Another description of foundation now very extensively used where the traffic is heavy, is that known as “Bituminous Concrete,” which is made as follows: The ground being excavated to the proper depth and contour, broken stone as for macadam is spread for a depth of 6 or 9 inches; this is then levelled and thoroughly rolled with a light roller, a boiling mixture of pitch and tar, or creosote oil is then poured over the whole surface until every interstice is filled, when a thin layer of small broken stone is spread upon it, and then well rolled until it consolidates. It may be well to observe here that in all works involving concrete foundations and paving in streets, the traffic should be entirely stopped if at all possible. Streets paved half at a time are never quite satisfactory, and the concrete should have at least a week to set before the pavement is placed upon it. Upon a foundation of either Portland cement or bituminous concrete, the granite setts themselves should be grouted with a bituminous mixture instead of cement or lime grouting. This renders the pavement more impervious to moisture, makes it less noisy, and adds considerably to its strength; the mode of applying it is nearly similar to that of ordinary grouting. The setts are placed on about an inch of sand and well rammed, the boiling mixture is then poured over the whole surface, which is then covered with a thin coating of small, sharp gravel. The following table of the proportions necessary for the bituminous mixture may here be of use. PROPORTIONS FOR BITUMINOUS MIXTURE.[64] Creosote Pitch. Tar. Pitch. oil. For grouting in pavements 1 to 1 or 3 to 1 For foundations or lower layer 3 to 1 or 3¹⁄₂ to ¹⁄₂ of asphalte macadam For upper layer of asphalte 2¹⁄₂ to 1¹⁄₂ or 3¹⁄₄ to ³⁄₄ macadam and for foot paths The objections to this method of paving are only temporary: the nuisance arising from the fumes of the boiling mixture whilst it is being applied, and the necessity for dry weather to make the operation successful. Healey’s Patent Pitch Boilers[65] are said to moderate, if not entirely to do away with the former, and the latter can be arranged by only doing the work at favourable seasons, or if the worst come to the worst, to cover the work with tarpaulins raised on trestles. Before closing this chapter I should like to draw attention to the question of provision for wheel tracks, or tramways paved with stone, asphalte, or other hard material, and a track for horses giving a firmer foot-hold, similar to those so highly spoken of in Milan and other Italian cities. An excellent description is given of them by Mr. P. le Neve Foster, Jun., in an appendix to a report on the Application of Science and Art to Street Paving and Street Cleansing of the Metropolis (1872).