ON DRAUGHT. 427 



The draught of a sledge, even upon the pavement, is about one-fifth of 

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

 four hundred weight; 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 ground, but with the load before mentioned, viz : one ton, the force of 

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

 strong 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 limited, 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 Avould 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 climates, on the contrary, not only are they now almost un- 

 known, 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 much 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 without 

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

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

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

 at which they are to" move. There is, it is well known, only one source of 

 resistance which is inseparable from the use of rollers, viz: the unevennesa 

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

 the same thing. 



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

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

 »r 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 ^ placed upon the roller. 



