177 
Steam Engine. 
of twenty pounds the square inch, if steam of that forcq 
be employed, the space, place, or vessel in which it is al- 
lowed or intended to expand itself, should be of the same 
temperature, or nearly so; and so with steam of any 
other power, as of thirty, forty, or fifty pounds the square 
inch, the space, place, or vessel in which it is to expand, 
should be at or about the same degree of heat as steam of 
the force employed requires for its existence ; in which 
case, as I have before stated, steam can expand itself 
about as many times as the number of pounds it could 
have sustained on a square inch as aforesaid, before it is 
allowed to expand or dilate itself. Here, however, it may 
be necessary to remark, that in stating this ratio, I only 
speak of the expansion of steam as it can be managed 
and commanded in practice, and not of the absolute ex- 
pansion which perhaps might be obtained, if mechanism 
could be made so perfect as to prevent all escape of steam, 
and all partial condensation of it and waste of heat ; for 
the real expansive force of steam, I am inclined to believe, 
from the experiments I have made, increases in a regular 
ratio a little beyond what I have stated, though I would 
not recommend that it should be calculated higher in ap- 
plying it to steam-engines, because the difficulty of con- 
fining and managing it increases also as the elasticity of 
the steam is increased, or as its temperature is increased. 
And here it may be of use to the public to state some 
facts respecting different degrees of temperature required 
to bring steam to, and maintain it at, different expansive 
forces above the weight of the atmosphere ; because the 
temperature of the steam indicated by a thermometer, 
having its bulb in the boiler which produces it, will indi- 
cate the expansive’ force of the steam, without the trouble 
and inconvenience of changing the weights on the valve,, 
by which its force is regulated for the work intended to 
be performed by it, and which valve acts as a common. 
Vol. II. Z 
