410 



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. 



*" - Tlie 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 oiT, as in 

 Jig. 30. As tb-e 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 tlie roads approach 

 nearer to perfection in point of hardness and flatness. When 

 the roads are good, a very little dishing will be suflicient, and 

 a slight inclination of the wlieel from the vertical, 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, wliicli de- 

 pends entirely upon the state of the road. 



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

 ing the upper surface of the road is one of the principal causes of resist- 

 ance. If the whole mass of the road were formed of a yielding substance, 

 into which the wheel would sink to a depth exactly proportionate to tiie 

 weight bearing upon it, it is probable that great breadth would be advan- 

 tageous, so that the wheel might form a roller, tending to consolidate 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, cov- 

 ered 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 pres- 

 sure. In considering, then, simply, 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 conse- 

 quently capable of bearing a greater load than before, 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 wnccls. 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 dilierent 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 pre- 

 servation 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 theso 

 wheels, says: " If the axle of a six-inch wheel is oi that lengtli to cause 

 the hind-wheels to make tracks five inches outside the tracks of the fore- 



