DYNAMICS 



149 



waggons commonly used are made with a detachable box, and 

 the barns are fitted with pulleys by means of which it can be 

 raised by horse-power in order to save the labour of pitching 

 up the sheaves when the grain is brought in. This kind of 

 arrangement is roughly indicated in the diagram. If we 



disregard for the moment 

 questions of leverage and 

 the friction of the pulleys, 

 it is clear that the tension 

 of the rope, and therefore 

 the pressure on the horse's 

 collar, is equal to the 

 weight of the box and 

 contents. This pressure, 

 called the draught, is ex- 

 pressed in pounds weight. 

 If the animal were con- 

 tinuously engaged in 

 labour of this kind for 

 some time, it would be 



easy to apply the formula 

 given above. 



FIG. 10. 



When the work is not done against gravity, but consists in 

 hauling a cart along the level, the draught is much less in pro- 

 portion to the mass of substance moved. In the case of a 

 properly lubricated tramway car on a well-laid, level track it is 

 very small. It arises chiefly from the friction of the wheels, 

 which increases with the load, and the resistance of the air. 

 In the case of an ordinary cart or waggon on a rough road, or 



