78 MECHANISM OF DRAUGHT. 



but as a rule it is so distributed as to cause the back-band to exert on 

 the " girth-place " an upward pressure, which, naturally, will more or 

 less aid in " lightening " the fore -hand. 



5. The force of propulsion given by a hind limb is dependent on a 

 series of levers which extend from the toe of the hind foot, along the 

 bones of the hind leg, pelvis, spinal column, up to the centre of pressure 

 on the inner side of the collar. It is manifest that the flatter this 

 irregular line of levers is with reference to the line of propulsion from 

 the toe of the hind foot to the collar, the greater will be the mechanical 

 advantage at which the hind limb will act. Consequently, we may infer 

 that the cart-horse should be long in the body, as compared to his 

 height ; that he should be lower over the croup than he is at the withers ; 

 and that he should not have a horizontal croup. We should not, how- 

 ever, desire his hind-quarters to be so dropping as to cause him to be 

 actually " goose-rumped " (Fig. 333). 



6. It is a well-known mathematical fact that the most advantageous 

 direction for a purely mechanical pull, such as that of a rope which is 

 attached to a traction engine in movement, is one which makes an angle 

 with the ground equal to the angle of friction. I may explain that if, 

 for instance, 10° was the greatest slope of ground upon which a body 

 could rest without sliding down, its angle of friction would be equal to 

 10°. From this law of mechanical traction, one is apt to wrongly 

 infer that the best direction of the traces would be at an angle to the 

 ground equal to the angle of friction. As Major Dwyer (Seats and 

 Saddles) justly points out, a more useful effect " may be attained with 

 traces which are so disposed as to enable the horse to e.xert his entire 

 strength under a theoretically less favourable angle of traction than 

 when the ease of the animal is sacrificed to a correct, but, in such a 

 case, inapplicable mathematical principle. In a word, our contention 

 is, that the angle of traction must be regulated with reference to the 

 horse, and not to the carriage exclusively." Major Griffiths [The 

 Artillerist's Manual, pubhshed many years ago) remarked that the best 

 disposition of the traces in draught is when they are perpendicular to 

 the collar, which very useful suggestion Major Dwyer greatly improved 

 by saying that they should be perpendicular to the horse's shoulder 

 blade. By accepting this thoroughly sound statement, and by bearing 

 in mind the mechanical law of traction to which I have referred, we 

 arrive at the important conclusion that the greater the angle of friction 

 of the ground, the more sloping should the shoulders of cart-horses 

 be. Hence, for work on arable land, we require a horse with more 

 sloping shoulders than on smooth roads. 



