THE CONQUEST OF NATURE 



to escape into the funnel of the engine to increase the 

 draught, an expedient which was so largely responsible 

 for Stephenson's success with his locomotive twenty 

 years later, and which retains its utility in the case of 

 the most highly developed modern locomotive. 



Trevithick was, however, entirely subordinated by 

 the great influence of Watt, and the use of high pressure 

 was in consequence discountenanced by the leading 

 mechanical engineers of England for some decades. 

 Meantime, in America, the initiative of Evans led to a 

 much earlier general use of high-pressure steam. In 

 due course, however, the advantages of steam under 

 high pressure became evident to engineers everywhere, 

 and its conquest was finally complete. 



The essential feature of super-heated steam is that it 

 contains, as the name implies, an excess of heat beyond 

 the quantity necessary to produce mere vaporiza- 

 tion, and that the amount of water represented in this 

 vapor is not the maximum possible under given con- 

 ditions. In other words, the vapor is not saturated. It 

 has been already explained that the amount of vapor 

 that can be taken up in a given space under a given 

 pressure varies with the, temperature of the space. 

 Under normal conditions, when a closed space exists 

 above a liquid, evaporation occurs from the surface of 

 the liquid until the space is saturated, and no further 

 evaporation can occur so long as the temperature and 

 pressure are unchanged . If now the same space is heated 

 to a higher degree, more vapor will be taken up until 

 again the point of saturation is attained. But, obviously, 

 if the space were disconnected with the liquid, and 



t"4] 



