310 



POWER AND THE PLOW 



(11) Tractive efficiency = Drawbar h.p. 



B.h.p. 



(12) Thermal efficiency = 2545 



B.t.u. supplied per h.p.-hr. 



(13) Roberts' formula for rating small gas engines: 

 B.h.p. DxLxRxN 

 18000 



for four-cycle engines* 



(14) and DxLxRxN 

 13600 



for two-cycle engines, 



in which D = diameter of cylinder in inches; L = length of stroke in inches; 

 R = revolutions of crankshaft per minute* N = number of cylinders. 



Draft of different numbers of 14-inch stubble plows in sandy 

 loam soil of an Illinois cornfield, based on thesis of C. A. Ocock, 

 at Urbana, 111., in 1904. Average of 90 tests at each depth, 

 30 in wet, 30 hi dry, and 30 in ideal soil conditions: 



Three-wheeled sulky plows used in all tests. For draft of 

 engine gang plows under the same condition add from 10 to 

 25 per cent. For heavier soils add from 25 to 200 per cent. 



Capacity of plowing outfits in acres per hour at different 

 speeds, for furrows of varying width and number. Formula: 

 Miles net furrow travel and width hi feet of strip plowed -5- 

 8.25 = Acres: 



