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V. On the Air-Engine. By James Prescott Joule, F.R.S., F.C.S., Corn. Mem. 
R.A. Turin, Sec. Lit. and Phil. Soc. Manchester, 8^c. 
Received May 13, — Read June 19, 1851. 
It has long been suspected that important advantages might be derived from the 
substitution of air for steam as a prime mover of machinery. It has been alleged 
that the air-engine would be safer, lighter, and more economical in the expenditure 
of fuel than the steam-engine. Until comparatively recent times, however, experi- 
mental science was hardly in the state of advancement requisite to enable the 
physicist, in his investigation of this important subject, to arrive at conclusions 
sufficiently certain to give confidence to the practical machinist. Professor Thomson, 
Mr. Rankine, and M. Clausius have of late, however, published papers of great 
value on the mechanical action of gases, and particularly of steam, founded on 
tolerably correct experimental data. I hope that the following remarks founded on 
the same general principles, but applied to a particular kind of air-engine, may be 
interesting to the Royal Society. 
The air-engine, the performance of which I propose to discuss, consists of two 
parts, in one of which the air is compressed into a receiver, where its elasticity is 
increased by the application of heat, and in the other it is allowed to escape again 
from the receiver into the atmosphere. By the former work is absorbed, by the 
latter it is evolved in a larger quantity, the excess constituting the work evolved by 
the engine on the whole. The simple question, therefore, is to determine the 
quantity of work so evolved, together with the heat applied to increase the elasticity 
of the air in the receiver. 
In Plate VI. fig. 1 let A be the pump by which air is forced into the receiver C, 
where heat may be communicated to it from an external source, and B the cylinder, 
by which the same quantity is allowed to escape again into the atmosphere. More- 
over, let the material of which the apparatus is made, with the exception of that part 
through which heat may be communicated to the air in C, be impervious to, and 
destitute of capacity for heat. Such a machine may be conceived to work in the 
following manner. 
The cylinder of the pump A being filled with air of the atmospheric temperature 
and pressure, the piston compresses the air until, at a point n, its pressure is rendered 
equal to that of the air in the receiver C, which has been previously filled with air 
of an elevated temperature and pressure. The work absorbed by this action will be 
that communicated to the air in the cylinder, minus the work due to the atmospheric 
MDCCCLII. 
K 
