Bteam Mngine* 73 
not be depended on, unless the engine contained a provi« 
sion for constantly keeping it up. Mr. Watt’s contri« 
vance in his simplest engine is as follows : The steam is 
conveyed from the boiler to the upper part of the cylinder 
through a pipe, which also communicates occasionally 
with the lower part, and beyond that space with a vessel 
immersed in a trough of water ; in which vessel the con-= 
densation is performed by an injected stream of cold wa» 
ter. This water is drawn off, not by an eduction-pipe 
but by a pump, of which the stroke is sufficiently capa- 
cious to leave room for the elastic fluid, separated during 
the injection, to follov/ and be carried out with the injec- 
tion water. Suppose now the piston to be at its greatest 
elevation, and the communication from the boiler to the 
upper as well jfe to the lower parts of the cylinder to be 
opened. The steam will then pass into the whole internal 
part of the engine, and will drive the air downwards into 
the condenser, and thence through the valves of the air- 
pump. In this situation, if the communication from the 
boiler to the lower part of the cylinder be stopped, and an 
injection be made into the condenser, a vacuum will be 
produced in that vessel, and the steam contained in the 
lower part of the cylinder and communication pipe will 
expand itself with wonderful rapidity towards the con^ 
denser, so that in a period of time too minute to be ap- 
preciated, the wffiole of the steam beneath the piston will 
be practically condensed. The steam which continues to 
act above the piston will immediately depress it into the 
vacuum beneath ; at the same time that by connection 
with the external apparatus the piston of the air-pump al- 
so descends in its barrel. When the stroke is nearly 
completed downwards, the requisite part of the apparatus 
shuts the communication with the boiler, opens that be- 
tween the upper and lower parts of the cylinder and con- 
densing vessel, and turns the iiijection-cock. At this ve^ 
VoL IL K 
