444 ON DRAUGHT. 



being" upon each pair of wheels, the whole force of the teann is applied suc- 

 cessively to each half of the load, consequently in any bad road the power 

 occasionally required is less, although the draught of the carriage, pro- 

 perly speaking, is greater than that of a two-wheeled cart. These various 

 arguments would appear to lead to the conclusion, that upon good roads, 

 and for short distances, with good horses, two-wheeled single-horse carts 

 are the best ;. but that, with inferior roads and ordinary horses, light four- 

 wheeled waggons, with a team of three or four horses, are the most 

 advantageous. 



Two-wheeled carts with two horses are decidedly inferior to either of 

 these : the shaft horse suffers all the inconveniences complained of in the 

 single-horse cart, and the leader does not produce more effect than when 

 in a waggon team. 



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

 different arrangements, because the result depends entirely upon the 

 circumstances 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 pro- 

 ceed 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. Impediments are continually met with which obstruct the progress 

 of the wheels, and the draught is constantly varying by the different in- 

 clinations 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, in the commencement of this treatise, proved, 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 im- 

 pediment, 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 etfect 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 ^y a rigid 

 resistance, is checked with ease by a very little degree of elastic'^y ; so with 

 a wheel meeting a small stone, if the load were so placed, ^" hung upon 

 the wheels, as to allow free or elastic action longitudinallv t^iat is, in the 

 direction of the movement, the wheel being stopped aG""s^ tne stone, the 

 whole load would be gradually checked, and broi'-^'' ^"^ ^ lull stop; 



