ON DRAUGHT. 



557 



immoveable 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 longi- 

 tudinal 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 sen- 

 sible 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 very considerable longitudinal movement in the body of the 

 carriage, are notoriously the most heavy to pull ; and cabriolets, which are hung 

 in this manner, are expressively called, in the stable, horse-murderers, and 

 require heavy powerful horses to drag them ; while lighter animals are able to 

 drag much greater 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 waggon. 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 smallness 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 consider- 

 able play in the parts, and except in new-built or very strong waggons, there is 

 never that firm connexion 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 bo found to produce a very considerable effect 

 in diminishing the draught. _ 



We have been very particular in confining our observations to longitudinal 

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

 the progressive movement takes place ; because elasticity in any other direction, 

 instead of increasing the draught, tends very much to diminish it. Let us sup- 

 pose 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 



