Steam Eny', 



507 



that it is necessary to provide tin- cylinder with relief 

 cocks at each end, shown at 8 in Fig. 250, which must be 

 opened at the start to allow the water to escape. If these 

 arc not opened at the start enough water may collect in the 

 cylinder to cause the piston to drive out the head of the 

 cylinder or do some other injury. 



the temperature of the cylinder gradually increases 

 less and less water is deposited and then the relief cocks 

 may be closed, the water which is condensed afterward 

 being so little that it is re-evaporated after the cut-oil' takes 

 place and during the exhaust stroke because, as the piston 

 travels, the space for the steam increases and this reduces 

 the pressure so that at the lower pressure the heat in the 

 walls of the cylinder is able to re-evaporate the water 

 which had been condensed. In this way a well protected 

 cylinder keeps itself empty after it has become heated. 



634. Condensation Due to Work During Expansion. - 

 When the steam expands and expends its energy in driv- 



FiQ. 243. Horizontal boiler. 



ing the piston forward its temperature is lowered in pro- 

 portion to the amount of work which it does and on this 



