Air Pump, a pump for exhausting air and other gases from a closed space, or for compressing air within an enclosed space (See AIR; AIR COMPRESSOR). The ordinary suction pump for raising water is constructed on the same plan and operates on the same principle as the air pump. In fact, before the water reaches the top of the pipe the air has been exhausted by the pump which pumps the water. An ordinary air pump consists of a cylinder A, connected by the tube D with a closed vessel with the receiver G. Within the cylinder is the piston P, on the upper surface of which is the valve E, opening upward. At the bottom of the piston is the second valve F, which also opens upward. H is a screw which opens and closes the connection between the cylinder and the receiver, and J is an air-tight tube containing a U-shaped tube, in which there is a quantity of mercury, connected with the receiver G. C is the plate upon which the receiver rests. In operating the pump the piston is forced downward and the valve E is opened, thus transferring the air from below the piston to the space above it. When the piston is raised, the valve E is closed and the air is forced out through the tube K. The space below the piston becomes a vacuum and the expansive force of the air in G opens the valve F and fills the cylinder. With the second stroke of the piston this air is forced through the tube K, and to on with each repeated stroke until the air in G becomes so rarified that it can no longer operate the valve F. The difference in height in the columns of mercury in J indicates how perfect a vacuum has been obtained. Many interesting experiments can be performed with the air pump. A lighted candle placed under the receiver immediately goes out when the air is exhausted, thus showing that air is necessary to combustion. A bell suspended from a silk thread can be heard when the receiver is filled with air, but when the air is exhausted it cannot be heard. This shows that air is necessary to the propagation of sound. If a glass of water be placed under the receiver, as the air is exhausted bubbles will rise to the surface of the water, showing that it contains air, which, as the pressure above is lessened, expands and rises. A shriveled apple or a prune placed under the receiver becomes plump as the air is exhausted, and a bladder filled with air will be expanded until it bursts, because of the expansive force of the air it contains. The air pump was invented by Otto von Guericke, about 1654.