440 ON DRAUGHT. 



the management of a considerable portion of a principal road in the middle 

 of England, the question was considered, and it was agreed to encourage 

 the use of conical wheels, as at least equal to, if not superior to cylindrical 

 ones, by allowing them to run at a less toll than that required by act of 

 Parliament. 



The cylindrical form is the only one which ought to be 

 admitted. As a wheel must, however, always be liable to 

 sink a little into the road, and cannot be expected always to 

 bear perfectly flat upon the ground, the surface of the tires 

 should be slightly curved, and the edges rounded off, as in 

 Jig. 36. As the rounding is rendered necessary by the yield- 

 ing of the road, its degree must depend upon the state of 

 the road, and the form of the wheel may approach more 

 nearly to the true cylinder, in proportion as the roads ap- 

 proach nearer to perfection in point of hardness and flatness. 

 When the roads are good, a very little dishing will be suffi- 

 cient, and a slight inclination of the wheel from the ver- 

 tical will make it correspond with the barrel or curve of the 

 road, which is now generally very trifling. 



Next to the form, the breadth of the wheel is the point 

 requiring most consideration : it is one, however, which 

 depends entirely upon the state of the road. 



We have seen, that the displacement or crushing of the materials 

 forming the upper surface of the road is one of the principal causes of 

 resistance. If the whole mass of the road were formed of a yielding sub- 

 stance, into which the wheel would sink to a depth exactly proportionate 

 to the weight bearing upon it, it is probable that great breadth would be 

 advantageous, so that the wheel might form a roller, tending to consoli- 

 date the materials rather than cause any permanent displacement ; but, 

 in the improved state of modern roads, it may safely be considered that 

 such is never the case. 



A road, as we have before stated, always consists of a hard bottom, 

 covered with a stratum, more or less thick, of soft, yielding material. A 

 wheel, even moderately loaded, will force its way through, and form a rut 

 in this upper coating. The resistance will be nearly proportionate to the 

 breadth of this rut ; the depth of it will not increase in the ratio of the 

 pressure. In considering, then, sim])ly, the case of a single wheel or a 

 pair of wheels forming two distinct ruts, it is evident that it should form 

 as narrow a rut as possible, but that it should not in any degree crush or 

 derange the core or hard basis of the road. When a rut is thus formed, 

 a small track or portion of the road is for a time rendered clean and hard, 

 and consequently capable of bearing a greater load than belbre, and with 

 less injury. It is, then, highly important in a four-wheel carriage that 

 the hind wheels should follow exactly in the track of the front wheels. 

 If rollers were necessary for the road, as if, for instance, it was merely a 

 bed of clay, then indeed, but only in such a case, would it be judicious to 

 cause the wheels to run in different tracks, as has been proposed, and was 

 carried into effect under the encouragement of an act of parliament. Such 

 wheels were called straddlers : they might have been necessary tools for 

 the preservation of such roads as then existed, but the increased draught 

 soon taught the public to evade the law which encouraged them. 



Mr. Deacon, one of the principal carriers in England, in an excellent 

 practical work on wheel-carriages, published in 1810, describing these 

 wheels, says, ' If the axle of a six-inch wheel is of that length to cause 

 the hiud wheels to make tracks tive inches outside, the tracks of the fore- 



