652 ON DRAUGHT. 



dragging of the load with the wheels locked a distance of four and a half 

 inches upon every fourteen. 



Every child knows that the front wheel of a carriage goes oftener round than 

 the hind wheel. If, then, the front wheel were obliged to make only one revo- 

 lution to every revolution of the other, but still impelled at the same rate, it 

 must be partly dragged over the road. If these wheels be placed side by side, 

 instead of one being in front of the other, the effect must be the same. Now, 

 suppose them to be the outer and inner tire of the same wheel, the circumstances 

 are not thereby altered : the smaller circle and the larger circle cannot both 

 roll upon the ground. A conical wheel is then constantly twisting the surface 

 upon which it rests, and hence arises a very considerable resistance, as well as 

 destruction to the roads. 



If these arguments are not sufficient to decide the point completely, let the 

 reader bear in mind simply, that a cone, when left to itself, will always roll in 

 a circle. The frustum of a cone, AB,fiy. 34, is only a portion of the entire 



cone, ABC, which will roll round 



Piff. 34. the point C ; if this entire cone 



be completely severed at the 

 point B, the two parts will still 



c ^ A continue to roll round the same 



point, and if the portion BC be 

 now abstracted, the motion of 

 the remainder will not be altered. 

 If a wine-glass or decanter anything which is not of the same size at the two 

 parts which 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 illus- 

 tration 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 cylindrical 

 mill-stone, as in fig. 35, which, if left to 

 itself, would proceed hi a straight line, is 

 compelled to follow a curved line, and is 

 constantly twisted round the centre C, 

 it would grind everything beneath it to 

 powder. Yet these travelling grind- 

 stones 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 employing 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 increase 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 liable 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 the wheel may approach 



