332 AGRICULTURAL ENGINEERING 



Referring to the angle of trace for the minimum draft, ib 

 is to be recognized that there are two distinct classes of imple- 

 ments to which horse labor is applied: (1) those intended 

 primarily for moving heavy weights from place to place; 



and (2) those designed to work 

 \ the soil. In the first case the draft 



\ is due chiefly to the friction of 



\ - the machine and the rolling or 



\ sliding resistance of the surface. 



\ Thus, in Fig. 213, it is to be noted 



that a certain force, W, will lift 

 the block, A, and another force, 

 F will slide it on the surface. 



Fiff 'th 2 e 3 an g t 'S^f- The least force, however, that will 



produce motion lies between 



these two, as D, and its direction depends upon the magni- 

 tude of each of the other two forces. In mechanics the 

 angle this force makes with the horizontal is called the angle 

 of repose. If the load is to be drawn up an incline, the 

 proper angle of trace should equal the ordinary angle plus 

 the angle of the grade. With an implement like the plow, the 

 line of least draft extends almost directly to the center of the 

 place where the work is being performed. 



The Length of Hitch. Lengthening the hitch does not 

 have the effect that it is generally supposed to have. The 

 principal effects are that the horse does not have as complete 

 control over the load and that the angle of trace is changed. 

 Lengthening a horizontal trace ten or even fifty feet has 

 practically no effect upon the capacity of the horse. Men 

 are often found who think they can hold a horse at the end of 

 a 50- or 100-foot rope. A trial is very convincing that they 

 cannot do so. 



