36 Steam Engine. 
voisier, with the aid of the calorimeter, makes it 1000® 
or a little more. (Ibid. 175.) 
IX. The same TV eight of steam contains^ Tvhatever may 
he its density., the same quantity of caloric ; its latent heat 
being increased in exact proportion as its sensible heat is 
diminished ; and the reverse . — This principle, though 
scarcely admitting of illustration by any easy experiment, 
is one of considerable importance ; and an ignorance 
of it has been the occasion of many fruitless attempts to 
improve the economy of fuel in the steam engine. The 
fact, so far as respects steam of lower density than that 
of 30 inches of mercury, was long ago determined experi- 
mentally by Mr. Watt. (Black, i. 190.) As the boiling 
jx)int of liquids is known to be considerably reduced un- 
der a diminished pressure, it seemed reasonable to suspect 
that, under these circumstances, steam might be obtained 
from them with a less expenditure of heat. W ater, Mr. 
Watt found, might easily be distilled in vacuo when in 
the temperature of only 70° Fahrenheit. But, by con- 
densing steam formed at this temperature, and observing 
the quantity of heat which it communicated to a given 
weight of water, he determined that its latent heat, instead 
of being only 950°, was between 1200° and 1300°. 
The same principle may be explained, also, by the fol- 
lowing illustration, which was suggested to me by Mr. 
Ewart. Let us suppose that in a cylinder, furnished with 
a piston, we have a certain quantity of steam, and that it 
is suddenly compressed, by a stroke of the piston, into 
half its bulk. None of the steam will in this case be con- 
densed ; but it wdll acquire double elasticity, and its tem- 
perature will be considerably increased. Now if w'e ei- 
ther suppose the cylinder incapable of transmitting heat, 
or take the moment instantly folloiving the compression 
before any heat has had time to escape, it must be evident 
that the sensible and latent heat of the steam, taken toge- 
