ceive also a layer of new material laid on to a sufficient thickness to make the outside of the roadway nine inches lower than the centre, by giving a slight convexity to the surface from the centre outwards. A coating of clean coarse gravel, one inch and a half thick, termed a binding, is spread over the surface, and the roadcovering is then ready to be thrown open to travelling.

The stone used for the pavement may be of an inferior quality, in hardness and strength, to that placed at the surface, as it is but little exposed to the wear and tear occasioned by travelling. The surface-stone should be of the hardest kind that can be procured. The gravel binding is laid over the surface to facilitate the travelling, whilst the under stratum of stone is still loose; it is, however, hurtful, as, by working in between the broken stones, it prevents them from setting as compactly as they would otherwise do.

If the roadway cannot be paved the entire width, it should, at least, receive a pavement for the width of nine feet on each side of the centre. The wings, in this case, may be formed entirely of clean gravel, or of chippings of stone.

For roads which are not much travelled, like the ordinary cross roads of the country, the pavement will not demand so much care; but may be made of any stone at hand, broken into fragments of such dimensions that no stone shall weigh over four pounds. The surface-coating may be formed in the manner just described.

657. In forming a road-covering of broken stone alone, the bed for the covering is arranged in the same manner as for the paved bottoming: a layer of the stone, four inches in thickness, is carefully spread over the bed, and the road is thrown open to vehicles, care being taken to fill the ruts, and preserve the surface in a uniform state until the layer has become compact; successive layers are laid on and treated in the same manner as the first, until the covering has received a thickness of about twelve inches in the centre, with the ordinary convexity at the surface.

658. Where good gravel can be procured the road-covering may be made of this material, which should be well screened, and all pebbles found in it over two and a half inches in diameter should be broken into fragments of not greater dimensions than these. A firm level form having been prepared, a layer of gravel, four inches in thickness, is laid on, and, when this has become compact from the travel, successive layers of about three inches in thickness are laid on and treated like the first, until the covering has received a thickness of sixteen inches in the centre and the ordinary convexity.

659. As has been already stated, the French civil engineers

do not regard a paved bottoming as essential for broken-stone road-coverings, except in cases of a very heavy traffic, or where the substratum of the road is of a very yielding character They also give less thickness to the road-covering than the English engineers of Telford's school deem necessary; allowing not more than six to eight inches to road-coverings for light traffic, and about ten inches only for the heaviest traffic.

660. If the soil upon which the road-covering is to be placed is not dry and firm, they compress it by rolling, which is done by passing over it several times an iron cylinder, about six feet in diameter, and four feet in length, the weight of which can be increased, by additional weights, from six thousand to about twenty thousand pounds. The road material is placed upon the bed, when well compressed and levelled, in layers of about four inches, each layer being compressed by passing the cylinder several times over it before a new one is laid on. If the operation of rolling is performed in dry weather, the layer of stone is watered, and some add a thin layer of clean sand, from four to eight tenths of an inch in thickness, over each layer before it is rolled, for the purpose of consolidating the surface of the layer, by filling the voids between the broken-stone fragments. After the surface has been well consolidated by rolling, the road is thrown open for travel, and all ruts and other displacement of the stone on the surface are carefully repaired, by adding fresh material, and levelling the ridges by ramming.

Great importance is attached by the French engineers to the use of the iron cylinder for compressing the materials of a new road, and to minute attention to daily repairs. It is stated that by the use of the cylinder the road is presented at once in a good travelling condition; the wear of the materials is less than by the old method of gradually consolidating them by the travel; the cost of repairs during the first years is diminished; it gives to the road-covering a more uniform thickness, and admits of its being thinner than in the usual method.

661. Materials and Repairs. The materials for broken-stone roads should be hard and durable. For the bottom layer a soft stone, or a mixture of hard and soft may be used, but on the surface none but the hardest stone will withstand the action of the wheels. The stone should be carefully broken into fragments of nearly as cubical a form as practicable, and be cleansed from dirt and of all very small fragments. The broken stone should be kept in depots at convenient points along the line of the road for repairs.

Too great attention cannot be bestowed upon keeping the road-surface free from an accumulation of mud and even of dust. It should be constantly cleaned by scraping and sweeping. The

repairs should be daily made by adding fresh material upon all points where hollows or ruts commence to form. It is recominended by some that when fresh material is added, the surface on which it is spread should be broken with a pick to the depth of half an inch to an inch, and the fresh material be well settled by ramming, a small quantity of clean sand being added to make the stone pack better. When not daily repaired by persons whose sole business it is to keep the road in good order, general repairs should be made in the months of October and April, by removing all accumulations of mud, cleaning out the side channels and other drains, and adding fresh material where requisite.

