52 POWER AND THE PLOW, 



light work, the double engine is hardly necessary. Prof. L. W. 

 Chase says, in "Farm Machinery and Farm Motors": "Al- 

 though a double engine is more easily handled than a single 

 one, there are only a few instances, such as plowing and heavy 

 traction work, where its use is recommended for farm work." 

 The single cylinder engine is the more economical of fuel and 

 has fewer parts to get out of order. The double engine with 

 cranks 90 apart can never be stopped on dead centre so as 

 to require turning over by hand. (An engine is said be be on 

 dead centre when a straight line will pass through the centres 

 of the cross-head, the crankpin, and the crankshaft, so the 

 thrust of the piston operates directly against solid metal 

 instead of turning the shaft.) At least one crank is always 

 in position to be acted upon by the connecting rod, and the 

 two cylinders working together are able to start a heavy load 

 easily and without damage to any part. The division of work 

 between two cylinders naturally gives better balance ' and 

 greater durability. 



As compared with the two-cylinder gas engine the double- 

 cylinder steam engine gives four impulses to the crankshaft 

 at every revolution of the flywheel, where the gas engine gives 

 but one. On this account the crankshaft is not exposed to 

 the same shock and vibration, hence is made much smaller. 



Tandem compound steam engine 



By way of illustration it may be said that the crankshaft on 

 a well designed two-cylinder gas tractor developing 70 b.h.p. 



