Prof. Barnard on the Elastic Force of Heated Air. 161 
m—t, »,167-8—671_, ' 
HH 1267 XH, 
as the measure of the economy of working with steam under 
those conditions. In like manner we obtain ‘0861 x H, for steam 
of three atmospheres. And Joule’s engine gives 
1220-7 — 915°5 
_ zai gd ‘433 XH. 
By properly combining these numbers with the foregoing, we 
obtain these ratios, steam being in both cases taken as unity 
ql 
i) 
= 
Py 
Me 
3 
& 
~ 
Steam. Joule. Max. m. flax. 1. 
5 atmospheres = 1 3°436 3631 2839 2502 
3 atmospheres = 1 5-031 5321 4153 3665 
There can therefore be no doubt of: the very great econumy 
which would attend the working of a hot-air engine, should suc- 
cessful ingenuity overcome the difficulties (entirely mechanical) 
that have hitherto, in a great measure, baflled the efforts of 
inventors. 
~ Tam willing to admit that the working temperature assumed 
by Mr. Joule is too high; or is at least higher than is to be de- 
sired: and I may add that it is higher than is necessary. But 
before examining what would be the effect of reducing this heat 
within limits to which no one could reasonably object, it may be 
well enough to examine one or two of the difficulties which have 
been mentioned earlier in this article, as having been esteemed in- 
separable from the use of this motive power, and insurmountable. 
Mr. Joule has provided against losses of heat by escape through 
chimneys, &c., by proposing to make the furnaces air-tight, and 
to pass the air through them in its way from the reservoir to the 
cylinder. The blast may be directed in part through the fuel ; 
but as the entire mass of air might create unnecessarily powerful 
combustion if driven through the fire, the greater portion may 
pass over it.* It may be added that, as the difficulty hitherto has 
which on supposition that the foregoing proportion holds, becomes, 
y-1 t 1 79 deere 1! — 4, 
u[—( -5)+,0-pJ=H( 7 =H zh 
which is Thompson’s formula. The demonstration of this proposition is reserved 
for another place. 
* By this arrangement the oxygen of t 
acid, but without change of volume, although with some small increase of specific 
heat. Should the plan be adupted of employing confined air previously compress- 
= 
ed, some fresh air will require to be regularly supplied, to sustain the combustion. 
This ma h th i 
he air is shortly converted into carbonic 
e 
1 
co 
be introduced beneath the fuel, while the rest passes over it. The amount 
ecessary mi ily b ted. According to Dr. Prout, 100 cubic inches of ai 
at the normal pressure and density weigh 31-0117 grs. ; of which ght 238 cent. 
is oxygen. Accordin ule, one grain of coal produces by it bustio 
heat enough to raise the temperatur pound of water 1°634 cording to 
ault, the specific heat of air at constant pressure is (taking a mean of his val- 
ues) = a ; d, t. equivalents of carbo is XY, - vf gt 8 
respectively, the composition of carbonic acid, COs, requires that eight grains of oxy- 
gen be supptied Sor the jpartie aeenbostion of three grains of carbon. From these — 
Secoxp Szniss, Vol. XVII, No. 50.—March, 1854. Se 
