Page:Popular Science Monthly Volume 80.djvu/332

328 road. To-day quite a different procedure is employed. The stone is applied in two courses and in two sizes. The first or lower course consists of larger stone than that used for the surface, from an inch and a half to two and a half inches in diameter. It is carefully and uniformly spread to such a thickness that it has, when compacted, a thickness of from three to five inches, depending on the character of the foundation and that of the traffic which the road is to carry. The second or upper course, which forms the surface or crust of the road, is composed of finer stone, one half to one and a half inches in diameter, to a thickness of at least three inches when compacted. Great advances in highway construction have taken place since Macadam's day, in that steam rollers have been available for some decades for compressing and putting in place in a proper manner, the broken stone after it is applied to the foundation. Each course is rolled separately until it ceases to move under the roller.

After the compression is completed a binder or filler of much finer stone is spread over the surface, all of it passing-an opening three eighths of an inch in diameter and a considerable portion being fine dust, for the purpose of filling the voids in the upper course of stone, closing up the surface and preventing infiltration of water through it. This is accomplished by the use of water, applied with a watering cart, which washes the fine material into the interstices in the stone. The road is then again rolled with the steam roller, to aid in forcing the filler into the surface, and to render it compact and waterproof. Skill is required in the manipulation of the roller to produce a surface of proper conformation and uniform density.

The method of constructing a water-bound broken-stone road, which has been described in a very general way, is still in use for work of this type, and was the only one employed up to the beginning of the present century, in building roads to meet the most trying conditions then existing. It was a very satisfactory form of construction and still is, under certain conditions and environment. The execution of such work was an art involving great skill and experience, but science contributed but little to perfecting it and placing it on a rational foundation, with the exception of methods of examining the character and availability, from this point of view, of the various rocks which are used in building roads, which were developed in France, and on a much more elaborate scale by the Massachusetts Highway Commission, and in the Office of Public Roads of the United States Department of Agriculture by Mr. Logan Waller Page, the present director of that office, who, with his assistants, has contributed largely to the application of science to the improvement of highway construction. These methods are of so much interest that they are worthy of description.

In determining the suitability of a rock for the construction of a