292 



Ploughs and Ploughing. 



pins or notches or other arrangement, so that the point of 

 attachment may be brought accurately into line with the line 

 of draught. The adjustment of the bridle needs the constant 

 attention of the ploughman quite as much as the setting of 

 the cutting-irons, as any change in the plough — such as taking 

 a different size of furrow-slice, substituting small horses for 

 large ones, putting in fresh cutting-irons for worn and blunt 

 ones, &c. — necessitates the moving of the bridle-pin right or 

 left to give more or less " land," or up or down to give more 

 or less " earth." 



The mechanical principles just mentioned involved in the 

 draught of the plough can be best explained by a diagram. 

 In giving a graphic representation of the effect of the 

 application of force to a body it is customary to employ 

 the " parallelogram of forces," and this in the case of the 

 plough is illustrated in Figure 1. The plough is intended 

 to move along horizontally, but the power of the horses is 



Sq-fac. L.vel I 



f*„,r.~ B.ll.m 



Fig. 1. — Draught of the Plough. 



applied obliquely. The most convenient angle for the 

 line of draught — i.e., of the plough chains — has been found 

 to be about 20 degrees with the ground line, and at this, 

 with ordinary horses, the distance from the bridle-hook to 

 the shoulder-hook on the hames will be about 10 feet. At 

 these figures the line of draught will adjust itself correctly, 

 the horses will pull comfortably, and the plough will swim 

 through the soil with very little interference from the plough- 

 man. But the oblique application of the force means that 

 there is a loss of power — a loss which can be worked out 

 mathematically. If the line of draught from the " centre of 

 resistance," A in the diagram, to the shoulder-hook C be taken 

 as the diagonal of a rectangular parallelogram, then if the 

 parallelogram be completed the line AB will represent by its 



