SCIENCE OF THRESHING. 119 



watching the fire bed and allowing no places on the 

 grate to be bare of burning coal and no part so 

 loaded as to prevent air from passing up through the 

 grate freely, a fierce and hot fire can be maintained 

 with a minimum expenditure of coal. 



The ashes below the grates should be kept cleared 

 away and the spaces between the grates open to allow 

 free passage of the air. 



Oftentimes the water used deposits lime or like 

 substances on the inside of the shell. These cake 

 over the heating surface, and as they are not ready 

 conductors of heat, the boiler will require more fuel 

 than it should in order to generate steam enough. 

 Consequently, if the engineer finds that an engine is 

 not supplying power enough, he should first find If 

 the boiler and its smoke passages are in proper order. 

 In the narrow spaces between the side plates of the 

 fire-box and outer shell, scale sometimes accumulates 

 to such an extent as to completely fill the interval 

 between the plates. The energy of the molecules of 

 the burning fuel is largely expended in heating this 

 scale and as a consequence the water is not turned to 

 steam rapidly enough. 



In order to keep the parts in direct contact with the 

 fire from burning out or leaking, the water must be 

 kept in the boiler at a level which will cover them. 

 The engineer is aided in this by the familiar water 

 glass, also by a row of gauge cocks placed one above 

 the other. By opening these the depth of the water 

 can be readily determined. 



In a traction engine, such as used with a threshing 



