AIR-GUN, an apparatus by means of which a closed vessel can have the air it contains removed from it. It consists essentially of two parts—a receiver, from which the air is to be exhausted; and a pump, to perform the work of exhaustion. The receiver is in general made of glass, in order that the condition of objects placed within it for the purpose of experiment may be readily seen by the opera-tor. It is open at the bottom, and has its lower edge accurately ground; when in its place in the air-pump it stands upon a smooth brass plate. The pump itself is a brass cylinder, having a piston in it, which can be moved backwards and forwards by means of a rod, in the usual way. At the end of the cylinder nearest the receiver is placed a small valve, in the piston itself is another (or some mechanism which serves the purpose of a valve), and there is frequently a third in the outer end of the cylinder. All these valves open outwards from thereceiver. The action of the pump, when arranged in this way, is exactly similar to the action of an ordinary well-pump, with air as the fluid instead of water. The air-pump was invented about 1654 by Otto von Guericke, a magistrate of Magdeburg, and a man who devoted great attention to various pro-blems in pneumatics. The first description of his pump was published in 1657 in the Mechanica Hydraulico-pneumatica of Gaspar Schottus, professor of mathematics at Wurtemberg. He used a spherical glass receiver, with a pumping syringe attached, and kept the whole of the working parts under water to prevent leakage. His pump was very imperfectly constructed, but he did eventually succeed in getting a very good vacuum with it. The method of producing the Torricellian vacuum, by filling a vessel with liquid and then removing the liquid without permitting ingress of air, was previously known; but a vacuum produced in this way was obviously useless for experiments with any objects but those which could pre-viously be immersed in the liquid used. Guericke was, however, the first to recognise that, by virtue of its perfect elasticity, or tendency to expand indefinitely, air could be pumped out of a closed space as well as water; and this is the principle of his and all succeeding air-pumps. Al-though the invention of the air-pump is due to a German, almost all the improvements made in it from time to time have come from Englishmen. Dr Boyle contributed so much to its perfection that for a long time the state of the air in an exhausted receiver was called vacuum Boyleanum, and the air-pump itself machina Boyleana. Dr Hook, Hawkesbee, John Smeaton, and others brought the air-pump externally to very much the same form as that in which it is commonly seen at present, and which is shown in the annexed woodcut. The pump here has two cylin-ders, which are worked by a winch handle, the pump rods having toothed racks on the upper part of their length. Professor Tate is the inventor of a double-action air-pump, now much used where a very perfect vacuum is required. It has two pistons in one barrel, the air being drawn from the receiver at the centre of the barrel, and discharged into the atmosphere at its extremities. Very complete air-pumps have two or three barrels, arranged as shown in the woodcut, for rapid exhaustion, until the pressure in the receiver is equal to (say) half-an-inch of mercury; and in addition to these a horizontal Tate's barrel, which can then be put into action to bring the vacuum down to inch of mercury (l-600th of the pressure of the atmosphere), or even less at low temperatures. See PNEUMATICS.

AIR-PUMP, in steam-engines, is the pump which draws the condensed steam, along with the air which is always mixed with it, and also the condensing water (except where a surface condenser is used), away from the condenser, and discharges it into the hot well. See STEAM-ENGINE, (A. B. W. K.)