Steam Engine, 187 
that the piston or pistons may perform the whole or a 
great part of the stroke by the time the intended quantity 
of steam has been admitted into the cylinder or cylinders : 
for example, when steam of forty pounds the square inch 
is used, such a quantity of the same as shall be equal in 
volume to one-fortieth of the capacity of the cylinder or 
cylinders, and so in proportion when steam of any other 
force is employed ; and when the requisite quantity has 
been admitted, the steam is to be shut off till the proper 
moment for admitting a fresh quantityc But if it is in» 
tended that advantage shall also be taken of the elastic 
force of the steam acting on one side of the piston or pis- 
tons, while condensation goes on on the other side, then 
the steam must be admitted more freely, but still with 
caution at the first, for the reason already mentioned. 
And in this, as well as in every other application of my 
said improvements, grounded on my said discovery of the 
law of expansibility of steam, due and effectual means 
must be used to keep up the requisite temperature of all 
the parts of the apparatus into which the steam is admit- 
ted, and in which it is not intended to be condensed. 
And here it may be proper to state, that, instead of the ob- 
vious and usual means for accomplishing this, namely, 
inclosing them in the boiler, or in a steam case or cases 
communicating with the boiler, the following method 
may sometimes be ysed with advantage, viz. a separate 
fire under the steam case or cases, which, in that event, 
will become a boiler or boilers, and must be furnished 
with a safety-valve or valves to regulate the temperature. 
By means of the last-mentioned arrangement, the steam 
from the smaller cylinder or steam measure (when either 
of these is employed) may be admitted into the larger cy? 
Jinder or cylinders kept at a higher temperature than the 
smaller, by which its power to expand itself may be in- 
creased $ and, on the contrary, by keeping the iarge^' at a 
