592 



PRACTICE OF AGRICULTURE. 



Part III. 



3666. The depth of solid materials recommended by M'Adam is one of ten inches, which 

 he thinks equal to carry any thing when well consolidated, and whether on a soft or hard 

 substratum ; he should prefer a soft one. (Examinations, ^c 1819.) 



3667. 27ie depth of metals, according to Paterson, should be regulated according to 

 their quality, the situation of the road, and the nature of its basis. On the generality of 

 turnpike roads it should be made from ten to twelve inches ; and upheld afterwards at 

 the depth of nine or ten inches. Yet, in some situations, even six or eight inches will 

 make a much better road than twelve or fourteen in other situations. 



3668. The depth, according to Stevenson, must depend a good deal on the quality of 

 the rock, but it should seldom be less than eight inches in all those parts of the road on 

 which carriage wheels may be supposed to pass. Towards the verge, it may be less. 

 {Ed. Encyc. art. Roads.) 



3669. With respect to the shape of the surface of the metals, almost all road-makers 

 agree that it should be convex, but they differ a little in the degree of convexity. It is 

 also allowed by most of them that on roads up ascents, the surface of the metals may be 

 flat, bevelled, or somewhat inclined to one side. Concave roads are not here taken into 

 account, as they require a different general plan, and may be considered as not resorted 

 to in preference, but from accidental circumstances. 



*-3670. Concave ruads {fig. 5^5.) were recommended, and to a certain extent adopted, by the celebrated 

 Bakewell of Uishley. Practically considered, such a road is in effect nothing more than a flat road with 

 a gutter in the middle, instead of a gutter at each side. 



545 



3671. Tlie proper convexity of a wet-weather road, according to Marshal, is to be 

 regulated by a variety of circumstances ; as, first, by the materials of which it is to be 

 formed : soft materials are most liable to be worn into ruts and hollows, and require to 

 be laid up with a quicker descent for rain-water than hard materials, which require less 

 elevation or rotundity of surface ; and least of all a firm even pavement. Secondly, a 

 convex road in the face of a steep is to be laid up higher, with a given material, than 

 one on more level grouiid, on which rain-water has no other tendency than to the sides : 

 whereas, in the face of a steep, it may have an equal or greater tendency along the line 

 of the road, and is liable to be caught by the slightest impressions of wheels ; and thus 

 to wear channels, as may too often be seen, from the top to the bottom of the hill. 

 Even where the surface of the road is perfectly smooth, it may have twi(^e the distance 

 to run, before it reaches the outer margin, that it has on a level. And, thirdly, the 

 degree of convexity is to be determined, in part, by the width of the road j the mate- 

 rials and descent being equal. A wide road requires to be formed with a greater side- 

 ways descent than a narrower one ; which more readily frees itself from rain-v/ater, 

 inasmuch as the distance is shorter from the crown to the outskirts of the road. Nor is 

 freeing a road from rain-water the only object to be kept in view, with regard to its 

 convexity. The ease and safety of carriages, and particularly those of burthen, whose 

 loads, being of light materials, are laid up high, require to be consulted. A carriage moves 

 most freely, and with the least exertion of draught, when the load lies evenly upon the 

 wheels on each side. In proportion as the weight is thrown on one side, or the other, 

 the resistance is increased ; especially on a road which is liable to impression. Hence 

 an inconveniency of a highly convex road in the face of a steep, and hence the utility 

 of breaks in long ascents. 



3672. It is evident that every part of a road should be equally and duly convex, should 

 be equally safe and easy for ceuriages of every description, otherwise it becomes more 

 partially worn ; the more level parts only are used, the steeper being in a degree useless. 

 Hence a road of even and due convexity is not only easy and safe, but may be formed 

 of a narrower width, than one whose steep sides are neither easy nor safe to be travelled, 

 and whose crown only is in use. On measuring different passages of roads which 

 appeared to lie in the most desirable form. Marshal found that their convexity, or the 

 elevation of the crown or middle of the road above the base line, in roads of twenty feet 

 in width, was about ten inches ; namely, one inch in every foot on each side : and he 

 is of opinion that this result may be taken as a general guide in forming roads ; this 

 middle degree of convexity being liable to be altered, according to the width of the road, 

 the nature of the materials, and other circumstances. 



3673. A whole barrel or convex road cannot easily be kept up in a narrow site, as in 

 the case of narrow lanes. If raised, it presently wears into a middle track and two 

 wheel-ruts, with foul drains on each side of them, and becomes, in wet weather, a dirty 

 trough, which is unfit for either carriages or horses, and in which a foot passenger has 

 not where to set his foot. But if such a lane be thrown into a shelving form, resembling 



