THE ANIMAL AS A MOTOR. 



63 



by the horse, or to balance a part of the weight of the 

 animal ; the latter either carrying part and hauling 

 part of the load, or hauling the load and, with it, a 

 part of its own weight, transferred, by the incHned 

 upward line of traction, to the load. The former dis- 

 position is obviously suitable for heavy loads and steep 

 gradients, the latter for light loads and level or falling 

 stretches of road. Heavy wagons should be handled in 

 such manner as to give the former, Hght carriages the 

 latter, adjustment. It would probably, in the case of the 

 heavy vehicle be well to provide, if practicable, for the 

 change of the line of pull to suit the load and gradient, 

 as has been practised by Mr. Brigg ; who finds, in some 

 cases, a loss of one half- the mechanical efficiency at- 

 tainable, due to inappropriate methods of attachment 

 of the animal to the vehicle.^ He concludes : 



" The resistance which a horse can overcome depends 

 upon the following conditions : (i) his own weight ; 

 (2) his grip ; (3) his height and length ; (4) direction of 

 trace ; (5) his muscular development, which determines 

 the power to straighten the bent lever represented by 

 his body and hind legs against the two resistances, the 

 vehicle through the trace attached to the shoulder and 

 the hind feet against the ground. 



To pull through a very low trace, or to have a man, 

 or even two or three men, on a horse's back is advisable 

 and even necessary if a horse is expected to haul a 

 load requiring the full force of his muscles at any par- 

 ticular moment — and for the moment, under such con- 

 ditions, he would be able to draw a much greater load 

 than without the added weight. But any person can 



Ibid. 



