ON DRAUGHT. 



439 



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

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

 less gTound than the development of its circumference, the latter nine 

 inches more — the one pushing back the ground, the other dragging it 

 forward. 



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

 than the hind wheel. If, then, the front wheel were obHged 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, Jig. 34, is only a portion of the 



entire cone, ABC, which will 

 Fiff. 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, anything which 

 is not of the same size at the two parts which are in contact with the sur- 

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

 familiar with the fact to require an illustration of it, will immediately see 

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



quit the straight line ; when com- 

 pelled to follow it, it is clear that 

 exactly the same effect must be pro- 

 duced as when a cylindrical mill- 

 stone, as in Jig. 35, which would 

 proceed in a straight line, is com- 

 pelled to follow a curved line, and 

 is constantly twisted round the 

 centre C, it would grind everything 

 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 

 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. 



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 parties interested in 



Fig. 35. 



