540 ON DRAUGHT. 



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 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 time very little in use. In some com- 

 mercial 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 draught. Low-wheeled trucks, how- 

 ever, in these cases, possess the same advantage, and have gradually been sub- 

 stituted 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 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 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 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 little 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 required 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 

 hundred 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 

 want of means, prevents the adoption of more improved vehicles j and these 

 cases are fortunately too rare in England to render it worth our while to bestow 

 much time upon its description. 



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

 feet apart, with their lower edges shod with iron : and transverse planks, bolted 



