74 Steam Engine^ 
ry instant the piston loses its tendency to descend, because 
the steam presses equally on both surfaces, and continues 
its equality of pressure while the condensation is perform- 
ed. It therefore rises ; the injection is stopped ; and the 
air-pump making its stroke, suifers the injection water 
and a considerable part of the elastic fluid to pass through 
its lot^ er valve. The vacuum is thus kept up through the 
whole internal capacity of the engine. As soon as the pis- 
ton has reached the upper part of the cylinder, the com- 
munication to the under part of the cylinder is stopped, 
and that with the boiler opened, as before ; the conse- 
quence of which is, that the piston again descends ; and 
in his manner the ?1 ernations repeatedly take place. 
The principal augmentation of power in this engine^ 
compared with that of Newcomen, arises from the cylin- 
der not being cooled by the injection water, from its being 
practicable to use steam, which is more powerful than the 
pressure of the atmosphere, and from the employing of 
this steam both to elevate and to depress the piston. In 
general, these engines are worked by steam, which would 
support a column of four or five inches of mercury be- 
sides the pressure of the atmosphere, and sometimes 
more ; for Mr. Nicholson says, he has sometimes seen the 
gage as high as eight inches. Mr. Watt has made seve- 
ral successive modiflcations and additions to the engine 
just described, some of which will be further spoken of in 
the course of this article. 
4. It has been customary, w^hen treating of steam-en- 
gines, to present several theorems for the computation of 
their power and effects. But as all which has hitherto 
been advanced on these points seems to us very vague 
and unsatisfactory, we shall not delude the student with 
an appearance of mathematical accuracy, when it is so far 
from being attained. It is obvious enough that the abso- 
lute povi er of a steam-eiigine is in the compound ratio of 
