Plank countries where lumber is plenty and cheap, plank roads are in use. They are commonly made by laying- down, lengthwise of the road, parallel rows of planks (called "stringers" or "sleepers") about 5 feet apart between centres, which serve to support cross-planks laid upon them. A section of such road is seen in Figure 2. Plank roads were in very common use in the United States twenty-five or thirty years ago, and are still by no means rare; they are in vogue also in Russia. When new, they afford great convenience for heavy haulage, and, though lacking in durability, as temporary expedients in newly-settled districts they have their raison an NIT.
Slone modern forms of road-construction a distinction is made, based on the character of the road-covering, between roads whose surface is formed of broken stone in small fragments and roads whose sur face is composed of prepared blocks of regular form. The first are used chiefly for country roads; the last, for the streets of cities. Roads of broken stone have been variously constructed at different periods and in different countries, as the following historical sinninary will exhibit.
Syston, introduced in France toward the close of the eigh teenth century and still in use in some districts, is shown in Figure 3. The road-bed is prepared by removing a section of earth of suitable width and depth to form the body of the road, which, after proper levelling and compacting with ran-liners, to give it a horizontal or slightly-arched form, constitutes the floor of the road-bed. At the side are laid border-stones, which serve to establish the surface-level, and between these is laid a sub pavement of wedge-shaped stones about 43/1. inches thick and inches high, flat side down and tapering parts upward. The spaces between the apices of these pyramidal stones are filled out with smaller stones, which are driven down with a heavy rammer until the top of the foundation-layer has asstnned a tolerably uniform surface. Upon this is placed a laver of rather coarse vet evenly-broken stone, which is gone over several times with a roller of about 6o cwt., and upon this, in turn, collies the top laver of finely-broken stone, evenly spread and levelled, first with a roller of 6o cwt., which for finishing should be followed by oue of r5o cwt., or even h eavier.
Macadami.Ted the early portion of the present century Eng land contributed greatly to the advancement of road-building through the labors of Macadam and Telford, who were the pioneers of good road-con struction in the United Kingdom, and whose methods have since been more or less closely copied. In the system recommended by Macadam, and
since known by his name, the road is constructed entirely of layers of stone broken into fragments of uniform size and approximating the cubical form (the largest of which should not exceed or 2 inches in its greatest cliag-onal length); the successive layers are then evenly rolled down. The present practice is to prepare at once a comparatively hard surface for traffic. A road of this construction is exhibited in Figure 4 (pz. 2o). If stone of the best quality cannot be had for the entire road-body, the hardest and toughest materials should be reserved for the upper layers—say to the depth of 6 inches. Macadamized roads are in very general use in Great Britain and on the Continent, and to some extent also in the United States. They cost comparatively little for maintenance, and they present a sur face so compact that water will penetrate it only with difficulty.
The Telford Road, in general principles of construction, is not unlike that of Tresaguet, above described, in being formed of a sub-pavement of stone blocks with layers of broken stone. The materials laid upon the bottom course were, like Macadam's materials, angular fragments of hard stone broken into small pieces, gradually decreasing in size toward the top, where they formed a fine, hard surface.
Charader of the thickness that should be given to a roadway of broken stone must be determined by such considerations as the supporting power of the soil, the climate, the hardness and toughness of the stone covering, and largely by the kind of traffic it is intended to sus tain. It is safe to assume that a road-bed of well-consolidated materials, if designed for heavy traffic, should have a thickness of from to to 12 inches; if designed for light traffic only, the thickness may be reduced to 6 or S inches. French road-engineers consider io inches sufficient in France for the most important roads. 'Macadam's practice varied from 6 inches as a minimum to ro inches as a maximum. The surface of the stone roadway, that it may quicklv shed water, must be given a suitable inclination from the centre to ward the sides; so that its cross-section may have a slightly-arched form, varying in degree of curvature according to the grade—flatter where the grades are steep, and rice versa.