ON DRAUGHT. 



445 



whereas, if this same load had been fixed firmly to the wheel, its impetus 

 would have carried the wheel over the stone, with very little loss of ve- 

 locity. 



In the first case, it would be necessary for the horses to drag the load 

 over the stone by main force ; in the latter, they would only have to make 

 up by degrees for the loss of velocity which the mass had sustained in 

 passing over the stone. The quantity of power required will indeed be 

 the same in either case ; but in the one, the horses must exert it in a sinMe 

 effort, while in the other, this momentary exertion is borrowed, as it were, 

 from the impetus of the mass in motion, and being spread over a o-reater 

 space of time, as far as the horses are concerned, only augments in a small 

 degree the average resistance. It is thus that the fly-wheel of a steam- 

 engine in a rolling-mill, accumulates power, sometimes for several minutes, 

 till it is able to roll, with apparent ease, a large mass of metal, which, with- 

 out the effect of the fly-wheel, would stop the engine immediately ; or, to 

 mention a case more to the point, in the operation of scotching a wheel, a 

 large stone, and even a brick, will render almost immoveable a wago-on, 

 which, when in motion, would pass over the same stone, without any sen- 

 sible alteration of speed. It is most essential, therefore, that the effect of 

 the momentum of the load should in no way be reduced by any longi- 

 tudinal elasticity, arising either from the injudicious application of spriufi-s, 

 or weakness in the construction of the carriage. 



The action of impetus, and the effect of an injudicious mode of hangino* 

 the load, is of course more sensible at high than at low velocities, and in a 

 carriage hung upon springs, than in a waggon without springs, but 

 although not so sensible to the eye, it nevertheless affects the draught ma- 

 terially even in the latter case. Carriages hung upon springs, as in^^. 39, 



which are called C springs, and which admit of very considerable longi- 

 tudinal movement in the body of the carriage, are notoriously the most 

 heavy to pull ; and cabriolets which are hung in this manner, are expres- 

 sively called in the stable, horse murderers, and require heavy powerfid 

 horses to drag them, while lighter animals are able to drag much greater 

 weights in Stanhopes and spring-carts, which do not admit of this elasticity. 

 This is one of the reasons why the draught of a two-wheeled cart is less 

 than that of a waggon. In a cart, the horse pulls at once on the shafts, 

 which are fixed immediately both to the load and to the axletree, so that 

 not only the impetus of the load, but also of the horse, acts directly and 

 without elasticity upon the wheel. In a waggon, owing to the smallness 

 of the front wheels, there is a considerable space between the fore-axle and 

 the floor of the waggon, which is tilled up with pieces of timber, called 



