Steam-Engine—Physics, §c. 165 
he second cock, and open the first, the water discharges 
from the chamber into the boiler; repeated by a movement 
from the engine, when in motion, the supply continues with 
more certainty than by a pump, because it is difficult to pump 
hot water, on account of the elasticity of the steam arising 
from it, which obstructs the operation of the valves. And itis 
important not to have to pump against the pressure of high 
steam.* é 
Plate ILL. Fig. 4. The mode of changing the passage of the 
steam tothe opposite sides of the valves, in order to get a re- 
versed motion of the engine. aa, the fixed part, or valves; 
ed, the pipes; fg, the cross pipes; e e, the cocks, which are 
represented open, to pipes ¢ and d—turn them half round, 
they close ¢ and d, and open f and g. Fig. 1 shows the side- 
rudders, d, e, &c. 
To this engine is conveniently applied the gas-fire, in the fol- 
lowing manner : : 
~The boilers being cylindrical, with an inside flue for fuel, 
two or three are placed close together, and set in the following 
manner: First, cross-bars of iron are laid on the timbers; a 
platform of sheet-iron is laid on these bars, coated over with 
clay mortar, or cemented, to keep out the air. Upon the sheet- 
iron, and over the bars below, are placed cast-iron blocks, in 
- shape to fit the curve of the boiler, so as to raise it three or four 
inches above the platform. ‘The sheet-iron is continued up the 
outsides of the outer boilers, so as to enclose them ; and at one 
end, between the boilers, there are small grates for coal or other 
fuel. 
The tar vessel or vessels, as the case may be, are lodged in 
the space between and upon the boilers, and a small fire may 
be made under them, if necessary. A pipe leads steam in at 
one end, two pipes at the other; one near the bottom, and one 
hear the top, lead out the tar and steam. These pipes unite 
low; the steam and tar, thus mingled in suitable proportions, 
flow to the main fire, or the flues of the boilers, as well as to 
: tt is found with very high steam, that the source of supply must be 
above the chamber, or a small quantity of cold water mtroduced to con- 
case the steam therein. 
