ON DRAUGHT. 541 



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 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 won- 

 derful. 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 unknown, 

 but the records which refer to periods so far removed as 3000 years make no 

 mention of snch conveyances. 



Rollers come next under consideration; they certainly afford 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, 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 more or 

 less the case, according to the perfection of workmanship in the formation of the 

 rollers, and, if cylindrical, the care with which they are placed at right angles 

 to the direction at which they are to move. There is only one source of resistance 

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

 or the yielding of the material, which amounts to nearly the same thing. 



Fig. 16. 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: conse- 

 quently, if the material of the roller, and the sur- 

 face on which it rolled, were perfectly hard and 

 inelastic, such would be their contact, whatever 

 weight might be placed upon the roller. 



But in practice no such material can be obtained, 

 and rollers, on the contrary, are generally made of 

 wood, and, when loaded, they must yield until the 

 surface AB, fig. 16, is proportionate to the pressure. 

 Still, if the substance were perfectly elastic; that 

 is to say, if it would return to its original form with 

 the same force and velocity which were required to 

 distort it, this alteration would not cause any re- 

 ' sistance ; the elasticity at E would tend to raise the 

 back of the roller with a force T)E,fig- 17, equal to, 

 and exactly similar, but opposite to CB, and would 



consequently balance it. , . 



Although perfect elasticity is unattainable, yet most hard substances possess 

 this quality to some extent; consequently, when the load is not sufficient to 

 crush the materials, the resistance is not much increased by even a con- 



