovery of the true cause of the phenomenon, and at the same time, the determination of the pressure of the atmosphere by the weight of the column of mercury supported in a tube closed at the upper extremity, are noticed under BARO ZIETER.
Soon after the commencement of the seventeenth century, the idea of ascertaining with precision the state of bodies with respect to temperature by the expansion of fluids when acted on by heat, to have occurred to the members of tho Academy del Cimento, at Florence ; and this gave rise to the invention of an instrument of great importance, as well for many occasions of ordinary life, as for the purposes of pneumatical science. [TilimmomErEn.) •
The desire of perfecting the theory for determining the heights of mountains by the barometer, led to numerous researches in order to ascertain the relations between the condensation of air and the forces compressing it, and also to discover the effects produced by variations of temperature. About the year 1650, Boyle in England, and about 1676, Marlette in France determined from experiments, that when the temperature was constant, the density of air was proportional to the compressing force ; but it was not till a century afterwards that De Luc determined the expansion consequent upon given increments of temperature.
In the beginning of the eighteenth century, steam began to be employed as a moving power for pumps and other machinery. The law of the resistance of the air to bodies moving in it was discovered by Sir Isaac Newton; but the intensity of this resistance against military projectiles was first determined, for the inferior velocities, by Robins, in 1740 ; and Dr. Hutton subsequently obtained a formula which, with perhaps sufficient correctness, may be employed with any velocity whatever. [GunrEnr.] The expansion of fired gunpowder has been determined from the researches of Robins, Euler, and Hutton; and that of steam bas been investigated by Dalton in Eng land, and by Prony, Arago, and other distinguished physicists in France. [Sreati.) If a fluid be conceived to be perfectly elastic, it is evident that it can be made to occupy a given volume only by being confined within a close vessel, or by the pressure of a circumambient fluid. In the latter case it is easy to perceive that while the temperature of tho elastic fluid remains the same, the spaces occupied by it will be diminished when the compressing force is increased, and increased when the latter is diminiehed; and that the density, or degree of close ness of the particles, will vary directly with the change of volume. Now the compressing forces which are made to act on the surface of an elastic fluid are usually estimated in terms of the pressure exerted by a vertical column of the atmosphere, its base being equal to some unit of superficies, as one square inch or foot. Whatever be the con stitution of such column, its weight or pressure is known, in a given state of the atmosphere, by direct experiment, and this is usually designated the pressure of one atmosphere : the same pressure is also frequently expressed by the height of the column of mercury whiCh is supported in a barometer-tube by the counteracting weight of the atmospheric column.