The importance of keeping the road-surface at all times free from an accumulation of mud and dust, and of preserving the surface in a uniform state of evenness, by the daily addition of fresh material wherever the wear is sufficient to call for it, cannot be too strongly insisted upon. Without this constant supervision, the best constructed road will, in a short time, be unfit for travel, and with it the weakest may at all times be kept in a tolerably fair state.

662. Cross dimensions of roads. A road thirty feet in width. is amply sufficient for the carriage-way of the most frequented thoroughfares between cities. A width of forty, or even sixty feet, may be given near cities, where the greater part of the Transportation is effected by land. For cross roads, and others of minor importance, the width may be reduced according to the nature of the case. The width should be at least sufficient to allow two of the ordinary carriages of the country to pass each other with safety. In all cases, it should be borne in mind, that any unnecessary width increases both the first cost of construc tion, and the expense of annual repairs.

Very wide roads have, in some cases, been used, the centre part only receiving a road-covering, and the wings, termed summer roads, being formed on the natural surface of the subsoil. The object of this system is to relieve the road-covering from the wear and tear occasioned by the lighter kind of vehicles during the summer, as the wings present a more pleasant surface for travelling in that season. But little is gained by this system under this point of view; and it has the inconvenience of forming during the winter a large quantity of mud which is very injurious to the road-covering.

There should be at least one foot-path, from five to six feet wide, and not more than nine inches higher than the bottom of the side channels. The surface of the foot-path should have a pitch of two inches, towards the side channels, to convey its surface water into them. When the natural soil is firm and

sandy, or gravelly, its surface will serve for the foot-path; but in other cases the natural soil must be thrown out to a depth of six inches, and the excavation be filled with fine clean gravel.

To prevent the foot-path from being damaged by the current of water in the side channels, its side slope, next to the side channel, must be protected by a facing of good sods, or of dry

stone.

As it is of the first importance, in keeping the road-way in a good travelling state, that its surface should be kept dry, it will Бе necessary to remove from it, as far as practicable, all objects that might obstruct the action of the wind and the sun on its surface. Fences and hedges along the road should not be higher than five feet; and no trees should be suffered to stand on the road-side of the side drains, for independently of shading the road-way, their roots would in time throw up the road-cov ering.

See Note B., Appendix.

RAILWAYS.

663. THE great resistance offered to the force of traction on common roads, where the traffic is of a heavy character, naturally suggested the idea of trying other means, which would afford a more even and durable track for the wheels than the road-coverings in ordinary use. Various methods have been resorted to, with greater or less success, to accomplish this object: in some instances tracks have been formed of long narrow stone blocks; in others, heavy beams of timber, covered on the surface with sheet iron to protect them from wear, have been used; and finally, both the stone and wooden ways were replaced by iron plates and bars, and that system of road-covering, now so well known as the railway, or railroad, has been the result.

For these successive stages of improvement, by which, in the short period of less than a quarter of a century, so great a revolution has been made both in the speed and the amount of transportation on land, by means which bid fair to supersede every other, the civilized world is indebted to England, in whose mining districts the railway system first sprung up.

664. A railway, or railroad, is a track for the wheels of vehicles to run on, which is formed of iron bars placed in two parallel lines and resting on firm supports.

665. Rails. The iron ways first laid down, and termed tramways, were made of narrow iron plates, cast in short lengths, with an upright flanch on the exterior to confine the wheel within the track. The plates were found to be deficient in strength, and were replaced by others to which a vertical rib was added under the plate. This rib was of uniform breadth, and of the shape of a semi-ellipse in elevation. This form of tramway, although superior in strength to the first, was still found not to work well, as the mud which accumulated between the flanch and the surface of the plate presented a considerable resistance to the force of traction. To obviate this defect, iron bars of a semi-elliptical shape in elevation, which received the name of Fig. 157-Represents a cross section a, of the fish-bellied rail of the Liverpool and Manchester Railway, and the method in which it is secured to its chair. The rail is formed with a slight projection at bottom, which fits into a corresponding notch in the side of the chair b. An iron wedge c is inserted into a notch on the opposite side of the chair, and confines the rail in its place.

edge rails, were substituted for the plates of the tramway. The cross sections of these rails were of the form shown in Fig. 157,