112 SCIENCE OF SUCCESSFUL THRESHING 



of the soap used as a lubricant on the taps in manufacture, 

 new boilers are liable to foam until they are washed out 

 two or three times. It is difficult to tell exactly how much 

 water there is in a foaming boiler, but it is probable that 

 some of it is being drawn over with the steam, and there- 

 fore, the pump should feed more than the usual amount. 

 Do not run too long with a foaming boiler, but close the 

 throttle occasionally to see how full the boiler is when 

 the water settles. The remedy for foaming is to keep the 

 boiler clean and to use clean water. Foaming often causes 

 priming. Foaming and priming are more apt to occur 

 with low than with high steam pressure. 



Priming. When water is drawn over into the cylinder 

 with the steam, the engine is said to "prime." A priming 

 engine appears to be working very hard, exhausting heav- 

 ily, throwing water from the stack and often making a 

 loud knocking or pounding noise in the cylinder. Prim- 

 ing may be caused by: I. Too high a level of water in 

 the boiler. 2. Too low steam pressure. 3. Engine work- 

 ing hard with the front of the boiler low. 4. Boiler work- 

 ing beyond its capacity. 5. Foaming. 6. Piston rings or 

 valve leaking. 7. Valve improperly set. 



In case the engine should begin to prime, the cylinder 

 cocks should be opened and the throttle partially closed, so 

 that the engine runs quite slowly, until dry steam comes 

 from the cylinder cocks. Priming is liable to knock out a 

 cylinder head, break the piston-head or cross-head, or do 



