THE STEAM ENGINE. 93 



play into it and condense the steam, and thereby pro- 

 duce a vacuum. When, immediately afterward, steam 

 is admitted below the piston, the latter has nothing on 

 the other side to drive out, and consequently rises more 

 easily. As less force is required, steam of lower pres- 

 sure may be used, with a corresponding economy of 

 heat and the fuel required in its producton. 



224. THE CONDENSER. In steam en- 

 we'and' Object g ines j as now made, the water used to con- 

 ey the con- dense the steam, does not come into the 



cylinder itself, but into a side vessel, called 

 the condenser. The steam expands into this vessel, and 

 is condensed, producing a vacuum in the cylinder 

 itself, as effectually as if the water were there intro- 

 duced. The object of the modification is to avoid 

 cooling the cylinder, and thereby diminish the ef- 

 fect of the steam subsequently entering from the 

 boiler. This engine is called the low pressure engine, 

 from the fact that steam of lower pressure may be em- 

 ployed to move it than is the case with the engine pre- 

 viously described. It may, indeed, be made to run 

 with vapor formed below 212, instead of steam. But 

 in practice, steam of from ten to thirty pounds effective 

 pressure is employed. 



225. ORIGINAL STEAM ENGINE. In the 

 original low original form of the steam engine, the pres- 

 presmrecn- sure o f t he atmosphere, instead of steam, 



was applied on one side of the piston, and 

 it therefore received the name of the atmospheric engine. 

 Suppose the cylinder in the last figure to be open at 

 the top, and the piston at its full height. On condens- 



