ON DRAUGHT. 



5G1 



We shall repeat the principal arguments which were made use of at the 



time of the enquiry mentioned. i n , i +v,o+ 



Mr. Cummins took great pains, by constructing models, to show that 



conical wheels were not adapted for roll- 

 ing in a straight line, by making a small 



conical wheel run over longitudinal bars, 



as in fig. 33. It was seen that if the 



middle part of the tire rolled upon the 



centre bar without moving it, the bar A 



was pushed backwards, while the bar C 



was pushed forwards ; clearly showmg 



if, instead of sliding bars, the wheel had 



moved upon a road, how much it must 



have ground the road, and what a small 

 portion of the tire was truly rolling. 



That such must have been the case is, 

 indeed, easily proved without a model. 

 We will take only three different parts of 

 the wheel, and consider them as indepen- n i x^ „^ +1,^ 



dent hoops of different diameter; if these hoops are compelled to go the 

 same nuinber of revolutions, the large one will evidently gam upon the 

 second, while the third will be left far behind Now, if mstead of being 

 independent of each other, they be fixed to the same axle, and compelled 

 to revolve together, the large one not bemg able to advance faster than the 

 other, must fear u^ the ground. The smaller one, on the con rary being 

 drao-ged forward faster than it would naturally roll, must drag up the 

 oTound ; and this is what must take place, and does mth any but a cylin- 

 drical wheel, and that to a very considerable extent. 



Suppose, ibr instance, a conical wheel, of an average diameter of four 

 feet six inches; that is to say, that the centre advances about fourteen 

 f^et to every r;volution of the wheel. If the inner tire be s- .nches 

 larger in diameter than the outer tire, the circumlerence of jf ;^^1 ^e 

 about eighteen inches greater; therefore, at each revolution f ^^^^ feel 

 the inner tire would naturally advance eighteen inches more than the 

 outer tire : but they are compelled to go over the same distance of ground 

 The one or the other, therefore, must have disturbed the ground, or, what 

 is nearer the truth, upon every fourteen feet of road run the former l^s 



passed over nine inches less ground than the d^Y^^^P?^^^* °^/J^!^^^^^ .'X 

 ference, the latter nine inches more_the one pushing back the grotmd, the 

 other dragging it forward, or, which would be equivalent to the dragging 

 of the load w^ith the wheel locked-a distance of four and a halt inches 



''^ EveTy diikl'knowa that the front wheel of a carriage goes oftener round 

 than the hind Avheel. If, then, the front wheel were obliged to ^ake on ly 

 one revolution to every revolution of the other, but «t^ll/^P^"f^^t p 

 same rate, it must be partly dragged over the road. If these -heels be 

 placed side by side, instead of one being m fr-ont of the other, the effects 

 must be the same. Now, suppose them to be the outer and inner tire of 

 same wheel, the circumstances are not thereby altered : the smaller circle 

 and the larger circle cannot both roll upon the ground. A conical wd.eel 

 is then constantly twisting the surface upon winch it rests, and hence 

 arises a very considerable resistance, as well as destruction to the 



'"'' If'these arg-uments 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, A B. fig. 34, is only a portion of 



O 



