CREAMERY BUTTER MAKING , 171 



steam directly, and (2) those run by the engine. The 

 latter is the more economical and handles hot water with 

 less trouble. It has one disadvantage, however, and tha't 

 is it does not work unless the engine is running. With 

 good pumps, especially those run by the engine, good 

 work may be expected when the feed water has been 

 heated to 200 F. with the exhaust steam from the engine. 

 With the injector such high temperatures are not per- 

 missible, hence the greater economy of the pump. The 

 great saving of fuel* by feeding water hot into the boiler 

 is illustrated by experiments made by Jacobus which 

 show that with a direct 'acting pump 12.1% fuel is saved 

 by heating the feed water from 60 to 200 before pump- 

 ing it into the boiler. With injectors the feed water used 

 usually has a temperature of about 60 F. 



STEAM, 



Water is practically a non-conductor of heat. This 

 means that it cannot conduct its heat to its neighboring 

 particles. When, therefore, heat is applied to the bottom 

 of a vessel containing water, the particles at the bottom 

 do not communicate their heat to the particles next above 

 them, but expand and rise, cool ones taking their places. 

 This gives rise to convection currents which tend to equal- 

 ize the temperature of the water in the vessel. When the 

 water has reached a uniform temperature of 212 F. the 

 particles begin to ily off at the 'surface in the form of 

 vapor, and this we call steam. To generate steam in a 

 boiler, then, it is necessary to impart to the water in it 

 a considerable amount of heat, which is produced by 

 burning fuel in the fire box. 



