•550 OX DRACGHT. 



be coutinuallj sinking into those lioles, and thus oppose considerable resist- 

 ance, and would also expose the load to frequent dang(3r of upsetting! 



It would appear, therefore, that over broken ground, or even upon' a 

 very bad uneven road, a sledge may sometimes be more advantageous than 

 wheels, _ and its extreme simplicity of construction renders it very 

 economical as regards first cost ; but the ground must indeed be very bad, 

 or the country be very poor and Httle cultivated, where the formation of 

 roads would not amply repay themselves by allowing the use of wheels ; 

 for the power required to draw a loaded sledge will be at least four or 

 five times greater than that requii-ed for an equally loaded cart upon a 

 tolerably good road. 



The draught 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 hundi-ed 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 Avant 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 descriptfon. 



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

 or five feet apart, with theii- lower edges shod Avith iron : and transverse 

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

 The traces should be more inclined than wth wheeled carnages, because 

 the friction bearing a greater proportion to the load, it is more advan- 

 tageous to throw a portion of that load upon the horse, and beino- used 

 upon uneven ground it is more important to be able to lift the fronfof 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 cover- 

 ing 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 ghde upon the thin frozen crust without 

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

 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 

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

 years make no mention of such conveyances. 



Rollers come next under consideration ; they certainly afibrd the means 

 of transporting a heavy weight 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, oan roll upon a plane without 

 any rubbmg of the surfaces whatever, and consequently without friction ; 

 and, m the same manner, a plane will roll upon this roller without friction • 

 m practice, this is more or less the case, according to the perfection of 

 workmanship in the formation of the rollers, and, if cylindrical, the care 

 A\ath which they are placed at right angles to the direction at which they 

 (ire to move. There is only one source of resistance which is inseparable 



