f,6fi ON DRAUGHT. 



the draught ; that provided a weight to be moved were placed upon the 

 wheels, and the wheels put in motion, that nothing more could be required. 

 Upon a perfectly level smooth plane, and with a constant force of trac- 

 tion, this would, indeed, be the case ; but, in practice, the conditions are 

 entirely altered. Impediments are continually met with, which obstruct 

 the progress of the wheels, and the draught is constantly varied by the 

 different inclinations of the road ; it is, therefore, necessary to study the 

 means by which impediments can be easiest overcome, and by which the 

 resistance thus caused will affect the animal, which is the source of power, 

 in the least disadvantageous manner. 



We have already stated that impetus is necessary to overcome an 

 obstniction, and that elasticity in the direction of the movement is destruc- 

 tive of the fall effect of impetus. 



When, therefore, the wheel of a carriage comes in contact with any 

 impediment, it is most essential that the whole of the impetus or momen- 

 tum which the carriage has ah-eady obtained should be brought into full 

 action, to force the wheel forward. To effect this, no elasticity should 

 intervene between the wheel and the load, at least in the direction of the 

 motion, that is, longitudinally ; otherwise, as we instanced in the case of 

 catching a cricket-ball, a force which would be quite irresistible if opposed 

 by a rigid resistance, is checked with ease by a very Httle degree of elas- 

 ticity ; so with a wheel meeting a small stone, if the load were so placed, 

 or hung upon the wheels, as to allow free or elastic action, longitudinally, 

 that is, in the direction of the movement, the Avheel being stopped against 

 the stone, the whole load would be gradually checked, and brought to a 

 full stop ; 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 velocity. 



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 total quantity 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 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, without the effect of the fly-wheel, would stop the engine imme- 

 diately ; 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 im- 

 moveable a waggon which, when in motion, would pass over the same stone 

 without any sensible 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 longitudinal elasticity, arising either from the injudicious appli- 

 cation 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 

 fig. 37, which are called C springs, and which admit of veiy consider- 

 able longitudinal movement in the body of the carriage, are notoriously 

 the most heavy to pull ; and cabiiolets, which are hung in this manner, 

 are expressively called, in the stable, horse-murderers, and require heavy 



