What is the best transverse form for a road is a much debated question among engineers. All agree that it should be higher in the middle than at the sides, but some think it should be much higher than others. As a road can be better kept clear of water by a slight inclination in the direction of its length than by any form which can be given to its cross-section it seems preferable that it should be as nearly flat as possible, because every part of its breadth will then be equally available for traffic; whereas it is almost necessary to keep on the center of a highly convex road, and consequently wear deep furrows there, by confining the wheels and horses to pretty much the same track. An approved form of road has a slope of 1 in 30, with a few feet in the center on a flat curve.
Different opinions are also held as to whether the bed upon which the road is to be formed should be flat or rounded; those who prefer it flat considering that there should be a greater depth of material at the center than at the sides, while others think that the depth should be uniform.
As respects the construction of the road itself, the first point to consider is the foun dation. The majority of roads have no artificial foundation. In such cases the surface on which the road-material is to be laid is generally made as solid as possible by means of efficient drainage, and by rolling and beating wherever there are embankments formed. It is the question whether or not a road should have a foundation of rough pavement below the broken stone covering, which is the essential point of difference between the two great rival systems of Telford and Macadam. Telford considered it of great import ance that there should be such a foundation. He made it of stones varying in depth from 9 in. at the center to 3 in. at the sides of the road, these being set with their broad est edge downward, and no stone being more than 4 in. broad upon the upper edge; upon were placed a (Tinting of &ken stones not exceeding 6 in. in thickness. The Glasgow and Carlisle and the Holyhead roads are excellent examples of the endur ing character of those made on Telford's plan.
In our biographical notice of Macadam (q.v.) will be found a reference to his method of road-making. Suffice it here to say that he preferred a yielding and soft foundation to one which was rigid and unyielding, so that even on boggy ground, if it were but firm enough to allow of a man walking over it, he considered an artificial bottoming quite unnecessary. His roads were formed entirely of anuilar pieces of stones, of such a size as to pass freely through a ring 24- in. in diameter. This plan has now fewer advocates
than Telford's, or than the one subsequently proposed by Mr. Thomas Hughes, where a concrete of gravel and lime is employed for the f, undation of the road. But experience has shown that Macadam's plan of employing angular pieces of stone is superior to every other as a mere covering for roads, whether they have an artificial foundation or not. So popular at one time was the system of macadamizing, that expensively paved streets, such as that between Edinburgh and Leith, were torn up to be reformed on the new plan. Dublin has been instanced as an example of the failure of Macadam's plan for the streets of a populous city. There the macadamized streets are in winter con stantly covered with mud, and iu summer profuse watering is required to keep them from being overwhelmed with dust. It is curious, however, that the French road engineers have, in recent years, come to the conclusion that a covering of broken stone alone is sufficient on the most frequented. roads and under all but the very heaviest traffic.
With regard to the kind of stone suitable for covering roads, granite and the differ ent kinds of greenstone and basalt, ordinarily called whinstones, are the only kinds admissible. Sandstone is too easily crushed, limestone is objectionable from its slight solubility in water. The stone employed should be totigh as well as hard. Flint is hard enough, but it is brittle, and easily crushed to powder. The object is to get it to hind into a firm mass, and not to roll about, after it has been laid down for some time.
Little need be said about the drainage of roads, notwithstanding its great importance, because it will be apparent from what. lras peen said, that it is in great part secured by the plan on which a road is made. What further drainage a road requires. can, in many situations, be effected by ditches on either side. Where this is not possible, as in the . case of portions situated in cuttings more or less deep, proper drains require to be con structed. In such circumstances a drain is either made down the center, with branch drains from the sides running into it; or drains are formed along the sides, with gratings at proper intervals to take in the surface-water. If the ground beneath the road is com, posed of clay or of any kind of wet soil, under drainage must be resorted to; and of course, wherever there are footpaths, small drains require to he placed under them, if there is no other means of carrying off the water from the channel between them and the road.
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