Prof. Barnard on the Elastic Force of Heated Air. 157 
air from the point at which it leaves the regenerator to its original 
temperature, is a great objection to such an arrangement in-an 
engine designed for locomotion either on the water or on land. 
If, on the other hand, we abandon the regenerator, we must 
look for our economy in enlarging the expansion in the working 
cylinder, and thus increasing the range of temperature through 
which the heated air descends in working. The disadvantage, 
already mentioned as accompanying this arrangement, of creating 
a great counteracting force, at the period at which the positive . 
power is reduced nearly or quite to zero, must be met and dis- 
posed of by mechanical expedients., This appears to have been 
the view taken of the subject by. Mr. Joule in the plau he has 
will presently appear, is not rigidly true, though nearly so. The 
which we will represent by 7. 
The general equation,t ; 
tT 
P =P ( le, 
* Lond, and Ed. Phil. Mag., Jan, 1853, In the Phil. Trans. Part I, 1852, Mr. 
Joule enters into some very elaborate calculations in regard to the power of air en- 
gines under various suppositions, but the plan given in the Magazine is more practi- 
cal than anything contained in the previous article. ' ghee 
+ He also proposes another and more economical mode of heating, which is no- 
ticed further on. ‘et aS 
t In this equation, p is the pressure at the absolute temperature 1, and density 1 ; 
while p’ is the pressure at empensture +0, and density ¢, The Reged at dis 
charge (i. e, ¢) must necessarily be to the original density in the inverse of 
eylinders. 
: ee 
