174 



Report on Trials of Plows. 



the amount of resistance to the motive force. The diagram, 

 Fio-. 82, will make this important matter still more intelligible. 

 Let a be the point of resistance of a plow's body, b the point 

 of the beam, c the position of the horses' shoulders, and a d the 

 horizontal line; then will c a d he the angle of draught, equal 



g 



J^/^. 82. 



20 deg. Let the circle e represent a wheel placed under the 

 beam, which is supported by a stem or shears, here represented 

 by the line e h. In this position the point of the beam, which is 

 also the point of draught, lies in the line of draught; the wheel, 

 therefore, bears no load, but is simply in place, and has no effect 

 on the draught; the motive force, therefore, continues to be 407 

 pounds. 



Suppose, now, the point of the beam to be raised to a, so that 

 the line of draught, g c, may be horizontal; and since the line of 

 drauo;ht lies now out of the oris^inal line, a b c, and has assumed 

 that oi a g c, g being now supported on the produced stem, e g, 

 of the wheel, draw g i perpendicular to « c and complete the 

 parallelogram, a i g k; the side, a i, will still represent the origi- 

 nal motive force of 407 pounds, but by the change of direction 

 of the line of draught, the required force will now be represented 

 by the diagonal, a g, of the parallelogram, equal to 425 pounds, 

 and .9^ c is a continuation of this force in a horizontal direction. 

 The draft is therefore increased by 18 pounds. Complete also 

 the parallelogram, a I g m, and as the diagonal, a g — the line of 

 draught last found — is equal to 425 pounds, the side, I g, of the 

 parallelogram will represent the vertical pressure of the beam 

 upon the wheel, e, equal to about 250 pounds, which, from Mr. 

 Stephens' experiments, may be valued at 50 pounds of additional 

 resistance, making the whole resistance to the motion 475 pounds, 

 and being a total increase arising from the introduction of a wheel 

 in this position of 68 pounds. Having here derived a maxiunim — 



