426 ON DRAUGHT. 



have shown the advantage of interposing rolling bodies, and thus may 

 rollers have been invented and first brought into use. 



These steps appear natural and likely to have led to these results ; they 

 are at any rate sufficient to account for the first introduction of these two 

 means of facilitating transport, but no steps of this kind appear capable of 

 leading to the beautiful yet simple contrivance of a wheel. 



A roller is by no means an imperfect wheel, as it may at first appear to 

 be ; they have nothing in common but their rotatory or revolving action, but 

 the effect of this motion is totally different in the two. In a roller, friction 

 is avoided altogether by it, in a wheel it exists as completely as in a sledge, 

 but the sliding surfaces being at the centre of the wheel, instead of on the 

 jrround, are always the same, and being under control, may be kept in 

 that state which shall cause as little friction as possible : moreover, the 

 friction is at a point where we have the means of overcoming it, by acting 

 with the power of a considerable lever, as we shall hereafter show. 



There is, indeed, a kind of roller, which partakes somewhat of the cha- 

 racter of the wheel, but without possessing the advantages of it. 



This species of roller may have been an intermediate step between the 

 two, and we shall therefore describe it, when we have dismissed the subject 

 of sledges and rollers. 



In England sledges are at the present time very little in use. In some 

 commercial towns the facility with which bulky and heavy articles can be 

 placed upon them, without being raised to the height of a cart, has caused 

 them still to be employed, but even in these cases, they are in general used 

 only upon the pavement where the friction is not considerable, and for 

 short distances, in which case the saving of labour, in loading and unload- 

 ing, more than compensates for the increase of power absorbed by the 

 draught. Low-wheeled trucks would, however, in these cases possess the 

 same advantage, and might be substituted for them, if this advantage is so 

 indispensable : for agricultural purposes they are almost become obsolete, 

 and for all purposes of traffic between distant points, they are quite aban- 

 doned. 



It is only in the North of England and in some parts of Cornwall, that 

 they are sometimes used in farms, but wherever good roads exist, and me- 

 chanical arts keep pace with the improvements of the age, they have given 

 place to wheel carriages. An examination into their nature and action 

 will immediately account for this. 



A sledge is merely a frame, generally of wood, upon which the load is 

 placed, and resting at once upon the ground, the friction between the under 

 surface of the sledge and the ground bears a considerable proportion to 

 the load ; but if the ground be very uneven and full of holes, the sledge, by 

 extendino- over a great surface, avoids the holes, and slides only upon the 

 eminences, which being naturally the stones or the hard portions of the 

 ground, cause less friction ; on such a road, a wheel would be continually 

 sinking into those holes, and thus oppose considerable resistance, and 

 would also expose the load to frequent danger of upsetting. 



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

 very bad uneven road, a sledge may 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%ery poor and little cultivated, where the formation of roads would not 

 amply repay themselves by allowing the use of wheels ; for the power re- 

 quired to draw a loaded sledge will be at least four or five times greater 

 than that required for an equally loaded cart upon a tolerably good road. 



