An instrument or machine for exhausting or rarefying the air in closed vessels, and very generally employed to illustrate the properties of air, and to explain the various phenomena connected with the science of pneumatics. The inventor of the air-pump was Otto Guericke, a magistrate of Magdeburgh; his machine was of a rude and inconvenient structure, and worked under water, but a description of it having been received by Mr. Boyle, he, with the assistance of Dr. Hook, introduced such improvements in the construction as to render the machine extremely serviceable in philosophical experiments upon the nature and properties of the atmosphere. Numerous improvements have since been made by Hawksbee, Gravesand, Smeaton, Prince, and others; but more especially by Mr. Cuthbertson, whose arrangements are so excellent as to claim a particular notice and description.
Fig. 1 represents a perspective view of the machine, with its two principal gauges screwed into their places, which need not, however, be used together except in cases requiring the utmost exactness; but in common experiments, one of them is removed, and a stop-cock put in its place. Fig. 2 is a section of one of the barrels, with all its internal parts} Fig. 3, a section of the piston; mi Figs. 4 and 5, parts of the piston, shown detached for the sake of perspicuity. In fig. 2, C D represents the barrel, F the collar of leathers, G a hollow cylindrical vessel to contain oil; R is also an oil-vessel to receive the oil which is drawn along with the air from the barrel when the piston is drawn upwards, and when this vessel is full the oil is carried over with the air along the tube T into the oil vessel G; c c is a wire which is driven upwards from the hole, by the passage of the air, and as soon as this has escaped, it falls down again by its own weight, shuts the hole, and prevents the return of the air into the Barrel; at d are fixed two pieces of brass to keep the wire cc in a vertical position, in order that it may accurately shut the hole.
H is the piston rod, having a rack on the upper end, and made hollow to receive a long wire g, which opens and shuts the hole L; on the lower end of this wire is screwed a nut o, which, by stopping in the narrowest part of the hole, prevents the wire g from being drawn up too far. This nut and screw are seen more distinctly in Fig. 3; the wire slides in a collar of leathers shown in Figs. 3 and 5.in the middle piece of the piston. Figs. 4 and 5 are the two main pieces which compose the piston, which is shown entire in Fig. 3; Fig. 5 is a conical piece of brass having a shoulder at bottom; a long hollow screw is cut about two-thirds of its length, and the remaining part of the hole, in which there is no screw, is about the same diameter as the screwed part, except a thin plate at the end, where the diameter of the hole is just equal to that of the wire g g. That part of the inside of the conical brass in which no screw is cut, is filled with oiled leathers, having holes in them, through which the wire can slide stiffly; a short external screw, working in the internal screw, and a washer with holes in them, through which the wire g passes, serve to compress the leathers, a a, Figs. 3 and 4, is the core of the piston, the inside of which is turned so as exactly to fit the outside of Fig. 5; b b, Fig. 3, are round leathers, about 60 in number of the same diameter as the barrel C D, and having holes in the centre to receive a a; a circular plate, or washer, c c, Fig. 3, is placed over the leather, and a nut d d, upon a screw cut upon the upper end of a a, serves to compress them.
The piston rod H is screwed into the middle piece of the piston, Fig. 5, and when drawn upwards, it will cause Fig. 5 to shut close into Fig. 4, and drive out the air above it; but when pushed down it will open as far as the shoulder on the rod will allow, and permit the air to pass between Figs. 5 and 4. Having thus described at large the several parts of this machine, we proceed to explain the process of rarefaction, which is carried on in the following man' ner: - Conceive the piston to be at the bottom of the barrel, the inside of which contains common air. Now when the rod is drawn up, the upper part of the piston sticks fast in the barrel, till the conical part connected with the rod shuts the conical hole, and its shoulder applies close to its bottom. The piston being now shut, the whole is drawn up by the rack-work driving the air before it through the aperture, into the oil vessel at r, and out into the atmosphere by the tube T. The piston will then be at the top of the barrel, and the wire g will stand nearly as shown in the figure, just raised from the hole L, where it is prevented from rising higher by the nut O.
During this motion, the air will expand in the receiver, and come along the bent tube into the barrel; by this means the barrel will be again filled with air, which, as the piston rises, will be rarefied in the proportion of the capacity of the receiver, pipes, and barrel together, to that of the latter alone. When the piston is moved down again by the rack-work, it will force the conical part, Fig. 5, out of the hollow part, Fig. 4, as far as the shoulders a a. Fig. 3 will rest on a a, Fig. 4, which will then be so far open as to permit the air to pass freely through it, while at the same time the end of g g is forced against the top of the hole, and, by shutting, prevents any air from returning into the receiver; thus the piston moving downwards suffers the air to pass out between the sides of Figs. 4 and 5, and when it is at the bottom of the barrel, it will have the column of air above it, and, consequently, when drawn up, it will shut and drive out the air, and by opening the hole at L, will at the same time give a free passage to more air from the receiver.
This process being continued, the air of the receiver will be rarefied as far as its expansive powers will permit, for in this machine there are no valves to be forced open by the elasticity of the air in the receiver, which at last it is unable to effect; there is, therefore, nothing to prevent the air from expanding to its utmost degree, which is the peculiar excellency of this construction.