Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/465

Rh A I R A I R 429 a constantly diminishing force. Sometimes the weapon is made in the form of a walking-stick, and is then called an air-cane. The air-gun is little else than a scientific toy, and has no practical value. The apparatus is costly, the process of condensation requires considerable labour, and the propulsive force of the air is, at its maximum, less than that of an ordinary charge of gunpowder. The only advantage it can be said to have in any way is the ques tionable one of its use being unattended by the explosive noise that accompanies the discharge of a common gun. AIR-PUMP, an apparatus by means of which a closed vessel can have the air it contains removed from it. It con sists 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 the receiver. 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. 1 The first description of his pump was published in 1657 in the Mechanica Hydraulico- pncumatica of Caspar 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 1 He was also the inventor of the &quot; Magdeburg hemispheres.&quot; 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 -fa inch of mercury (1 -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 con denser, and discharges it into the hot well. See STEAM- ENGINE. (A. B. w. K.) AIR, or ASBEN, a country of central Africa, lying be tween 15 and 19 N. lat. and 6 and 10 E. long. The northern and best known portion of this region is of a very diversified character. It has numerous mountain ranges, some of which rise to a height of 5000 feet, with richly- wooded hollows and extensive plains interspersed. The mimosa, the durn-palm, and the date are abundant ; and the valleys are covered with the exuberant vegetation of the tropics. Some of the plains afford good pasturage for camels, asses, goats, and cattle; others are desert table lands. In the less frequented districts wild animals abound, notably the lion and the gazelle. The country generally is of sandstone or granite formation, with occasional trachyte and basaltic ranges. There are no permanent rivers; but during the rainy season, from August to October, very heavy floods convert the water-courses in the hollows of the mountains into broad and rapid streams. Numerous wells supply the wants of the people and their cattle. To the south of this variegated region lies a desert plateau, 2000 feet above the level of the sea, destitute of water, and tenanted only by the wild ox, the ostrich, and the giraffe. Still further south is the district of Damerghu, nominally tributary to Air, undulating and fertile, and yielding rich crops. Notwithstanding the fertility of the valleys in the northern portion of the country, there is little of the soil under cultivation except in the neighbourhood of the vil lages, where slaves are employed in tillage. Millet, dates, indigo, and senna are the principal productions. The great bulk of the food supplies is brought from Damerghu, and the whole materials for clothing are also imported. Were it not for the traffic in salt between Bilma and the Hansa states of Soudan, the country could scarcely maintain its present limited number of inhabitants. A great caravan annually passes through Air, consisting of several thousand camels, carrying salt from Bilma to Sokoto. Air was called Asben by the native tribes until they were conquered by the Berbers. The present inhabitants are for the most