40 
bteam Engine. 
iiiid particularly of one class (the gases), which are not 
easily compared in any other way. (See Leslie on Heat, 
chap. XX i.) 
■ON THE ELASTICITY OF STEAM. 
2 Gregory'* s System of Mechanics. 
Since the important invention of the Steam -engine, ano» 
ther species of first movers has come under the considera- 
tion of the mechanical investigator, namely, such as arise 
from the volatilisation of different ffuids. Of these the 
one most commonly chosen is the steam raised from hot 
water, which is an elastic ffuid, and which when raised 
with the ordinary heat of boiling water, is almost 3000 
times rarer than water, or more than 3j times rarer than 
air, and then has its elasticity equal to that of the common 
atmospheric air : by great heat it has been found that the 
steam may be expanded into 14000 times the space of 
water, and then exerts a force of nearly 5 times the pres- 
sure of the atmosphere : and there is no reason to suppose 
this is the limit : indeed some accidents which have hap- 
pened, prove clearly, that the elastic force of steam may at 
least equal that of gunpowder. 
The observations on the different degrees of tempera- 
ture acquired by water in boiling, under different pressures 
of the atmosphere, and the formation of the vapour from 
water under the receiver of an air-pump, when with the 
common temperatures the pressure is diminished to a 
certain degree, shew clearly that the expansive force of 
vapour or steam is different in the different temperatures,, 
and that in general it increases in a variable ratio as the 
temperature is raised. Previous to describing the method 
which has been adopted to measure the force of steam 
* Abridged from Prony^s Architecture hydraulique : Prony's 
account of Betancourt’s experiments, Journal de PEcoie poly- 
technique : ! Phil. Mag. T. C. 
