These experiments uniformly show, that the force of trac tion is in every ease in an exact proportion to the strength and hardness of a road. The following are the results on a well-made pavement :—The power required to draw a wag gon is 34 lbs.; on a road made with 6 inches of broken stones of great hardness, laid on a foundation of large stones set in the form of a pavement, the power required is 46 lbs.; on a road made with a thick coating of broken stone, laid on earth, the power required is 65 lbs.; and on a road made with a thick coating of gravel, the power required is 147 lbs. : thus it appears that the results of actual experiments fully correspond with those deduced from the laws of science.
Sir John, in his examination before a Cotnmitee of the House of Commons in 1836, says, "The great advantage of the roads appearing by the machine is certainly in proportion to their solidity and their strength, and their want of yield ing. Wit could be a perfectly solid mass of stone or metal, the least resistance would be presented ; that is shown both on stone tramways and on and metal rails. " This foundation should be composed either of a rough paving of large stones, or of a sufficient bed of concrete, the former being the practice of Mr. Telford, and the latter of many engineers of the present day ; the selection of either of these materials will depend upon their economy in various places ; whichever is most accessible, may he safely used ; but whatever material be used, it is necessary that the sub stratum should be of uniform strength and solidity over the entire surfitee, otherwise, after the pavement has been laid, the weaker parts will give way, while the stronger maintain their position, and so the surface of the road becomes broken and uneven, rugged, and full of hollows; for this reason we prefer the use of concrete, which offers greater security against this defect than foundations made of large stones or rubble, though in many roads even concrete is not sufficient entirely to prevent it. We are inclined to think that the value of concrete for this purpose has not been fairly tested, for the work, especially in London roads, is usually got through in so hurried a manner as not to allow a sufficient time for setting : we imagine, that with a somewhat thicker bed of concrete, the materials of which have been properly mixed, and allowed a sufficient time to set before the metal ling is put on, we should be enabled to construct roads suffi ciently firm to bear the extraordinary wear and tear of Lon don traffic, and that, too, at a less cost than is expended on the present roadways. We are aware that there are many disadvantages to contend with in London roads ; that they are continually being pulled up, and thereby destroyed, for works connected with water, gas, or sewers, and that this acts as a great discouragement to the construction of good roads. That any necessity of this nature should be allowed
to exist is much to be lamented, but we feel confident that such practices might be restricted to a very considerable extent, without doing any injury to the public convenience. The concrete foundation should be slightly curved, falling from the centre towards both sides, which form will assist to drain it, and also to give the proper form to the surface of the roadway. It is necessary that this lower surface should be properly drained, otherwise the water will lie there, and destroy the road. If the concrete were laid IS inches thick, we should not deem the extra expense ill laid out for a good road. With respect to the other kind of foundations, Mr. Parnell says—" In streets where the traffic is not very great, the foundation should be made in the following manner : A bed should be formed, with a convexity of inches to 10 feet, so as to admit of 1'2 inches of broken stone being laid upon it : these should be put on in layers of four inches at a time. After the first layer is put on, the street should be kept open for carriages to pass over it. When the layer has become firm and consolidated, then another layer of 4 inches should be laid on, and worked in as before, care being taken to rake the ruts and tracks of the wheels of carriages, so that the surface may become smooth and con solidated. The same process should be repeated with the third layer of stones, by which means a solid and firm foun dation will be established, of 1:2 inches in thickness, for the dressed paving-stoncs to lie upon." Such directions deserve greater attention than we fear they usually receive; in all cases, let your foundation be secure before you lay your roadway.
Various kinds of materials are used for the purpose of forming the upper coat of the road, and amongst those most frequently employed may be mentioned broken Guernsey granite, flints, gravel, and cubes of granite of various sizes. Of these the broken granite and granite cubes are most useful, the one for general purposes, the other for roads which arc subject to great wear and tear, such as those in the principal thoroughfares of London, for although both descriptions of material are used for this purpose, yet we are . inclined to think that the broken stone is scarcely durable enough for such work : and there is this further objection to it, that it is constantly requiring repairs, which must, to a certain extent, inconvenience public business. Besides this, as usually managed, they occasion a great deal of dust and dirt, while the draught of vehicles upon them is very heavy, and their maintenance expensive. It would be scarcely fair, however, to attribute the faults of management to the quality of the material, and we believe that many of the above objections may be removed by skilful management.