ON DRAUGHT. 



435 



aboui the proportion in a large two-wheeled cart, the wliole resistance 

 arising from friction at the axle will be equalled to i- of y'g , or of ^V? which 

 is equal to -jIj and yi^ respectively. So tiiat to move one ton wjuld not, 

 in the latter case, require a force of traction greater than 181 \bs.; and 

 liaving overcome this resistance, the force of traction required remains 

 nearly the same at all velocities ; that is to say, friction is not materially 

 affected by velocity: therefore the resistance arising from it is not sensibly 

 augmented by a considerable increase in the speed. In practice, however, 

 the friction at the axle is far from being the greatest impediment to the 

 motion of a carriage. "We have hitherto, for the purpose of considering 

 friction alone, supposed the surface upon which the wheel moved as per- 

 fectly hard, smooth, level, and plane: we need hardly say that such can 

 never be the case in a road. The friction, however, remains, praclicallv 

 speaking, the same, and the laws which govern the amount and the effects 

 of it remain unaltered; and we have only to ascertain what is the addi- 

 tional resistance, arising from other sources, to obtain the whole draught of 

 the carriage. We have already stated, when pointing out the ditTerence 

 between the roller and the wheel, that the movement of the latter was 

 attended with two sources of resistance, viz : friction at the centre, which we 

 have considered, and another, which is common both to the wheel and the 

 roller, arising from impediments in the road, or the yielding of the materials. 



The laws which affect the amount of this latter are, of course, the same 

 in a wheel as in a roller. 



We have found that the power required to overcome it is inversely as the 

 square root of the diameter ; therefore, by increasing the diameter of tlie 

 wheel, the effect of friction, which is inversely as the diameter, diminishes 

 much more rapidly than that caused by impediments in the roads; and on 

 ordinary roads, with common carts, the amount of the latter is about three 

 times as great as that of the former, and when the I'oads are at all injured 

 by weather or by neglect, or if they are naturally heavy or sandy, it bears 

 a much greater proportion. A light four-wheeled cart, weighing, with its 

 load, 1000 lbs.* was repeatedly drawn upon different sorts of roads, the 

 average of a number of experiments gave the following results : 



Description of Road. Fo'-ce of Traction required 



' to move the carnage. 



Turnpike road — hard, dry, 301 lbs. 



Ditto dirty, 39 



Hard, compact loam, . 53 



Ordinary by-road, 106 



Turnpike road — new gravelled, . . . . 143 



Loose, sandy road, 204 



The friction at the axles, which were of wood, was, of course, nearly 

 constant, and probably absorbed at least j\ of the weight, or 12i lbs. of the 

 force of traction, leaving, therefore, for the resistance caused by the road in 

 •he different cases, as under: 



Descrintion of Road Force of Traction required to move the 



" ■ Carriage, independent of tlie Friction at tlie Axles. 



Turnpike road — hard, dry, about ... 18 lbs. 



Ditto dirty, 26-1 



Ditto new gravelled, .... 1301 



Loose, sandy road, 19l| 



* The experiment was not made with a load of exactly 1000 lbs., but the proportions of 

 the results are calculated to this standard. The public are indebted to Mr. Bevan for 

 these as well as a g-reat number of other hig-hly useful and practical experiments upon 

 the effects of power in various cases. 



