STEAM ENGINES 367 



One is the heat required to overcome the molecular re- 

 sistance of water to changing from the liquid state to the 

 gaseous. This is known as internal latent heat. The 

 other component is the heat required to overcome the 

 external resistance or pressure. 



510. Volume and weight of steam. The weight of a 

 cubic foot of steam at 212 F. is 0.03758. If the tempera- 

 ture be increased to 337, which corresponds to a gauge 

 pressure of 100 pounds, the weight of a cubic foot will 

 be 0.2589 pounds. By increasing the weight of steam 

 we decrease the volume; i.e., the volume of I pound of 

 steam at 212 is 26.64 cubic feet, but at 337 it is only 

 3.86 cubic feet. Hence when it is stated that steam has 

 a volume of so many times the volume of an equal weight 

 of water the temperature or pressure of the steam must 

 be known. Often in testing a steam boiler it is assumed 

 that as many pounds of steam are evaporated as there 

 have been pounds of water fed to the boiler. This is an 

 erroneous assumption, for there is always a certain per 

 cent of the steam which is not steam but water in sus- 

 pension. This, of course, will make the boiler appear 

 to be generating more steam than it really is, but when 

 this wet steam comes to the engine it will be charged 

 against the engine as using all steam and consequently 

 much more than is necessary, when in fact it is not using 

 so much steam as is recorded, but is passing water 

 through the cylinder. 



511. Expansion of steam. When saturated steam is 

 used in an engine without expansion only about 8 per 

 cent of the heat expended is converted into useful work. 

 By not admitting steam into the cylinder for the full 

 length of stroke, as shown in a previous part of this 

 chapter, but by cutting it off during the first part of the 

 stroke and allowing it to expand during the remaining 



