94 HEAT. 



ing the steam below it, the piston would evidently sink, 

 in consequence of the pressure of the atmosphere. By 

 thus employing steam pressure on one side, and atmo- 

 spheric pressure on the other, a constant motion would 

 be realized. But the effective power would evidently 

 be less than in the low pressure engine, because part 

 it would have to be expended each time in lifting 

 the piston, in opposition to the pressure of the atmo- 

 sphere. 



226. A test-tube provided with a pis- 



Tfrrtlfiin the 



. f ton made of cork, or better of w^ood w^ound 



figure. 



with cotton, suffices perfectly to illus- 

 trate the source of power in the steam engine. On 

 boiling a little water in the tube, the 

 piston rises. On dipping it into cool 

 water, and thus condensing the steam, 

 the piston is forced down to the bot- 

 tom, as in the original form of the 

 low pressure engine. In the ascent 

 of the piston, the analogy is not 

 perfect ; for it is, in this case, the 

 production of new steam, and not, as in the steam en- 

 gine, the expansion of steam already produced, that 

 causes the piston to ascend. 



227. STEAM USED EXPANSIVELY. It is 



Explain the 



action of steam not necessary in the steam engine, that 

 steam be made to flow from the boiler du- 

 ring the whole movement of the piston, 

 from one end of the cylinder to the other. When the 

 cylinder is partly filled, the supply is cut off, and the 

 steam already introduced forces the piston through the 



