Book II. FORM AND IVJfATERIALS OF ROADS. 533 



3423. The degree of convexity proposed by Clarke, a young Irish road surveyor, is still less 

 than that of Telford. Were it not absolutely necessary, he says, to let the rain-water run 

 off quickly, the best shape for a road would be a flat surface, and, therefore, the nearer 

 we can approach to that form the better ; for, if the road is much elevated in the centre 

 wheel carriages will all run in the middle, and, of course, very soon wear that part into 

 deep ruts ; and if they are then forced to go upon the sides, almost the whole weight will 

 press upon the lower wheel, which will, of course, sink deeper, and occasion a distressing 

 resistance to the shoulder of the horse at that side ; therefore, as before observed, the 

 flatter a road can be made, consistently with a moderate fall for the rain-water to escape 

 the more convenient and durable it will be ; for a road should be as hard and as smooth as 

 possible. An idea of a perfect road may be formed from a frozen canal, where flatness 

 smoothness, and hardness are combined: in imitation of such a surface railways were 

 invented, and fully illustrate the principles assumed. Roads cannot be made so as fully 

 to attain those perfections ; but we should always have them in our view ; for the nearer 

 we approach to such a standard, the less will be the friction, and tlie greater the facility of 

 draught. On a site of sixtj'-three feet he forms a metalled road of thirty-four feet with a 

 rise of nine inches in the middle ; a six-feet path at one side, and a ditch and bank at 

 each side, occupying ten feet six inches. (Jig- 457.) 



3424. The convexiti/ preferred by Telford is no more than is just suflScient to permit the 

 water to pass from the centre towards the sides of the road ; the declivity may increase 

 towards the sides, and the general section form a very flat ellipsis, so that the side, at the 

 time, should (upon a road of about thirty feet in width) be nine inches below the surface 

 in the middle. 



3425. The degree of convexity preferred by M'^dam, is less than that approved of by 

 any of the road engineers mentioned, unless perhaps Edgeworth. " I consider," he says, 

 " that a road should be as flat as possible without regard to allowing the water to run off at 

 all, because a carriage ought to stand upright in travelling as much as possible. I have 

 generally made roads three inches higher in the centre than I have at the sides, when they 

 are eighteen feet wide ; if the road be smooth and well made, the water will run off very 

 easily in such a slope. When a road is made flat, people will not follow the middle of 

 it as they do when it is made extremely convex, which is the only place where a carriage 

 can run upright, by which means three furrows are made by the horses and the wheels, 

 and the water continually stands there : and I think that more water actually stands 

 upon a very convex road, than on one which is reasonably flat." 



3426. If a road be high and convex inthe middle. Fry observes, no care of the surveyor 

 can prevent the formation of a pair of ruts along the ridge of the road ; from an 

 instinctive operation of fear every driver will take this track, as being the only part of the 

 road where his carriage can stand upright ; and even if it be not so convex as to ex- 

 cite fear, yet the inconvenience of travelling on a sloping road will always produce the 

 same effect. 



3427. The convexity recommended by Paterson on the level ground, where the bottom 

 is dry, should be from one inch to one inch and a half in the yard. From this, the de- 

 clivity may increase even to three inches in the yard, just in proportion as the ground 

 increases in wetness ; but beyond that declivity it would probably be improper to carry it 

 in any instance. If the bottom, however, is dry sand or gravel, the convexity should be 

 very little indeed. But, in all cases, whether wet or dry, a road formed on sloping 

 ground, should be very near level from side to side. The reasons are obvious. In the 

 first place, it is well known that carriages running quickly over a hill, are more easily 

 overturned than on level ground ; it would therefore be dangerous, in this respect alone, 

 were the road to have much slope on the sides. In the next place, as the great end in 

 giving it the convex shape is to run off the water and prevent it from lodging, this is 

 not so necessary on a road formed upon sloping ground, as there the water will not lodge 

 so as to injure it. In his second work, (Letters, c^c.) Paterson observes of the above 

 directions, " In my treatise respecting the form of the road, I proposed the slope from 

 the edges of the materials, to the side ditches, to be from one inch to an inch and a half 

 in the yard, where dry ; and to increase the slope a little, where wet. But by adopting 

 those drains under the road, no greater slope will be required, in any situation, than an 

 inch to the yard. 



Mm 3 



