Air Pump, in natural philosophy, a machine for exhausting the air from a vessel. The J first machine of this kind was made in 1650 by Otto von Guericke, burgomaster of Magdeburg, shortly after Galileo had discovered that air was ponderable. Since then this instrument has been much improved, principally by Hook, Papin, Boyle, Babinet, Richard, and Deleuil. In its most approved form it consists of a circular brass plate, on which is placed a bell-shaped glass vessel. The interior of this vessel communicates through a tube opening in the centre of the plate with the pump cylinders. The rim of the glass vessel, called the reservoir, is ground perfectly flat, and a little lard is rubbed upon the edge before it is applied on the brass plate, which is likewise ground flat. Thus an air-proof joint is formed. Valves, placed either on the piston or on the cylinders, a stopcock on the pipe, and a mercury vacuum gauge, communicating with the reservoir, complete the machine. At each stroke of the piston, a cylinder full of air is ex- pelled on one side the piston, and the air of the reservoir expands to fill the space on the; other side; at the return stroke, this air is ex- pelled in its turn, and so on.

The air of the reservoir becomes more and more dilated till the moment when a full cylinder of it, com-. pressed into the small space necessarily left between the piston and the cylinder bottom, has not a sufficient pressure to open the valve; that is to say, when this pressure is less than 14 pounds to the square inch, which is the pressure of the atmosphere acting on the other side of the valve. For this reason these valves are made as light and delicate as possible, and since the beginning of this century they have been made simply of a strip of oiled silk stretched over one or more of the small holes, through which the air can thus only pass in one direction. As by every stroke of the piston the air divides itself equally between the reservoir and the pump cylinder, it is evident that every stroke takes out of the reservoir only a certain fraction of that which is left in it; if, for instance, the capacity of the cylinder is 1/30 of that of the reservoir, the 30th stroke will by no means take out the last 30th part, but only the 30th part of the air left by the former stroke. As a consequence of this principle of action, a perfect vacuum cannot be produced by any air pump, but it will be approximated more or less according to the perfection of its workmanship.

In this workmanship an important point to be observed is, that if at the lowest portion of the piston there is any space left between it and the cylinder, this space will of course remain filled with some air after the downward stroke of the piston; at the succeeding upward stroke this air will expand, and may fill the cylinder to the same extent as the vessel to be exhausted; this will prevent the opening of the valves, and any further motion of the pump will become useless. It is therefore necessary to avoid this so-called "dead space" in air pumps; a precaution not necessary in pumps for liquids, as these are solid and not elastic like air.

Fig. 1. - Simpbst Form of Air Pump.

An apparatus so extensively used for a great variety of purposes has of course undergone a great many modifications and improvements. We will only describe some of the principal types, commencing with the oldest and most simple. In fig. 1, C is the brass cylinder in which the piston is moved up and down, by means of the handle II. This cylinder is attached to a wooden base A A, which carries also the plate P P, on which the vessel to be exhausted is placed. The hole O in the centre of the plate connects by means of a tube with the lower part of the cylinder C. At the bottom of this cylinder is a cock to admit the air again into the exhausted vessel; there is a valve above this cock, and another valve in the piston, both opening upward. The first air pumps made by Von Guericke were similar, only his cylinder was horizontal and his piston solid, and in place of valves he had stopcocks which had to be turned at every stroke. It is evident that in this pump only the up stroke requires labor, a portion of which is lost in the beginning of the down stroke, when the atmospheric pressure causes the piston forcibly to descend for a portion of its motion. To prevent this loss of labor the double-barrel air pump has been contrived, represented in our second figure.

By means of a ratchet wheel moved through about 180° by the double handle H C, the pistons are alternately raised and depressed in the cylinders C C attached to the base A, which also carries the plate P, with the so-called receiver R, which is the strong glass vessel into which the objects to be experimented upon are placed. An important improvement in such air pumps was the imitation of the manner of working of the steam piston and cylinder, namely, closing the top of the cylinder hermetically, by means of a packing box surrounding the piston rod, and the placing of a valve in this upper cylinder head, so as to give a pump with a single cylinder three valves, all opening upward - one in the piston, and two at the top and bottom of the cylinder. In this way the atmospheric pressure does not act on the piston during the downward stroke, as then the top valve closes, and a partial vacuum is formed over the piston, which is tilled by the air under the piston passing through its valve. Such an air pump is represented in fig. 3. A is the base; 0 the cylinder; P P the plate; S the support for the handle II; E an oil cup, attached to a cap over the top valve, to receive the lubricating fluid which the upward motion of the piston may throw out of the top valve; and R is the receiver, containing the apparatus prepared to demonstrate that the sound of a bell is not transmitted through a vacuum.