5<j4 PRACTICE OF AGRICULTURE. Part III. 



thoughl well-farmed land, much after wbal I would recommend for die form of a rood. 



Tlic object of forming the land into ridges, raised a little in the middle, is the same as 

 that of raising the middle of a road, to prevent the water from settling upon it ; and what 

 is sufficient tor the ploughed land, is certainly enough for a road. If the road is of good 

 stone, four to five inches rise in ten feet is sufficient ; gravel and other inferior material 

 will allow a little more. This shape not only assists the water to pass from the centre 

 towards the sides, but greatly contributes to the drying of the road, by allowing the action 

 of the sun and air to produce a great degree of evaporation. Surveyors ought to use a 

 level in giving road-, a proper shape, in order that the surface may be of one uniform 

 curvature, without the smallest deviation, in any one spot, from the prescribed line of the 

 cross section." 



3680. The degree of convexity preferred by M'Adam 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 oft' 

 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 one which is reasonably flat." 



3681. If a road be high and convex in the 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 

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

 same effect. 



3682. 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 gr 

 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 nearly 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. ) 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 an 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. 



3683. The convexity recommended by Stevenson is, where the road passes through a level 

 track of country, an ellipsis, " falling from the centre to the verges on either side, at a rate 

 not exceeding an inch and a half perpendicular to a yard horizontal, {fig. 547.) But 



547 



when an acclivity in the line of draught occurs, where carriages are in the greatest 

 danger of being upset, the surface of the road should be kept flat, or with a fall not 

 exceeding three quarters of an inch to the yard, to take the water gently oft' toward the 

 sides, and prevent it, during heavy rains, from rutting the road in a lateral direction." 

 ( Ed. Encyc. art. Roads. ) 



3684. With respect to the order and mode of laying out the materials, there is some dif- 

 ference of opinion. Some begin with the largest, and finish with the very smallest, or 

 with gravel ; some lay on the whole at once, and others in two or more strata, and so on. 

 That such a mode of depositing materials could never make a good road is evident, 

 for the reasons given by M'Adam and Clarke: the larger stones would soon rise to the 

 surface, and roll about loose on it; the strata, being thus broken up, would admit and 



