The air-pump is only occasionally required to be set to work.

Fig.2.

Pneumatic Transport 32

In 1828, Messrs. Parkinson and Crossley took out a patent for an air-engine, which differs considerably in the arrangement of its parts from the one just described; and as it appears to be of a somewhat simpler construction, we shall lay a description of it before our readers. Fig. 1 shows a front elevation of so much of the engine as is necessary to explain the invention. Fig. 2 is an end elevation, and Fig. 3 a section (upon an enlarged scale) of a differential vessel, and its transferrer, exhibiting also a mode of heating and cooling the differential vessel. The same letters in each figure where they occur, refer to the same parts. The differential vessel a a is of the form of a hollow cylinder with convex ends, of such a length as to preserve an essential difference of temperature between one end and the other, and nearly one-half being exposed to a hot and the other half to a cold, medium. The vessel has a stuffing-box at the end f, and at the other end is an opening or pipe l m or l n, for the purpose of forming a communication with the working cylinder and piston.

The transferrer b b is a hollow vessel, air-tight, and so much shorter, as to leave a sufficient space in the differential vessel for containing a volume of air, which, when expanded by heat passing through the pipes l m or l n, will also fill the working cylinder, and force the piston from one end of it to the other. The transferrer is also made only so much less in diameter than the differential vessel as to allow it to move freely from one end of the differential vessel to the other. To one end of the transferrer is fixed a rod e, passing through a stuffing-box f, for the purpose of moving it from one end of the differential vessel to the other, thereby causing the air to pass in a thin stratum against its hot and cold parts alternately, thus producing the force or power to be employed against the working piston. The rod g, Fig. 3, which is fixed on the upper part of the differential vessel, is intended to guide the transferrer in its proper direction, by means of a tube which is inserted in the upper end of the transferrer, the lower end of the tube being made air-tight. Motion is given to the transferrer by means of the eccentric on the shaft p being connected with the beam r, which beam is connected to the rod e of the transferrers by the links s s.

The working cylinder w with its piston, side rods, cranks, shafts, fly-wheel, and eccentric motion, are the same as those commonly used hi steam-engines, and therefore require no particular description. The pipe I m forms a communication between the differential vessel, No. 1, and the top of the cylinder; and the pipe I n connects the differential vessel, No. 2, with the bottom of the cylinder. The operation of the engine will be as follows: - Supposing the eccentric disconnected from the beam r, and the upper part of the differential vessels heated, and their lower parts cold, and the transferrers of the two differential vessels placed by hand in the situations shown in the figure, and the volume of air occupying the hot part of the differential vessel, No. 2, and being increased in elasticity in proportion to its temperature, whilst the volume of air in the differential vessel No 1, is occupying the coldest part, the working piston will be forced upwards by a power corresponding with the difference of the elastic force of the air in the two differential vessels; and when the working piston has been forced to the top, the situation of the transferrers should be reversed by hand, so that the air in the differential vessel, No. 1, will occupy the hot part, and communicate its force to the upper side of the working piston, and thereby produce a returning stroke; and the eccentric being then by hand re-connected with the beams, the alternate expansion and contraction of the air in the two differential vessels will keep the engine in motion; and then, by working the transferrer in the same way as the valves in steam-engines, the engine may be either stopped or put in motion.

For the purpose of heating the differential vessels, the inventors prefer the employment of inflammable gas, a mode of applying which is shown at Fig. 3, where d d is a hollow ring, surrounding the differential vessel, and communicating with the tube by which the gas is supplied; this ring is perforated for the emission of jets of gas, to flow, when ignited, all round and against the differential vessel, or nearly so; c c is an iron vessel, for directing the heat to the differential vessel, which casing is open at bottom for the admission of air, having also an opening at top, to serve as a chimney or flue; k is an outer covering of polished metal, of about two or three more inches in diameter than the casing c c, for the purpose of lessening the radiation of heat The working cylinder h may be kept hot by means of a current of heated air being conducted to it from the flues of the differential vessels, t t represent the differential vessel placed in a cistern of cold water, with a constant current running in at the bottom u against the differential vessel, and passing off at the top v. We are not aware that the engine just described has been brought into practical operation, but that invented by Mr. Stirling was employed in a stone quarry; it has, however, we learn, been replaced by a steam-engine, in consequence of its inferiority to the latter in the economy of working, particularly as respects the consumption of fuel. One objection to air-engines is, that the changes of volume do not take place with sufficient rapidity, and that when water is employed to accelerate such changes, the quantity necessary for that purpose is greater than would be required to supply the boiler of a high-pressure steam-engine; so that in situations where either water is scarce, or the weight of it an objection, the latter engines would, on those accounts, be found superior to the former.

Fig. 1.

Pneumatic Transport 33

No.1

No.2

Fig. 2.

Pneumatic Transport 34

Fig. 3.

Pneumatic Transport 35