166 Prof. Barnard on the Elastic Force of Heated Air. 
under the ordinary arrangements, extends over all that period 
when the driving power is approaching zero, is easily disposed of, . 
and the remaining resistance only becomes serious when the com- 
panion piston possesses a powerful mechanical advantage over it. 
The mechanism which I would propose is simple and liable to 
no objection at present discoverable; yet I have no doubt that 
other contrivances might be devised to accomplish the same end 
as well or better. I shall consider, therefore, the resistance of 
the supply cylinder as no longer entitled to very grave considera- 
tion; and, if it were, a heavy fly wheel would be a sufficient 
regulator for stationary engines of moderate power. For as the 
engine may be started with a cut-off which produces no counter 
resistance, there can be no difficulty in putting the regulator in 
motion. 
It is sufficiently obvious that, for all the ordinary purposes to 
which stationary engines are applied—that is, for all purposes In 
which immense power within limited bulk is not an indispensa- 
ble condition—the air-engine may take the place of steam with 
an economical advantage of at least two, and probably three, to 
one. It remains only to examine what dimensions would be ne- 
cessary to create a power equal to that employed to propel the 
great ocean steamers. ; 
There can be no doubt that the effectual modes of heating 
which may be made to supersede the very imperfect arrangements 
of the “ Ericsson,” will'render possible a material increase in the 
velocity of the piston. That at least twelve revolutions may 
be obtained with an eight foot stroke is a moderate estimate. 
Under the lowest of the foregoing assumed working temperatures, 
(482° F’.) a mean pressure of more than five pounds is obtained; 
but we will assume one lower than the lowest, that is; 43 |bs- 
per sq. inch. Now one double-action six foot cylinder with 4 
stroke of eight feet, twelve revolutions, and a mean pressure of 
four and a half pounds, will give an aggregate horse power 0 
1063. Let the air be drawn froma chamber in which it has been 
previously compressed to the density of four times that of the at- 
mosphere, and the power will be 425. Two such cylinders will 
* If the air he heated without coming into actual contact with the fuel, the three 
cylinders may all be supplied from the reservoir, and by making them only five feet 
each in diameter they will give an aggregate horse power of 855. This doubled will 
