Book IL FORM AND MATERI/xLS OF ROADS. 5Si 



good way, but I always make my surveyors carry a pair of scales, and a six-ounce weight 

 in their pocket, and when they come to a heap of stones, they weigh one or two of the 

 largest, and if they are reasonably about the weight, they will do ; it is impossible to 

 make them come exactly to it." 



3410. JVith respect to the size of stones, Paterson disapproves of six ounces being 

 made the maximum as proposed by M'Adam. " I find," says he, " there are many 

 under the weight that are yet of a very improper shape and size ; even from three to 

 four inches between the extreme points. Besides, scales for weighing are not so 

 portable nor convenient, as gauging rings for the size. The ring 1 generally use is 

 two inches and a half in diameter ; and the stones should be broken so that the largest 

 may pass, in any direction, through it. On this plan you have the materials smaller, 

 more equal, and more square in shape than on his plan. An inexperienced person, on 

 the first view of it, may think otherwise ; but it is a fact, that taking my ring as a guage, 

 you will not have five stones in a tliousand that will exceed four ounces in weight ; and 

 none of improper shape or dimensions : while on Mr. M* Adam's plan you will have 

 more than twenty in a thousand that will not pass longitudinally, even through a three 

 inch ring. It is now nearly three years since I first heard of his standard weight. 

 During that time I have had people both working to it, and also to my ring-gauge ; 

 but I have uniformly found, that mine are so much smaller that they cost about a. fifth 

 more in breaking than his. Upon the whole, then, I would recommend the ring as 

 every way preferable to the scales : and I have no doubt that it would be an improvement 

 even to reduce the ring a little where the ground under the road is so completely dried 

 by the method I have described." 



3411. With respect to the depth of metals, Marshal mentions twelve inches; but 

 Edgeworth considers an average of nine inches as suflicient for any road on a good 

 basis ; and two thirds of the quantity, he says, will make an excellent road at a distance 

 from any great town. 



3412. The dejHh of materials, according to JFalker, depends so much upon the soil 

 and the nature of the materials themselves, that it is impossible to lay down any general 

 rules for them. The thickness ought to be such that the greatest weight will not affect 

 more than the surface of the shell, and it is for this purpose chiefly, that thickness is 

 required, in order to spread the weight which comes upon a small part only of the road 

 over a large portion of the foundation. 



3413. The depth of solid materials recommended hy M^Adam is 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.) 



3414. The 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. 



3415. With respect tojhe 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 re- 

 sorted to in preference, but from accidental circumstances. 



3416. The proper conveodty 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 ground, 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 twice the 

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

 tlie degree of convexity is to be determined, in part, by the width of the road ; the 

 materials and descent being equal. A wide road re(|uires to be formed with a greater 

 sideways descent, tlian a narrower one ; which more readily frees itself from rain-water ; 

 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 burden, whose 

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



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