42G ON DRAUGHT. 



liave shown tho advantage of interposing rolling bodies, and thus rollers 

 may have been invented and first brought into use. 



These steps appear natural, and likely to have led to these results: ihcy 

 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 wjieel, as it may at first appear to 

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

 the elUct of tliis 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 

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

 that stale 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 tiie power of a considerable lever, as we shall hereafter show. 



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

 acter of the wheel, but without possessing tlie 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 tow^ns the facility with which bulky and heavy articles can be 

 placed upon them, without being raised to the heigiit 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 easily be substituted for them, if this advantage 

 is so indispensable : for agricultural purposes, they are almost become obso- 

 lete ; and for all purposes of traffic between distant points, they are quite 

 abandoned. 



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

 they are sometimes used in farming; but wherever good roads exist, and 

 mechanical 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, tiic 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 extending over a great surface, avoids the holes, and slides only upon 

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

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

 sinking into those holes, thus opposing 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 veiy economical, as regards 

 first cost ; but the ground must indeed be very bad, or the country be very 

 poor, and little cultivated, where the formation of roads would not amply 

 repay themselves by allowing the use of wheels; for the power reqoired 

 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. 



