in different forms of the Air-Engine. 163 
temperature and equally expanded by heat under that constant 
pressure, perform equal quantities of work, and therefore convert 
into mechanical effect equal quantities of heat. 
6th. That equal volumes of different gases, taken at the same 
pressure and temperature, and maintained during expansion at the 
same unvarying temperature, convert into mechanical effect dif- 
ferent quantities of heat, proportioned to their specific heat esti- 
tated according to volume. 
7th. That different gases expanding against pressure, without 
any accession of heat from without, convert into mechanical ef- 
fect a portion of the heat which they previously held sensibly, 
the temperature at the same time falling ; and that this depression 
of temperature is the more rapid in proportion as the specific heat 
of the gas is less. This proposition and the last are convertible. 
These propositions, with the aid of the specific heats of the 
gases furnished by Regnault, and of the formule of Poisson for 
the effect upon pressure and temperature produced by change of 
volume without any transfer of heat to or from the gas undergo- 
ing change, furnish all the data necessary for computing the 
power of any species of engine driven by the elastic force of 
heated gas, 
Pression. In some projects, a resort to both these expedients is 
aimed at; but in general one or the other will give a distinctive 
character to the engine. 
at possessed by these different forms of construction, this dif- 
erence is unimporta e may suppose, in every case, that the 
inass of air is constantly unchanged ; for in hose 1n wien a 
charge takes place, we may suppose the discharge to be directly 
Counter no greater resistance than that the a 
self would ie sige free discharge. This being premised, 
We shall simplify the theory of the air engine, by supposing that 
all the fluctuations of temperature and pressure which the air un- 
0€s, take place within the working cylinder itself. The fol- 
