CH. X 147 



CHAPTER X 



DRAUGHT 



The amount of power, usually called the draught, 

 which is required to overcome the resistance to 

 motion of a vehicle, may be examined from two 

 points of view : first, as to the actual mechanical 

 force expended ; and, secondly, as to the way in 

 which that force should be applied by an animal. 



The resistance to movement of a sled, is simply 

 that due to the friction between the surface of the 

 ground and the under side of the runner, but when 

 wheels are used, this resistance is composed of the 

 rolling friction between the tire and the ground, and 

 the friction of the axle in its box. 



The rolling friction results from the necessity of 

 surmounting obstacles in the way of the wheel, as 

 shown in Fig. 7 3, where the 

 power to move the load is act- 

 ing in the direction CP parallel 

 to the road, and the weight of 

 the vehicle is acting vertically 

 downward through the centre 



« > 



of the wheel and the point A, 



at which the wheel touches 



the ground. In order to surmount the obstacle B, 



he power acting on the bent lever CBA must lift 



