50 POWER AND THE PLOW 



Otherwise the sudden conversion into steam of the thin layer 

 of water above the crown sheet might result in an explosion. 

 The crown sheet must be kept covered under all circumstances. 

 The difficulty of doing this on a descending grade has led to 

 the adoption of special devices in some boilers to maintain 

 water at a higher level over the crown sheet at such times 

 than in the rest of the boiler. 



The boiler requires frequent cleaning unless very pure, soft 

 water is used. This is especially true in the alkali districts 

 of the West, where boilers become incrusted with a heavy 

 scale in a week's time. Th : s scale prevents the rapid conduc- 

 tion of heat to the water. It also increases the danger of over- 

 heating the metal, and it is thought that boiler explosions are 

 frequently caused when a large mass of scale drops away from 

 some overheated part, and a large quantity of steam is sud- 

 denly produced. Convenient hand holes are placed at various 

 points around the boiler and fire box, especially in the lower 

 portion of the water legs, as the sediment naturally drops to 

 the lowest points. In order to facilitate frequent cleaning 

 of the water legs, blow-off cocks are placed so that the sedi- 

 ment may be blown out by steam. A large door on the front 

 end of the smoke box enables the operator to get at the flues 

 for cleaning them of soot and cinders. A spark arrester in 

 the top of the stack, or else a sharp angle in the smoke box 

 through which the smoke must pass, will collect the cinders. 

 Some such provision is necessary for safety to surrounding 

 objects. 



Boiler Power 



Boilers are rated as to horse power according to the amount 

 of water which they will evaporate. The standard of the 

 American Society of Mechanical Engineers requires the evapo- 

 ration on one hour of thirty pounds of water from 100 F. 

 under a pressure of seventy pounds, which is considered 

 equivalent to evaporating thirty-four and one half from and 



