ON DRAUGH1 



445 



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

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

 velocity. 



In the fi.rst case, it would be necessary for the horses to drag the load 

 over the stone by main force; in the latter, they v/ould only have to make 

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

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

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

 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 greater 

 degree the average resistance. It is thus that the fiy-wheel of a steanj- 

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

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

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

 out the effect of the fly-wlieel, 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 waggon, 

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

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

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

 nal elasticity, arising either from the injudicious application of springs, or 

 weakness in the construction of the carriage. 



The action of impetus, and the effect of an injudicious mode of hanging 

 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 

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

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



gitudinal 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 tiie stable, horse murderers, and require heavy, powerful 

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

 weiirhts 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 tlie shafts 

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

 Mot 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 stnallness 

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

 .'tie floor of the waggon, which is filled up with pieces of timber, called 



