80 Steam Engine^ 
particularly coiliiected with the working gear, that by al- 
tering the place of a pin it is shut sooner or later, as occa» 
sion requires ; and when the piston has proceeded half 
way down, if this valve is shut at that instant, the piston 
is carried through the remainder of the stroke, partly by 
the momentum it has already acquired, and partly by the 
remaining expansion of the steam j which notwithstanding 
its growing rarer* and rarer, is sufficient With its momen- 
tum to complete the stroke. 
We have observed, that in order to give action to the 
engine at its first onset, and indeed at every succeeding 
stroke, the valve i is to be lifted. This valve is kept down 
by a weight equal to the pressure of the atmosphere, add- 
ed to the elasticity of the steam above that pressure ; there 
being a vacuum beneath it, and the action of the steam 
upon it. Hence, in large engines it requires a great force 
to lift it up. 
Watt’s double Engine. ^ W e have represented the 
mode of working this engine at fig. 2. where we must 
deviate largely from the practical application, in order to 
give a comprehensive explanation. A is the cylinder, as in 
the single engine ; B is a pipe appropriated to deliver the 
steam from the boiler through the branch L ; from thence 
it enters the cylinder alternately by the valves u, ry, both 
opening towards the steam side. The pipe C has also 
two valves, one at the upper end of the pipe in the box 
and one which we must designate at the joint r, both open- 
ing toward the cylinder. The condenser D has a blow- 
ing-valve, like the former figure in the single engine ; 
but the injection is made by a cock G, which has a pipe 
reaching near the bottom of the cistern ; and when the en- 
gine is at work this cock is always open, and the injec- 
tion always running in, because the steam is constantiv 
* Which Homblower derides. T. C. 
