FARM MOTORS 379 



tight by calking or battering the edges of the seams down 

 with a special tool designed for the purpose. The flat plates of 

 the fire box are supported by bolts or studs running from one 

 plate to the other. These are called stay bolts, except those 

 over the crown sheet, which are called crown bolts. The 

 boiler is usually provided with a valve at the lowest point, 

 which may be opened to allow any sediment in the boiler to 

 be blown out. 



In the management of the locomotive type of boiler, great 

 care should be taken to keep the water over the crown sheet 

 at all times. 



Return-Flue Boilers. The return-flue boiler has a large 

 cylindrical shell in which a comparatively large flue is placed, 

 large enough to contain 

 the furnace. The heated 

 gases pass to the front end 

 and then back through 

 tubes to the smoke box in 

 the back end. One ob- 

 jection to this type of boiler 

 is the limited amount of 



grate Surface Which Can Fi &. 242 - A sectional view of a return- 

 flue boiler. 



be provided. This type, 



however, is regarded as one of the safest, and is very eco- 

 nomical in the consumption of fuel. 



Capacity of Boilers. The capacity of a boiler is usually 

 designated in horsepower. Formerly this meant the capacity 

 to supply enough steam for an engine of the designated 

 horsepower. Now boiler horsepower means the capacity 

 to absorb a certain amount of heat in a given time. The 

 standard horsepower as established in this country is the 

 capacity to evaporate 30 Ibs. of water per hour into steam at 



