ON DRAUGHT. 



439 



therefore, must nave disturbed the ground, or, what is nearer the truth, 

 upon every fifteen feet run of road, the former has passed over nine inches 

 less ground than the development of its circumference ; the latter, nine 

 inches more — the one continually pushing back the ground, the other drag^ 

 ging it forward. 



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

 than the hind wheel. If, then, the front wheel were obliged to make only 

 one revolution 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 frustrum of a cone, AB,fg. JM, is only a portion of the 



entire cone, ABC, which will 

 Fii/. 34. 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 BC be now abstracted, 

 the motion of the remainder will 

 not be altered. If a wine-glass or decanter, any thing 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 illustrstion 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 tlie same ef- 



Ij I /^^^^^ahl feet must be produced as when a cylin- 



[| \j f^Bm drical mill-stone, as in fg. 35, which 



would proceed in a straight line, is 

 compelled to follow a curved line, and 

 is constantly twisted round the centre 

 C, it would grind every thinff beneath 

 it to powder. Yet these travelling 

 grindstones have been 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 

 u^eel to be twisted outwards, with the consequent friction, is a .source of 

 resistance absorbed and rendered comparatively inconsiderable, by the fl\r 

 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 justi- 

 fies, we think, our assertion, that such a wheel is as injudicious a contriv- 

 ance as could po.ssibly be invented. We trust they will not long continue 

 to di.sgrace our wheelwrights, and injure our roads. 



We hope that none of our readers will consider that we have wasted our 

 arguments upon a point too self-evident to require proof. In reply to this, 

 however, we will state that, at the last meeting of the parlies interested ir 



Fi^.35. 



