The cost of foundations is included in this table, but their thickness is not mentioned; the excavation was done for the contractors.

With the one serious objection of slipperiness, compressed asphalte seems a most suitable material for the surface of a roadway, but that objection is of considerable weight when we reflect that the great object of roadways is that of "traffic," and it is for that purpose they are constructed; still, in cities where a heavy business traffic is going on, this class of roadway has so many advantages that where cheap horses are driven it might be used; where, however, valuable horses are used for pleasure driving, as in the west end of London and the corresponding better parts of cities, some other description of roadway should be maintained.

Mastic asphalte will be described in the chapter on footpaths.

Specimen Specification for a Compressed Asphalte Roadway.

*

Excavation and Concrete.—The excavation and concrete foundation may be specified to be executed in a manner similar to that contained in the specimen specification for wood paving,† except of course that the excavation will be shallower in this case.

Asphalte.-The asphalte to be used shall be the pure unadulterated natural rock known as the Val de Travers, and be unmixed with any foreign or other matter whatever. The rock after being properly broken, shall be ground in a Carr's disintegrator to a powder of such fineness, that not more than meshes to

per cent. shall be left on a sieve containing

the square inch and decrepitation by heat will not be accepted. This powder shall be heated to 240° F. or such other

* 46 "Lime concrete ruins compressed work." Vide' Asphalte and its Application to Street Paving,' by B. Ellice Clarke. 'Proceedings of the Association of Municipal and Sanitary Engineers and Surveyors,' vol. vi. p. 46.

See p. 94 ante.

temperature as shall be found desirable, so as to eliminate all moisture, and carefully transported to the street in covered iron carts, in order that not more than 20° F. of heat shall be lost in transit. The powder must be spread upon the concrete inches in thickness* and carefully raked so as to have regularity of depth and surface.

Ramming. The powder must then be rammed with iron punners of not less weight than 10 lb. heated so as to prevent the adhesion of the asphalte. The ramming must be done lightly at first, so as to ensure equality of thickness, and afterwards augmented to heavy blows. Where the rammers are not available a T tool must be employed.

To meet some of the objections to compressed asphalte as a material for roadways the "Imperishable Stone Paving Blocks" have been introduced in America; they consist of asphalte formed into rectangular blocks under pressure of about one ton to the square inch, these are laid close together without any grouting, and a pavement of this description is said to combine all the advantages of wood and asphalte, though sufficient time has not yet elapsed to prove this.

In Salford, Manchester, etc., I believe "Woodward's Patent Molten Ironstone Blocks" are used with some success where there is not any very exceptionally heavy traffic.

One of the principal reasons of durability in asphalte pavement is its elasticity, and it should be remembered that compressed asphalte does not begin to "wear" until all compression has ceased; this is the case with no other system of pavement-stone and wood both begin "wearing" from the day the traffic commences. Under ordinarily heavy traffic it may be estimated that it will take two years to complete the compression of asphalte, and the weight of a square foot of this pavement will at the expiration of that time be nearly the same as on the day it was laid, though the thickness is

*It must be ths more in thickness than that specified as finished.

reduced during the first two years as much as it will be in the following eight.

Much is said about the advisability of good and dry concrete, but it may be as well to explain the reasons that necessitate so much care in the foundation. First, it should be always borne in mind that asphalte pavement is nothing more than a tough" carpet," and has no power of itself of offering resistance to heavy traffic; consequently, if the substratum or concrete is not thoroughly solid and resisting, the weight of traffic will crush it, and the asphalte will at once give way in all directions. The concrete should be made strong enough to resist the traffic, and the asphalte is a simple covering to protect the concrete from direct contact with the wear and friction caused by the traffic. So much for the strength, but the dryness is of even still greater importance; for the best asphalte, laid by skilled workmen, on thoroughly first-rate but damp concrete, will rapidly go to pieces—a phenomenon takes place, which, although quite natural, is little realised by most. engineers. When the hot asphalte is laid, the water is immediately sucked up and turned into steam, which tries to escape through the heated powder, and the result is that although the surface of the asphalte is smooth, the mass is really disintegrated from underneath by its bitter enemy "water," and as soon as the surface begins to wear, the fissures formed by the passing of the steam appear on the surface and the whole pavement falls to pieces: thus accounting for some of the failures this description of roadway has met with under unskilled treatment.

This completes the subject of roadways; I will turn to that of footpaths in the next chapter.

CHAPTER XI.

FOOTPATHS.

As in the case of roadways, so with footpaths: the foundation is of primary importance, whatever material may be used for the surface. Where this material may be classed under the head of "Paving," concrete makes the best foundation. For gravel, tar paving, or other similar surface, a hard core bottom well drained is sufficient. The materials of which a footpath can be formed are almost innumerable, but the following may be given as embodying most of them :

(1.) Natural asphalte, compressed and mastic. (2.) Yorkshire flagging-Caithness flagging.

(3.) Blue lias, and Devonian limestone flagging. (4.) Concrete.

(5.) Bricks.

(6.) Granite slabs.

(7.) Artificial asphaltes, including tar pavement.
(8.) Gravel.

First on the list stands. natural asphalte, compressed and mastic.

The compressed has been thoroughly described in the preceding chapter; it is sometimes used for footpaths where there is a very heavy traffic, and answers admirably.

I will, however, now deal solely with mastic asphalte, which means the rock ground to powder, mixed with a certain proportion of bitumen to act as a flux, and then subjected to

* 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 The concrete should be surface would be very unsightly as well as hold water. perfectly dry and thoroughly set before the asphalte is laid on it.

heat; this is sometimes used in conjunction with fine sharp clean river sand, but more often with finely crushed stone about the size of peppercorns, and is styled "gritted asphalte."

As long ago as the year 1838, Mr. F. W. Simms speaks of asphalte mastic from Pyrimont, near Seyssel, and says "it may be considered a species of mineral leather"-a very good description of its surprisingly tough, hard, durable and pliant properties.

The usual method of the preparation of the mastic is as follows:

According to the amount of bitumen contained in the natural stone, from 5 to 8 per cent. of refined Trinidad bitument is placed in a large caldron which is usually provided with agitators driven by steam power; when this is thoroughly melted, the powdered asphalte is added little by little, the heat being raised to between 390° and 480° F., the mixture kept well stirred and "cooked" for about five hours. It is then turned out into iron moulds, most companies having a special pattern with a trade mark for this purpose.

The caldrons generally used contain from 1 to 2 tons of mastic.

Some companies in large towns are provided with caldrons on wheels, commonly called "Locomobiles," in which case the grit is mixed with the mastic in the fixed caldrons, and the whole mass run out into the locomobiles (which are also provided with agitators worked by an endless chain attached to the axle of the wheels), and transported direct to where the work has to be done. This system, though undoubtedly the best, is not practicable except in large towns; the more usual

* Vide' Proceedings of the Institution of Civil Engineers,' vol. i. p. 6.

† 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.