184 
Steam Engine i 
communicate with a condensing vessel, with valves, cocks, 
or any other contrivance to open or shut the different com- 
munications when necessary. Things being thus arrang- 
ed, suppose the engine at work, and the pistons in botli 
cylinders at the top of their respective cylinders, and sup- 
pose the steam admitted to enter the small cylinder above 
the piston ; while the steam is thus entering, let the com- 
munication between the bottom of the small cylinder and 
the top of the large one be open, while the bottom of the 
large one is open to the condenser ; then both pistons 
will descend. When they have reached the bottom of 
their respective cylinders, the communications between 
the boiler and the small cylinder, between the small cylin- 
der and large cylinder, and between the latter and the 
condenser, must all be shut off ; and the communications 
must be opened between the two sides of the smaller pis- 
ton and between the two sides of the larger piston : then, 
as the pistons ascend, the steam which was last admitted 
above the small piston will pass to its under side, while 
the steam which was last admitted to the top of the large 
from the lower part of the small cylinder will pass into 
the lower part of the large cylinder under its piston, and 
both pistons will at the same time ascend by the action of 
the counter- weight, or the momentum of the fly, as the 
case may be. When both pistons have reached the top 
of their respective cylinders, the last mentioned commu - 
nications are shut, and the others are opened as before, 
and both will descend ; the steam which w^as last admit- 
ted to the under side of the small piston passing into the 
upper end of the large cylinder, while the steam last intro- 
duced below the piston of the large cylinder goes off to 
tlie condenser. Or an arrangement similar to the follow- 
ing may be adopted t only the bottom part of the 
smaller cylinder have a communication wnth the boiler, 
and let there be also a communication between the bottom 
