ON DRAUGHT. 



567 



and powerful horses to drag them ; while lighter animals are able to drag 

 much gi-eater 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 Avaggon. 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 small- 

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

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

 bolsters : this admits of considerable play in the parts, and except in new- 

 built or very strong waggons, there is never that firm connection between 

 the load and the wheels which we have stated to be necessary. Large 

 wheels would bring the axletrees much nearer the floors of the waggons, 

 and, therefore, admit of a much stronger and firmer mode of attachment, 

 which would be found to produce a very considerable effect in dimmishmg 



the draught. , , • - i u 



We have been very particular in confining our observations to longitu- 

 dinal elasticity, or yielding in the direction in which the power is applied, 

 and in which the progressive movement takes place ; because elasticity m 

 any other direction, instead of increasing the draught, tends very much 

 to diminish it. Let us suppose the load placed upon perfectly easy springs, 

 which allow it to move freely in every direction, except longitudinally, 

 when any one of the wheels comes in contact with a stone, the elasticity 

 of the spring will allow it to run over the stone without sensibly raising 

 the load which is upon it ; and the force which is required to pull the 

 wheel over the stone will be restored again by the descent of the wheel 

 from the stone, which will tend to impel the mass forward, with exactly 

 the same force as was required to draw it up to the top of this impedi- 

 ment Without this elasticity, it would be necessary to raise the whole 

 load Avith a sudden jerk, and thus instantaneously impart rapid movement 

 to the whole mass, wliich would absorb much power, and which would by 

 no means be returned by the load falling down from the stone. We see, 

 therefore, that the use of springs is to enable the wheels to nse and tail 

 according to the inequalities of the ground, while the load continues one 

 constant equable motion. The advantages of this action are very clearly 

 pointed out in a letter addressed to the Committee on the Highways ot 

 the Kingdom, by Mr. D. Giddy, and given in the Appendix to their first 

 Report, 'pr™^^^^ ^^ ^^^^ ^^^^ "^^^^ ' ^^^ *^"^ ^^**^^ explains so clearly, and 

 in such few words, the whole theory of wheels, as Avell as springs, that 

 we think Ave cannot do better than quote it at length : — 



