THE CONQUEST OF NATURE 



engine. It appears, then, that when steam enters the 

 cylinder and begins to thrust back the piston of the steam 

 engine, a portion of the steam is immediately condensed 

 on the walls of the cylinder, owing to the fact that 

 previous condensation of steam has cooled these walls 

 to a certain extent. We have already pointed out that 

 Watt endeavored in his earlier experiments to over- 

 come this difficulty, by equalizing the temperature of the 

 cylinder walls to the greatest practicable extent. 



Notwithstanding his efforts, however, and those of 

 numberless later experimenters, it still remains true 

 that under ordinary conditions, particularly if steam 

 enters the cylinder at the saturation point, a very 

 considerable condensation occurs. Indeed this may 

 amount to from thirty to fifty per cent, of the entire 

 bulk of water contained in the quantity of steam that 

 enters the cylinder. This condensation obviously mili- 

 tates against the expansive or working power of the steam. 

 But now as the steam expands, pushing forward the 

 cylinder, it becomes correspondingly rarefied, and im- 

 mediately a portion of the condensed steam becomes 

 again vaporized, and in so doing it takes up a certain 

 amount of heat and renders it latent. This disadvan- 

 tageous cycle of molecular transformations is very 

 much modified in the case of super-heated steam, for 

 the obvious reason that such steam may be very much 

 below the saturation point, and hence requires a very 

 much greater lowering of temperature in order to produce 

 condensation of any portion of its mass. Without 

 elaborating details, it suffices to note that in all highly 

 efficient modern engines, steam is employed at a rela- 



