On the intermediate course is to be laid the surface layer of broken stone.[17] It must be 2¹⁄₂ inches in depth, and the stones must be practically uniform in quality, and as near an approach to a cube in form as possible. Each stone used in this layer must have passed through a 2¹⁄₂-inch circular hole, and all stones that are wedge-shaped, and do not approach uniformity of measurement on their sides, are to be taken from the road with properly shaped rakes, and no stones allowed to remain which are not sound, strong, and equable in size and quality of material. The stones are to be raked into an even layer, and the steam roller passed over them twice or thrice. After this a quantity of fine screened gravel or sand is to be thrown on and sufficiently sprinkled to moisten the mass without “licking up.” The rolling is then to be continued (working the roller backwards and forwards, gradually from the gutter to the crown), with an occasional light watering of the pavement, until the cross-section be exact according to specification, the interstices filled in, the roadway firmly compacted and solid, and all excess of binding removed from the surface of the finished pavement.[18] Telford’s object was the complete separation of the road metal from the subsoil by a firm and regular foundation, and this system has ever since held its ground. The advantages to be gained in constructing a roadway in this manner may be summed up as follows:-- (1.) Economy of construction, as a considerable quantity of metalling is saved; only 3 inches of properly broken stone and a little binding material being necessary, the foundation of the roadway (which really carries the traffic) may be made of a quality of stone unsuitable for road metal, or even of bricks or stones from old buildings that are being pulled down. (2.) The prevention of the rising up or “spewing” of the clay or other soft material on which the roadway rests. (3.) A solid foundation is secured which will successfully resist the weight and percussion of the traffic. (4.) The increased facility for the drainage of the roadway water being ruinous to it.[19] Instead of forming a paved or “pinned” foundation for macadamised roadways, sometimes what is called “hard core” is placed at the bottom of the road upon the surface formation. This “hard core” is made of very heterogenous materials, often the waste products of the house refuse depôt, and consists of ashes, old pots and pans, meat tins, old bottles, shells, and a variety of similar articles; sometimes the core is made of burnt ballast, but in no case does it make so good a foundation as stones set by hand.