MR. J. P. JOULE ON THE AIR-ENGINE. 
71 
employed to evaporate water in the boiler is afterwards evolved in the form of work, 
in consequence of the liquefaction, in the cylinder, of a portion of the expanding 
vapour. This fact would induce the hope that a great portion of the latent heat of 
evaporation, which is at present almost entirely lost, might by an increase of tempe- 
rature, and by extending the principle of expansion, be converted into mechanical 
effect. 
If, as would appear from the experiments of De la Rive and Marcet, Haycraft 
and Dulong, the capacity for heat of a given volume is the same in all gases taken 
at the same pressure and temperature, the results of the above Tables will be equally 
true whatever elastic fluid be employed. 
It now only remains to offer a few observations, with a view to facilitate the labours 
of those who may be desirous of constructing a good practical air-engine. 
It may be remarked, in the first place, that the receiver C need not be of much 
greater capacity than the cylinder B. For in the reciprocating engine, the air could 
be introduced from the pump A, at the same time that an equal amount would be 
expelled into the cylinder B. It would therefore be only requisite to pass the air 
through tubes heated by a proper furnace, as in Neilson’s hot-blast] the tubes them- 
selves constituting the receiver C. For a temperature under the red heat, these 
tubes might be constructed of wrought or cast iron. They might be either straight, 
like the tubes of a locomotive boiler, or arranged in the form of a coil, as represented 
by fig. 2, in which a is the pipe which conveys the air from the pump, c, c, c, &c. is 
the coil of wrought or cast-iron tubing, and b is the pipe which conveys the heated 
air to the cylinder. The coil is surrounded by a massive arch of brickwork, which 
serves at once to support the pipes, and to prevent waste of heat. To prevent the 
temperature exceeding the proper limits, the pipe b might, as it expands by the heat 
of the inclosed air, move a piece of mechanism in connection with the damper of the 
flue. I may remark that, on the scales adopted, fig. 2 represents the size of receiver 
which would be required for an engine the cylinder of which is 3 feet in diameter. 
I would here venture to suggest whether the combustion of the fuel could not, by 
suitable mechanical arrangements, be carried on within the receiver C ; if this could 
be accomplished, the heat, which in the form of receiver already described is lost up 
the chimney, would be economized, and a great saving of weight and space would 
be effected. An engine furnished with a receiver of this kind would be strikingly 
analogous to the electro-magnetic engine, and present a beautiful illustration of the 
evolution of mechanical effect from chemical forces. 
In both of the above forms of receiver, it would be desirable, as already hinted, 
that the introduetion of the air into the receiver should be simultaneous with the 
expulsion of the same quantity into the cylinder. This is necessary in order both to 
keep the pressure in the receiver uniform and to promote the smooth action of the 
engine. For this purpose the piston-rods of the pump and cylinder, a and b (fig. 3), 
must be attached to cranks on diffeient parts of the circumference of the revolving 
shaft cc, so contrived that the piston shall arrive at the top or bottom of the cylinder 
