Prof. Barnard on the Elastic Force of Heated Air. 155 
4. That a very large additional waste must occur through 
chimneys and flues, and by radiation from the eylinders. 
5. That the leakage must be enormous, as compared with - 
what occurs in engines driven by steam. 
6. And finally, that air cannot be heated in such quantity, with 
such rapidity, and to such a degree as the exigencies of the case 
Not one of these propositions is necessarily or universally true; 
and it will presently be seen that the plan of Mr. Joule is in de- 
fiance of all of them. On the other hand, it must be admitted 
that there are some disadvantages quite inseparable from all air 
engines of whatever description. It’ is certainly true, 
1. That the mean effective pressure must, in the case of every 
such engine, be moderate compared with the absolute tension of 
the air in the cylinders. . 
2. That the mean effective pressure, if the engine is worked 
strictly with a view to economy, bears a less ratio to the absolute 
pressure, than it might do with less economical arrangements. 
3. That a large apparatus is necessary for the development of 
high power, even though the power be obtained economically. 
4. That the highest economy ‘equires so large expansion of 
the air in the working cylinder, as to make the counteracting 
pressure in the supply cylinder a great inconvenience, though not 
dn insuperable difficulty. 
Professor Thompson, of Glasgow, has shown that the frac- 
tional portion of the heat which in any thermo-dynamie engine 
