ON DRAUGHT. 427 



The drauG:lit of a sledge, even upon the pavement, is about one-fifth of 

 " the load, so that to draw a ton weight, requires a force of traction of about 

 four hundred wei^-ht ; upon roads the friction will be much greater : it is 

 difficult to state its amount, as it must depend so much upon the nature of 

 the o-round, but with the load before mentioned, viz. one ton, the force of 

 traction will probably vary from five to seven hundred weight : over a 

 strono- rocky surface the resistance of a sledge will be much the same as 

 on pavement. Its use, therefore, must be confined to very particular 

 cases, where the absence of roads, or the want of means, prevents the 

 adoption of more improved vehicles ; and these cases are fortunately too 

 rare in England to render it worth our while to bestow much time upon 

 its description. 



Sledges are generally formed of two longitudinal pieces of timber, four 

 or five feet apart, with their lower edges shod with iron ; and transverse 

 planks, bolted to these, form the floor, and they are thus easily constructed. 

 The traces should be more inclined than with wheeled carriages, because 

 the friction bearing a greater proportion to the load it is more advantageous 

 to throw a portion of that load upon the horse, and being used upon uneven 

 ground it is more important to be able to lift the front of the sledge 

 over obstacles. 



Although in this country the use of sledges is very Hmited, in many 

 parts of the world they constitute the best, and, indeed, the only means of 

 conveyance. Upon ice the friction is so trifling that they oppose less 

 resistance even than wheels, for the reasons before stated of their covering 

 a larger surface, and thereby sliding over those asperities which would 

 impede the progress of a wheel ; upon snow the advantage is still ^more 

 decided: where a wheel would sink a considerable depth and become 

 almost immoveable, a sledge will glide upon the thin frozen crust without 

 leaving a trace, and with an ease truly wonderful. In all cold climates 

 they are consequently in general use ; and the depth of winter is there the 

 season for the transport of merchandise. 



The Esquimaux with their dogs, the Laplanders with their rein-deer, and 

 the Russians with horses, use the sledge to a great extent in the winter, 

 over the frozen rivers or the hard snow. 



In the warm chmates, on the contrary, not only are they now almost 

 unknown, but the records which refer to periods so far removed as 3000 

 years make no mention of such conveyance. 



Rollers come next under consideration ; they certainly afford the means 

 of transporting a heavy weight upon land with less power than any other 

 means with which we are acquainted ; their motion is not necessarily 

 attended with any friction. A cylinder, or a sphere, can roll upon a plane 

 without any rubbing of the surfaces whatever, and consequently without 

 friction ; and, in the same manner, a plane will roll upon this roller with- 

 out friction : in practice, this is more or less the case, according to the 

 perfection of workmanship in the formation of the rollers, and, if the 

 cylindrical, the care with which they are placed at right angles to 

 direction at which they are to move. There is only one s.ource of resistance 

 which is inseparable from the use of rollers, viz., the unevenness of the 

 surfaces, or the yieldings of the material, which amounts to nearly the 

 same thing. 



A circle resting upon a straight Hue can only touch it in a single point, 

 and the contact of a cylinder with a plane is merely a line: consequently, 

 if the material of the roller, and the surface on which it rolled, were per- 

 fectly hard and inelastic, such would be their contact, whatever weight 

 might be placed upon the roller. 



