ON DRAUGHT. 



437 



Firj. 30, 



In jig. 30, 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 flat- 

 tened without bursting the felloes, or tires. The dishing, therefore, givt'si 

 the wheel a great degree of stiffness and strength, which it would noi 

 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 m jg. .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 never- 

 theless the form of this surfoce, 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 it may be 

 proper we should 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 very 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 Jig. 31,) 

 that it in fact constituted the wheel, the others 

 ''•^ ^ ^ ^ ^ being merely to give a nominal, and not a real 



width. The enormous loads which it was found 

 advantageous to place on these wheels rendered 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 

 creat 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. Jig. 32. 



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

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

 or drag, to the waggon, it might, indeed, iiave been judicious, but as 



