Air-Pump, a device for exhausting, compressing, or transmitting air. In the exhausting form of air-pump, the receiver is connected by an air-tight passage with an air-tight cylinder. Each cylinder head, as well as the piston, is fitted with a delicate valve, all three valves opening upward or away from the receiver. When the piston, which we shall suppose to be at the upper or outer end of the cylinder, is pushed down, the air in the cylinder is forced through the piston valve. When the piston is drawn up, the air in the cylinder is forced out of the cylinder through the upper valve and cannot return, as in the attempt to do so it closes the valve. In the meantime the expansive force of the air in the receiver has opened the lower valve, and air has passed from the receiver into the cylinder. The air now in the cylinder is driven out by the next downward and upward stroke, and so on until the air in the receiver becomes too rare to open the lower valve. Other features not considered, the efficiency of an air-pump, that is to say, the perfection of the vacuum produced, depends on the delicacy, lightness, and fit of the lower valve. An interesting problem can be made relative to the number of strokes required to form a theoretical vacuum. If we assume that there is no leakage and that the cylinder has, for illustration, a capacity 1/10 as great as that of the receiver, the first complete down and up stroke will remove the 1/11 part of the air, leaving 10/11 in the receiver. The second stroke removes 1/11 of this remainder, that is 1/11 of 10/11 or 10/121, leaving 100/121, and so on. So far as computation goes, a constantly diminishing remainder, but always a remainder, would be left in the receiver. Practically, a good pump will reduce the air to an inappreciable quantity. A more perfect vacuum is obtained by connecting the receiver with a small upright tube through which globules of mercury are falling. Successive portions of air are carried away between the globules as they pass the point of connection. There being no valve to open, the air is carried away as long as there is any left to expand. It is claimed that, by this method, the air may be removed until only 1/420,000,000 of the original amount is left in the receiver. Many interesting experiments are performed in the vacuum of a receiver. Water boils with very little heat when relieved from atmospheric pressure. A shriveled apple swells up and becomes plump with the expansion of the air with in it. A lighted candle goes out for want of air to burn. A mouse dies for want of air to breathe. The invention of the air-pump is credited to Otto von Guericke, a German physicist, about the year 1650. He was stimulated by the experiments of Galileo and Pascal. He was trying to produce a vacuum. He first attempted to pump the water out of a stout barrel, but found that air pressed in through the crevices. He then experimented by pumping water out of a copper globe. He soon found that, with a pump of sufficient delicacy and strength, a vacuum could be formed by pumping out the air directly. The bicycle pump is a familiar example of a pump used for compressing air. See PUMP; AIR