Mr. Stevenson’s specification of the Regent Road in Edinburgh is fuller, and is in the following terms:—
“The cross section (shown in [Fig. 12]) of the metalled road to be the same in all respects as that already described for the causewayed roadway. But the cross section is to rise from the interior brows or slopes of the paved channels to the centre of the roadway, at the rate of 1 in 25. The bottoming of the road is to be of broken stones from the excavated matters of the Calton Hill works; the pieces of stone not to exceed five or six lbs. in weight; to be laid by hand in a compact manner to the depth necessary for preparing the road for the upper strata, viz., a layer or stratum of clean sharp sand four inches in thickness, laid all over the surface, and forming a bed for the upper or road-metal stratum, which is to be seven inches in thickness, and to consist of broken stones taken from the quarries of Salisbury Crags, or the lands of Heriot’s Hospital, as may be finally agreed upon. The road-metal is to be broken into pieces of such dimensions as to pass freely through a screen, to be provided by the Commissioners, the meshes of which shall not exceed one inch and a half square. The whole to be finished with a ‘top-dressing’ of sea-gravel, in such a manner that none of the road-metal shall appear on the surface of the roadway when it is completed.”
These extracts, so far as I have been able to discover, contain the earliest proposals and precise specification of the construction of road now known by the familiar name of “macadamising,” and I dismiss the subject with the following candid quotation from Mr. Stevenson’s Memoranda, in which he says:—
“It may be well to notice that in 1811 I specified road materials of the size as nearly as may be of road-metal, which afterwards became what is called ‘macadamised roads.’ I am not sure if I was before Mr. M’Adam in this respect; at all events he had the great merit of introducing the system of smooth roads. When I first proposed this method, I think, to the Trustees of Marykirk, they objected to it upon the score of expense.”
As regards the iron cart-tracks suggested for trial by Mr. Stevenson in his report to the Edinburgh Road Trustees, already quoted, he subsequently matured his views and described them in the article “Roads” in the Edinburgh Encyclopædia, where he proposed to use stone tracks as a “smooth and durable city road,” which he describes as follows:—
“The individual component stones of the wheel-tracks, hitherto, very partially in use, extend from three to four feet in length, are about ten or twelve inches in breadth, and eight or ten inches in depth. The stones of the tracks recommended by me, on the other hand, are of a cubical form, measuring only from six to eight inches in the lengthway of the track, and twelve to fourteen inches in depth, eighteen inches in breadth at the base, and twelve inches at the top or wheel-track. The stones are therefore proportionate in all their dimensions, for unless they contain a mass of matter corresponding to their length, they will be found to want strength and stability. It would hardly be possible to keep slender stone-rails in their places, and hence the chief benefit of a connected railway would be lost. On the other hand, very large materials are difficult to be got, and are also more expensive in carriage and in workmanship than stones of a smaller size. The Italian wheel-tracks are composed of stones two feet in breadth, and of various lengths. To lessen the risk of horses falling, these broad stones are kept in a rough state, by occasionally cutting grooves with a pick-axe upon their upper surface. A mode of paving with large blocks of granite, chequered or cut in this manner, has been tried in some of the streets in London. In order, however, to give pavement of this kind the necessary stability, the blocks would require to have their dimensions equally large on all sides, the expense of which would be too great. But cubical stones of the size now recommended may be procured at a moderate price, and throughout a great range of country; while the tracks, if properly laid, will actually be more stable than if blocks of larger dimensions were employed. For we may notice that a carriage-wheel rests or impinges even upon a less surface than one inch of its track at a time, in the course of each revolution round its axis; hence, it may be conceived to produce a kind of compensating effect, connected with the use of small stones, which prevents the tremor from being communicated beyond the limited sphere of each particular block, and, consequently, extending only a few inches. This system of paving I originally proposed for the main street of Linlithgow, forming part of the great western road from Edinburgh to Stirlingshire, and a correct idea of the proposal will at once be acquired by examining [Fig. 13]. By using tracks of this description—giving the stones a proportionally broad bed, and laying them upon a firm foundation (which is indispensable)—we should have our streets and the acclivities of our highways rendered smooth and durable, avoiding the expense and inconvenience of the common road, and also the irksome noise and jolting motion of the causeway.
“The tracks may be formed of granite, whinstone, or any of the hard varieties of rock capable of being hammer-dressed.”
Fig. 13.—Section for Road Metal. Section for Causeway.
Specimens of these stone tracks were laid in Edinburgh, in terms of Mr. Stevenson’s specification, on South Bridge Street opposite to the College, and in the Pleasance, and a third specimen was laid by the Road Trustees on Liberton Hill, which still remains after a lapse of half a century.
Subsequently to this Mr. Walker laid similar tramways in the Commercial Road, London, and as is well known, they have been pretty largely used in the principal towns in Italy.