Steam-Engine — Physics, ^c. ' 165 



shut the 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 it 

 is important not to have to pump against the pressure of high 

 steam.* 



•" Plate III. Fig. 4. The mode of changing the passage of 

 the steam to the opposite sides of the valves, in order to get a 

 reversed motion of the engine, a a, the fixed part, or valves ; 

 c d, the pipes ; f g, the cross pipes ; e e, the cocks, which 

 are represented open, to pipes c and d — turn them half round, 

 they close c and d, and open / and g. Fig. 1 shows the side- 

 rudders, d, e, &c. 



To this engine is conveniently applied the gas-fire, in the 

 following 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 

 pliitform 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 near the top, lead out the tar and steam. These 

 pipes unite below ; the steam and tar, thus mingled in suitable 

 proportions, flow to the main fire, or the flues of the boilers, 



* It is found with rery high steam that the source of supply must be above the 

 chamber^ or a small quantity of cold water introduced to condense the steam 

 therein. 



