ON DRAUGHT. 



437 



In ficr 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, gives 

 the wheel a very great degree of stiffness and strength, which it would not 



^^ In'^on^sra^'uence 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 



Ficf. 30. 



axle bent downwards, as \\\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 neverthe- 

 less the form of this surface, 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 



""^ Abroad, 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 ot 

 loose stuff, more or less deep, according to the material used, and the tre- 

 quency of repair or the quantity of wet to which it may be exposed. 1 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 oi 

 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 orig'in in the bad state 

 of the roads. A certain width of tire proportionate to the load was there- 

 fore required by law. The endeavour to evade this law was the cause ot 

 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/g-. 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 great width of floor to the carriage, still keep- 

 ing the vehicle in the common tracks or ruts, so that the wheels ultimately 

 assumed the form represented, ^g". 32. 



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

 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 



-Ry. 3 



