656 ON DRAUGHT. 



It is impossible to decide generally upon the comparative merits of the 

 different arrangements, because the result depends entirely upon the circum- 

 stances of the case. 



We may, however, endeavour to unite in some degree the advantages claimed 

 by both. The draught of a cart is less than that of a waggon for several reasons: 

 amongst others, because the wheels are larger and the horse produces more 

 effect, because his force is applied immediately to the resistance. A light waggon 

 with large front wheels would not be much inferior in point of draught to the 

 cart, and two horses abreast in double shafts would work with equal advantage 

 to the single horse ; while an additional horse may always be applied when an 

 excessive load or the state of the roads should require it. 



All that we have said with respect to the size and contrivance of wheels is 

 equally applicable to light carriages as to heavy, and we shall now proceed to 

 consider the different modes of placing the loads upon the wheels. 



It might appear at first sight that this would not affect the amount of 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 traction, this would, 

 indeed, be the case ; but, in practice, the conditions are entirely altered. Im- 

 pediments 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 obstruction, 

 and that elasticity in the direction of the movement is destructive of the full 

 effect of impetus. 



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

 ment, it is most essential that the whole of the impetus or momentum which 

 the carriage has already 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 little degree of elasticity ; 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 wheel 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 immediately ; or, to mention a case more to the point, in the opera- 

 tion of scotching a wheel, a large stone, and even a brick, will render almost 



