ON DRAUGHT. 435 



about the proportion in a large two-wheeled cart, the whole rcsistanco 

 arising from friction at the axl'^ will be equalled to i of Jj, or of 3"^) wiiicl: 

 is equal to yig and j^^ respectively. So that to move one ton wjuld not, 

 in the latter case, require a force of traction greater than 181 lbs.; ana 

 having 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 thp purpose of considering 

 friction alone, supposed the surface upon which the wlieel 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, practically 

 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 difference 

 between the roller and the v/heel, 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 the 

 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 foriner, and when the roads 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 ditferent sorts of roads, the 

 average of a number of experiments gave the following results: 



-- ... r T, A Force of Traction required 



Descnption of Road. ^^ ^^^.^ 1,^^ carriage. 



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 -g\ 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: 



„ ... „„ , Force of Traction required to rr.ovf tlie 



Description of Road. Carriage, independent of tlie Friction at tlie Axles. 



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



Ditto dirty, ^ 261 



Ditto new gravelled, .... 130^ 



Loose, sandy road, 191 J 



* The experiment was not marie with a load of exactly 1000 ll>s., l)ut the proportic^ns ol 

 the results are calculated to this standard. The public are iiidcbtod to Mr. Hevan foi 

 those as well as a ijreat number of other highly useful and practical experinfienta upon 

 the effects of power in various cases. 



