Gravel footpaths are sometimes tarred over when thoroughly consolidated. This must be done only when the weather is quite settled and fine: the least rain will spoil the whole operation; it consists in simply tarring over the surface of the footpath in the same manner that a gate or wall or any other substance would be tarred. The surface of the path must previously have been swept perfectly clean, and immediately as the tarring is completed, fine stone dust must be sprinkled on its surface; the traffic should be diverted from it for a few hours, and it is then ready. Care must be taken that the tar is not too thin in consistency, and that the coat is not put on too thick. Treating a path in this manner saves gravel, which is washed or kicked off it if left with an ordinary surface; but a cold night, a slight shower, or inferior tar will make the whole process abortive, and the path will be in a fearful mess in the winter. [101] Concrete is especially necessary as a foundation for asphalte, as it has little or no power of resistance to vertical pressure in itself, and indentations in its surface would be very unsightly as well as hold water. The concrete should be perfectly dry and thoroughly set before the asphalte is laid on it. [102] _Vide_ ‘Proceedings of the Institution of Civil Engineers,’ vol. i. p. 6. [103] Trinidad bitumen is best, but it has to be refined before it can be used. This is done by cooking it with shale oil, then straining it and decanting it, which is a troublesome and tedious process, and there is great danger of fraud being practised. Good bitumen can be detected by its elasticity and softness when rolled between the finger and thumb, and also by its smell. [104] _Vide_ ‘Proceedings of the Institution of Civil Engineers,’ vol. xliii. p. 293. [105] _Ibid_, vol. xliii. p. 293. [106] A rule is sometimes made that York flags should be ¹⁄₂-inch thick for every square foot of surface, but they should never be less than 2 inches thick. [107] _Vide_ ‘Roads, Streets, and Pavements,’ by Q. A. Gillmore, p. 208. [108] Sawdust 2 feet in thickness has sometimes been used, well rammed and rolled. [109] The town of Brighton is an instance of this. Ordinary red bricks used to be laid as a footpath, no doubt to give a rural appearance for the eye of the jaded Londoner, but these are giving place to more modern materials. CHAPTER XII. KERBING AND CHANNELLING, ETC. For all footpaths both urban and sub-urban a kerb of some description is necessary to be fixed on the outside of the footpath, for the following reasons: (1.) It acts as a sill against which the material of which the footpath is paved may butt. (2.) It retains both the foundation and surface of the footpath. (3.) Whether there is a paved channel gutter or not, a kerb is necessary in order to finish the haunches of the roadway. (4.) The appearance of a footpath without kerbing is very unsightly.