377 
Steam Engine* 
above another ; C and D, their respective pistons con- 
nected by one rod. F is a cylinder^ containing a fire in 
a vessel within it in such a manner, that any air passing 
between the upper and lower portions of it must go 
through the fire. This vessel also contains a long cylin- 
der, open at the bottom and directly over the centre of the 
fire, for the purpose of holding coke or other fuel. This 
cylinder is covered at the top, and packed air tight when 
it has served the purpose of permitting the fire to be kind- 
led through it : and has been filled with fuel. 
The cylinder B is fitted up to answer the purpose of a 
double stroke forcing pump, or bellows, to drive the air 
into the upper portion of the vessel F, from whence it 
passes downwards through the fire for the purpose of con- 
suming the smoke (the fresh fuel being supplied from the 
reservoir above) in its passage through the more com- 
pletely ignited cinders below. In this act the air is ex- 
panded ; and, by means of pipes from the lower portion 
of F, it is conveyed alternately above and below the pis- 
ton of the cylinder A, In each pipe is fixed a stop cock 
or valve, so constructed as to open a passage to the ex- 
ternal air, when it shuts the connection with the fire ves- 
sel. These cocks are worked by a plug frame. 
From this construction it will appear evident, that 
whatever expansion the air receives, its pressure will ope- 
rate alike upon the piston of the bellows and of the re- 
ceiver ; and that always in opposition to each other : 
Hence the power of the stroke will be in proportion to the 
excess of the area of the receiving piston, over that of the 
feeding one, multiplied by the expansive force of the con- 
tained air, and by the length of the movement. 
If, when the engine is well constructed, the expansion 
of the air in keeping up the fire be not found sufficiently sen 
sible, still the form of the engine is such as to admit oi 
either inflammable gas^ oil of tar, or other inflammable 
