112 SCIENCE OF THRESHING. 



For anthracite coal the spaces should be from three- 

 eighths of an inch to half an inch wide; while for 

 coals that cake much, the width of the spaces may be 

 from five-eighths to three- fourths inches. For wood 

 or straw the spaces must be still wider. The bars are 

 about three-fourths inches wide at the top and taper 

 toward the bottom. 



SAFETY VALVE. 



A safety valve is attached to a boiler to prevent the 

 steam from rising above working pressure. When 

 steam is generated more rapidly than it is used, its 

 pressure must of necessity rise, and if no means of 

 escape are afforded it, the result must be an explosion. 



The ordinary form of valve used on traction 

 engines merely comprises a circular plug or closure 

 held to its seat against the outward thrust of the 

 steam pressure by a suitably disposed spring. There 

 are many different forms, but they all amount to the 

 same thing, differing only in their minor details. 



The area of the valve is the area of the opening in 

 the valve seat on which the spring pressed valve plug 

 rests, or is the projected area of the valve plug sur- 

 face which is in direct contact with the steam when 

 the valve is closed. The area should be large enough 

 to discharge the steam as fast as the boiler will 

 generate it, as otherwise the pressure will rise even 

 when the valve is open. The following rule is a safe 

 one to follow in computing the area necessary for 

 the boiler: 



Divide one-half of the number of pounds of steam 

 generated per hour by the blow-off pressure increased 



