562 



ON DRAUGHT. 



the entire cone, ABC, which will roll round the point C ; if this entire 

 cone be completely severed at the point B, the two parts will still continue 



to roll round the same point, and 

 if the portion B C be now ab- 

 stracted, the motion of the re- 

 mainder will not be altered. If 

 " '^ ^ ^ a wine-glass or decanter — 



anything which is not of the 

 same size at the two parts whicli 

 are in contact with the surface 

 on which it rests, be rolled upon a table, those who are not already too 

 familiar with the fact to require an illusti^ation of it, will immediately see 

 the truth of this statement. If, then, a wheel thus formed would naturally 



quit the straight line; when compelled to 

 follow it, it is clear that exactly the same 

 effect must be produced as when a cylin- 

 drical mill-stone, as in Jig. 35, which, if 

 left to itself, would proceed in a straight 

 line, is compelled to follow a straight 

 line, and is constantly twisted round the 

 centre C, it would grind everything be- 

 neath it to powder. Yet these travel- 

 ling grindstones were in use upwards of 

 twenty years to the destruction of the 

 roads, and at a great expense of power to those who have persisted in em- 

 ploying them. 



The increased strain upon the axles, from this constant tendency of the 

 wheel to be twisted outwards, with the consequent friction, is a source of 

 resistance absorbed and rendered comparatively inconsiderable, by the far 

 greater friction on the ground ; but it is not the less a cause of great in- 

 crease of draught, and the union of all these serious disadvantages justifies, 

 we think, our assertion, that such a wheel is as injudicious a contrivance 

 as could possibly be invented. We trust they will not long continue to 

 disgrace our wheelwrights and injure our roads. 



The cylindrical form is the only one which ought to be admitted. As a 

 wheel must, however, always be hable to sink a little into the road, and 

 cannot be expected always to bear perfectly flat upon the ground, the 

 surface of the tires should be slightly curved, and the edges rounded off", as 

 in fig. 36. As the rounding is rendered necessary by the yielding of the 

 road, its degree must depend upon the state of the road, and the form of 

 Fig. 36. the wheel may approach more nearly to the true cylinder, in 

 « 6 proportion as the roads approach nearer to perfection in point 



of hardness and flatness. Wlien the roads are good, a very 

 little dishing will be sufficient, and a shght inclination of the 

 wheel from the vertical will make it correspond vntli the 

 barrel or curve of the road, which is now generally very 

 trifling". 



Next to the form, the breadth of the wheel is the point 

 requiring most consideration ; it is one, however, which 

 depends entirely upon the state of the road. 



We have seen, that the displacement or crushing of the 

 materials forming the upper surface of the road is one of the 

 principal causes of resistance. If the whole mass of the road 

 were formed of a yielding substance, into which the wheel 

 a b would sink to a depth exactly proportionate to the weight 

 bearing upon it, it is probable that great breadth would be advantageous, 



