ON DKAUGllT. 549 



article from one spot to another, he doubtless endeavoured to lift or carry 

 it: if it proved too heavy for him to carry, he would naturally endeavour 

 to drag it. Here frequent experiments would soon show him how much 

 less labour was required to drag a body with a smooth surface in contact 

 with the ground, than when the contrary was the case ; and if the body 

 to be moved did not itself present a smooth surface on any of its sides, but 

 was, on the contrary, rough and angular in all directions, he would 

 naturally be led to interpose between it and the ground some plane surface 

 wliich should prevent the angles and projections of the body from entering 

 the ground and impeding the progress ; and we may presume that sledges 

 were thus very early brought into use. Wlien attempting to transport 

 still heavier niasses, the accidental presence of round stones, or of a piece 

 of timber, may 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 ; the}- 

 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 diflerent in the two. in a roller, 

 friction is avoided altogether by it, in a wheel this friction exists as com- 

 pletely as in a sledge, but the sHding surfaces, being at the centre of the 

 wheel instead of on the ground, are always the same, and being imder 

 control, may be kept in that state which shall cause as httle friction as 

 possible ; moreover, the friction is at a point Avhere 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 

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



This species of roller might 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 tiine 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 

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

 the di-aught. Low-wheeled trucks, however, in these cases, possess the 

 same advantage, and have gradually been substituted for them, where this 

 advantage was indispensable : for agricultural purposes they are almost 

 become obsolete, 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 farms ; 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, the fiiction between the 

 under surface of the sledge and the ground bears a considerable propor- 

 tion to the load ; but if the ground be very uneven and full of holes, the 

 sledge, by extending over a great sui'face, avoids the holes, and slides 

 only upon the eminences, which being naturally the stones of the hard 

 portions of the ground, cause less friction ; on such a road, a wheel would 



