Pazrai Roads.—Where stone is used for the purpose, the material should possess the needful hardness and durability for its intended service. It should likewise permit of being readily dressed into blocks of regular form. Granite and the hard and dense varieties of sandstone and limestone, in the order named, are found best suited for this service. Where these are not available, rubble and cobble-stones are frequently substituted therefor, thong-it these inak-e a much inferior pavement. A good stone pavement is more costly to lay than a macadamized roadway, but affords an easier sur face for traction and is less expensive to maintain.
Paving-stones.—The quality of a stone-paved street depends largely upon the form, size, and mode of setting the paving-stones employed in its construction. In the most approved system, where the stones are laid iu regular lines or courses, they are given the form of rectangular blocks, sometimes slightly tapering-, toward the lower end (pi. 20, figs. 5, 6). The smoother the blocks, the more intimate will be the contact between them, and consequently the more solid the resulting structure. In respect of size there is some difference in practice, but it may safely be stated that, to afford the best possible foothold for horses, the individual blocks should be no broader (in the direction of the draught) than the length of a horse's shoe—that is to say, from 4 to 4 inches, as a maximum. Their depth from top to bottom should be sufficient to avoid any disposition to tilt to one side when a weight comes upon the other—and for this dimension a little more than twice the horizontal breadth has been found a good proportion —and their length crosswise of the street, to insure an ample area of bear ing-surface on the foundation, should be at least equal to their depth, and may advantageously be somewhat greater than this.
From these data the most approved dimensions for a paving--block would therefore be as follows: Breadth (measured along the street), from 3;4 to d.;> inches; length (measured across the street), from 9 to 12 inches, or even 15 inches; depth, from S to ro inches. The stones are laid closely in contact on edge and in continuous courses, the direction of the individual courses being usually directly across the street (fig. S). Sometimes they are laid at an angle of from 45° to 6o° with the axis of the street, with the object of lessening the tendency to wear into a convex surface. On the steep g-racles the plan of laying the blocks in two sets of diagonal courses is sometimes followed, as is shown in Figure r 21). The courses meet in the centre of the street, with the angle pointing up the incline. The joints slope downward and facilitate the drainage by directing the surface water, right and left, into the gutters. In all cases the successive courses nmst be laid so as to " break joints" (pi. 20, fig. S).
Foundation of Pavea' respect of the foundation, this may be concrete, or rubble-stone filled in with coucrete, which case it is recom mended by the best constructors to lay each stone solidly in a bed of cement mortar, and to use the same material or bituminous mortar between the joints; or the foundation may be of sand or gravel, or of broken stone. In
the latter case the paving-stones are set in a layer of sand or gravel spread evenly on the foundation, after which they are rammed with a heavy wooden rammer, and the finished surface is then covered with a thin layer of clean sand or fine gravel, which gradually works its way between the joints in the pavement (pi. 2o,figs. 5, 6). Where, from scarcity of suitable material or for economic reasons, stone blocks are inadmissible, recourse is bad to cobble-stone or rubble-stone as a substitute.
The Pavement is formed of rounded, water-worn stones, usually egg-shaped, which are selected so as to be as nearly uniform in size as possible; the best material averaging from 6 to ro inches in length and from 3 to 6 inches in width. These stones are set as close to one another as possible, with the smaller ends down, in a foundation formed of a bed of sand or fine gravel, which should be from 8 to io inches deep. After thev have been set in position the cobbles are rammed down firmly, to give a uniform and slightly convex surface, and over the surface is spread a layer of sand or fine gravel, which works its way in between the stones. This form of pavement is in very common use throughout the United States, but is much inferior to the stone-block pavement.
The Pavement is composed of stone fragments of varions irregular shapes and sizes laid as closely together as possible; the appear ance of such pavement is shown in Figure 2 (fii. 21). Stone suitable for this purpose may generally be obtained with little difficulty from the ref use of stone-quarries. The flat surfaces of such material afford a bet ter foothold for draught-animals than the rounded surfaces of cobbles, and less tractive power is required than on the latter. As compared with the cobble-stone, therefore, the rubble pavement is preferable. The best sizes for the rubble-stone are a breadth of from 3 to 6 inches, a length of from 6 to 12 inches, and, as a minimum for stability, a depth of 6 inches. The rubble pavement is laid in the same manner as the cobble pavement, on a foundation of sand or g-ravel, each stone being carefully placed in position by hand and the entire surface afterward rammed down to the required level. The stone pavement is terminated laterally by the curb, which serves both as the limit for the sidewalk and as the side-wall for the glitters, which run along the sides of the street and serve to drain it through openings, placed at proper intervals, which communicate with the sewers. To facili tate drainage the street is given a slightly-arched form in cross-section.