FARM MOTORS 



same principle enters into the discussion of the draft of a 

 horse. As long as the trace is horizontal, the horse has 

 to depend upon his grip and his weight only to furnish 

 enough resistance to enable him to pull the load. But if 

 the trace be lower than horizontal the tendency is then to 

 draw the horse on to the ground and thus give him 

 greater adhesion. If the horse has sufficient adhesion to 

 pull a load without lowering the trace it is to his ad- 

 vantage because the draft is often less in this case than 

 any other. 



409. Line of least draft. When the road bed is level 

 and hard, the line of least draft to a loaded carriage is 

 nearly horizontal because the axle friction is but a small 

 part of the weight. 



FIG. 198 



Thus in Fig. 198, if AO represent by direction and mag- 

 nitude the weight upon the axle, and OB in like manner 

 the resistance of friction, the direction of the least force 

 required to produce motion will be perpendicular to AB, 

 a line joining the two forces. The angle that the line of 

 least draft makes with the horizontal is named in me- 

 chanics, the angle of repose. If the resistance of friction 

 be that of sliding friction and not that of axle friction, the 

 angle of repose will be much greater. 



