ON DRAUGHT. 



559 



than that on which the last experiment was tried, various attempts were 

 made to reduce the resistance, by using narrow wheels. These attempts, 

 and the laws which it was found necessary to enact to prevent the entire 

 destruction of the roads, led, at last, to curious results, having- gradually 

 caused the introduction of the worst-formed wheel which could probably 

 be invented, either as regards increasing the draught or the destruction of 

 the roads. 



To understand these alterations clearly, we must describe the principal 

 features of the wheel now in use. 



The general construction of it presents a striking instance of strength 

 arising from the judicious union of substances of very different qualities — • 

 wood and iron. 



A strong circular frame of wood, composed of different segments, called 

 felloes, is bound together by a hoop, or several hoops of iron, called tires, 

 which thus, at the same time that it gives great strength, protects the 

 outer surface from wear. 



The nave, a circular block of wood, is sustained in the centre of this 

 frame by the spokes, which, instead of being in the plane of the felloes, 

 form a cone ; this is called the dishing of the wheel. The object of it is to 

 give stiffiiess, to resist lateral shocks, as when the wheel slips sideways, 

 into a rut or hole. A reference to a comparative view of the wheel, -with 

 and -without dishing, will more clearly explain our meaning. Fig. 28 is 

 a wheel with the spokes all in one plane ; Jig, 29 a wheel with a 

 considerable degree of dishino-. 



Fig. 28. 



F/c/. 29. 



Fig. 30 



Here it is e\'ident that a small pressure on the nave in fig. 28 would have 

 a tendency to push it through, and Avould meet with but little resistance. 

 Iwfiq. 29, on the contrary, this force would be op- 

 posed at once by the direction of the spokes, which 

 form an arch, or dome, that cannot be flattened 

 without bursting the felloes, or tires. The dishing, 

 therefore, gives the wheel a very great degree of 

 stiffiiess and strength, which it would not otherwise 

 possess. 



In consequence of this conical form, the necessity 

 of keeping the loAver spokes which support the weight 

 as vertical as possible, has required that the whole 

 wheel should be placed oblique, and the axle bent 

 downwards, as in fig. 30 ; this, as we shall hereafter 

 show, is attended with very serious evils. As a 

 wheel is intended to roll upon the ground, without friction, it is natural to 



