240 



DRAUGHT AND HARNESS. 



that with the larger wheel the most advantageous line 

 of traction Q lies nearer to the horizontal line O H 

 than does the line of traction O' Q' of the smaller 

 wheel, and it is agreed on all hands that the line of 

 traction of a horse attached to a wheeled vehicle must 

 be somewhere near the horizontal line O H, so that 

 this shows that hindrances of a given height are more 

 easily overcome by a larger wheel than by a smaller 



Fig. 22. 



one, and that in exact proportion to their relative 

 heights. Nevertheless there must be always an advantage 

 to the motive power when it is exercised somewhere between 

 O H and Q ; and as H can, supposing an obstacle 

 to exist, never be at right-angles to O M, the advantage 

 with a wheel of a given height will always be greater 

 when the line of traction lies nearer Q than to H 

 that is to say, when the traction takes place under an 

 angle with the horizon. 



