AIR-PUMP. The air-pump is an instrument by which a vacuum can be produced in a given space, or rather by which air can be greatly rarefied, for an absolute vacuum cannot be produced by its means. Fig. 159 represents a simple form of this machine. Through the centre of the brass plate there is drilled an orifice A, from which orifice there is led a pipe A B, forming a communication between the receiver R and the interior of the cylinder BP V, which communication may be opened or closed by means of a stopcock at G. The cylinder or barrel BPV is furnished with a piston BP accurately fitted to the cylinder, but capable of free motion up and down, which motion is effected by means of a piston-rod D C, which moves through a stuffed or air-tight collar at D. The bottom of the cylinder or barrel is furnished with a valve V opening outward. This cylinder communicates with another BXPV, constructed and furnished in a similar manner; and the two piston-rods are provided with racks C C at the top, the teeth of which are acted upon by those of a wheel placed between them, as may be seen in the figure. Let us now attend to the mode of action. Suppose the stop-cock at G open, and the pistons as they are in the figure. The piston B P being at the top, a free communication is formed between the receiver R and the first cylinder, and the piston being pushed down past the orifice at B, the air contained in the cylinder or barrel will be forced into less space or compressed, and, of course, its elastic force increased. In consequence of this increased elasticity, the valve at V will be opened and the air expelled. When the piston is lifted, this valve will be shut by the pressure of the atmospheric air without; thus a portion of the air which was contained in the receiver, cornmunication-pipe, and barrel, has been expelled, and that which remains will consequently be less dense; another stroke of the piston will diminish the density still more; and this process may be continued until the density be so diminished that, when compressed by the descent of the piston to the bottom of the barrel, its elastic force is only sufficient to open the valve V. It will be easily seen that the exhaustion of the air in the receiver depends on the elasticity of the air; for when the piston descends and expels the air contained within the barrel, which it will do completely if it go to the bottom, and then, in returning, the valve V being shut, a vacuum will be formed in the barrel until the piston in its ascent passes the orifice B, when the air within the receiver will expand and fill the whole cavity. The operation of the second barrel and piston is precisely similar to that of the first, so that when the one is understood, the other requires no explanation.

The degree of exhaustion will depend upon the workmanship of the pump, the number of strokes of the piston, and the relative capacities of the receiver and barrels; but perhaps in no case can the vacuum in the receiver be made perfect. For the purpose of determining the degree of exhaustion, a mercurial gauge is employed, which acts on a similar principle with the common barometer. A glass tube E F rests in a basin of mercury F; and its upper orifice opens into the brass plate SS. When the exhaustion of the receiver has commenced, the pressure of the air in the receiver must be less than that of the atmosphere without. Wherefore, since the air in the receiver presses the mercury down the tube, and the atmosphere pressing on the mercury in the basin forces it up the tube, with the greater force the mercury will rise in the tube, and it will rise the higher according to the difference of the density, and consequently elastic force, of the air in the receiver, and that of the atmosphere.

Two examples of the latest improved air-pumps are given herewith. Fig. 160 is the free-piston air-pump of M. J. A. Deleuil. The peculiarity of the machine is that the piston works out of contact with the barrel of the pump, and, of course, without friction. The film of air between piston and cylinder-wall forms a kind of lubricating cushion. The piston is driven by an epicycloidal combination operated by crank and fly-wheel, and is guided by its rod, as shown. There are two valves at each end of the cylinder, one opening inward, the other outward. The outward-opening valves both communicate with the same tube, which is secured and united with the cylinder at both extremities. At the middle point of this tube a branch leading from it may be connected with a condensing apparatus ; so that the pump may be used for condensation as well as rarefaction. When used for the ordinary purposes of an air-pump this branch is open to the atmosphere.

On the other side, the two inward-opening valves are similarly connected, and the branch tube on that side establishes communication with the receiver to be exhausted. The valves are opened and shut mechanically by the piston itself in a manner not shown in the figure. For this purpose two cylindrical rods are introduced passing through the piston, and reaching from end to end of the cylinder, but capable of a slight longitudinal movement as the piston changes its direction. This movement opens a valve at one end, and simultaneously closes the corresponding one at the opposite end ; but this change having been effected, the rod remains stationary, the piston sliding on it in continuing its movement. With a machine of this kind, having a cylinder 4 1/2 inches in diameter, a 20-gallon receiver may be exhausted down to a pressure of less than half an inch of mercury in five minutes.

Fig. 161 represents an air-pump devised by M. de las Marismas, which may be cheaply constructed. Two reservoirs A A counterpoise each other, and are supported by the pulley B. They communicate with two glass balloons C by means of the glass tubes D, and of the Indiarubber tubes E. They are filled with mercury, which, when one of the reservoirs is lifted, passes into the balloon and drives the air out of it through the capillary tube F, which is soldered to the top, at the same time that the other reservoir, in falling lower then 29.64 inches, causes the mercury to quit the other balloon, thus forming a barometric vacuum. The balloons communicate with the plate G by the glass tubes H, which plunge to within 0.39 inch of the bottom of the balloons. They are automatically closed as soon as the mercury rises within the balloons to drive out the air, and opened as soon as it retires to produce a vacuum. The air cannot reenter the balloons by the tubes F after having been once driven out, because, in order to escape by the orifice I, it is obliged to pass through a slight layer of mercury contained in the curved tube J; and when the vacuum is formed in the balloons, the atmospheric pressure causes the mercury to mount up again in the tubes, and thus prevents the return of air. In order to receive the air or gas contained in the plate, all that is to be done is to place the required recipient in communication with the orifice I The degree of vacuum produced is indicated by the barometer K, which communicates with the plate by tube L. The return of the air is effected through the tube M, which communicates on one side with the plate, and on the other plunges into the mercury contained in the bent tube N.

Bunsen's Air-Pump is represented in Fig. 162. Falling water is employed to carry the surrounding air with it, and in this way a steady exhaustion is produced. The device consists of a wide glass tube D in which a narrower tube reaches downward to N, connected at the top by a well-fitting cork M. Water is carried in by a side branch C, connected by means of an India-rubber tube B, closed by a spring H, with a tube A drawing water from a reservoir. The current of this water going down in the tube D around the inner tube draws the air from T and S and from any vessel connected with S. To increase the effect, the wide tube D is connected below with a lead tube F which reaches 20 or 30 feet down; so that this long descending column of water acts like a powerful continuous piston.