208 POWER AND THE PLOW 



right or left of the centre, an opposed force must be exerted 

 to make the plow run evenly. 



The true line of draft is always from this centre to the 

 point where power is applied; hence any enforced angle, such 

 as caused by sagging of the traces, holding up the traces by 

 straps, extending or shortening the beam, and raising or lower- 

 ing the draft pin in the clevis, disturbs the adjustment. The 

 determination of the centre of resistance is, therefore, impor- 

 tant. 



Since the greatest effort is expended by the shin and share in 

 severing the furrow slice from the land, the centre must lie 

 much nearer the landside than the furrow side of the plow. 

 Again, since the resistance of the cutting edge of the share 

 and the sole are greater than that of the moldboard, the centre 

 must lie nearer the sole than to the crest of the moldboard. 

 A third plane of resistance stand perpendicular, at right angles 

 to the landside. Cross sections of the plow in planes at differ- 

 ent distances from the point show triangular areas increasing 

 more rapidly in size than the cutting edges do in length. The 

 force required to drive the plow into the ground does not 

 increase in proportion to the area cut, and the centre of resis- 

 tance will lie in a smaller cross section than that of the whole 

 furrow. In other words, it will lie much nearer the point than 

 the heel of the share, being farther forward in stiff soils, in 

 shallow plowing, and on a rather blunt plow point, than under 

 reverse conditions. The centre lies at the intersection of the 

 three planes, and its approximate location must be known in 

 order to adjust the point of hitch so that it will fall naturally 

 within the line of draft. On stubble and general purpose plows 

 it will lie close to the shin and to the junction of share and 

 moldboard. On plows designed for breaking tough sod at a 

 shallow depth it will lie closer to the point and the sole. Pro- 

 fessor Gilmore, of Cornell University, states that it is located 

 behind the moldboard and two and one half to three inches from 

 the wall and sole of the furrow. No other condition within the 



