550 



ON DRAUGHT. 



Fig. 30 



Here it is evident that a small pressure on the nave in fig. 28, would have a 

 tendency to push it through, and would meet with but little resistance, \afiy. 

 29, on the contrary, this force would be opposed 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 stiffness and strength, which it would not otherwise possess. 



In consequence of this conical form, the necessity of keeping the lower 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 suppose that the outer 

 surface of the tires should be cylindrical, as it is the 

 only form which admits of the wheel rolling freely in a 

 straight line ; but it is nevertheless the form of this sur- 

 face, its breadth, and the degree of dishing which have 

 varied so much from the causes before mentioned, viz., 

 the state of the roads, and to the consideration of which 

 we will now return. 

 A road, however much neglected and out of repair, will generally have, at a 

 certain depth, a hard bottom ; above this will be a coat of mud of loose stuff, 

 more or less deep, according to the material used, and the frequency of repair 

 or the quantity of wet to which it may be exposed. It is sinking through this, 

 until it reaches the hard bottom, that causes the resistance to the progress of 

 the wheel : whether the wheel be wide or narrow, it must squeeze or grind its 

 way to the bottom of this mud ; a narrow wheel evidently displaces less, and 

 therefore offers less resistance. The great object of carriers, then, was very 

 naturally to place as great a load as they could upon wheels which were as 

 narrow as possible, consistent with the necessary strength. 



It was soon perceived that the entire destruction of the roads would be the 

 consequence of this system, which had its origin in the bad state of the roads. 

 A certain width of tire proportionate to the load was therefore required by law. 

 The endeavour to evade this law was the cause of the absurd form of wheel 

 we are about to describe and to condemn. 

 In apparent obedience to the law, the felloes of the wheels were made of an 

 excessive breadth ; but to retain the advantages 

 of the narrow wheel, the middle tire was made 

 to project so far beyond the others, (see fig. 31,) 

 that it in fact constituted the wheel, the others 

 being added merely to give a nominal, and not a 

 real width. The enormous loads which it was 

 found advantageous to place on these wheels ren- 

 dered it necessary to give them a considerable 

 degree of dishing, to resist lateral shocks, and, 

 besides, the carriers were by this means enabled 

 to give a great width of floor to the carriage, 

 still keeping the vehicle in the common tracks or ruts, so that the wheels 

 ultimately assumed the form represented, fig. 32. 



If such a machine had been constructed for the express purpose of grinding 

 the materials of the road to powder, or of serving as a check or drag to the 

 waggon, it might, indeed, have been judicious, but as a wheel it was mon- 

 strous. Yet this is the form of wheel upon which the contradictory opinions 

 referred to in the first page of this treatise were given before a Committee, of 



Fig. 31. 



