Another description of paved road, the origin of which is commonly referred to the Romans, is the ehaussie, or roughly-paved causeway used In the principal highways of France and some other parts of the Continent. This kind of *road has been much recommended for its durability when well made, but, unless laid with a degree of care that would render It too expensive for general adoption, it causes a very unpleasant awl fatiguing jolting. In such roads the pavement tonally covers only a part of the breadth of the road, leaving the sides avail able for the 1110 of the light carriages in dry weather ; and it has been suggested, that where the width of the roadway would allow, it might prove advantageous to form, in all great roads, a track of pavement or hard broken stone for winter use, and another of inferior materials for summer, both to save the wear of the hard mad and increase the comfort of passengers. Such an arrangement is convenient in the principal approaches to great towns, where it is considered best to have the pavement ate the sides, that carters may walk either on or near the footpaths, and that footpassengera may not be incommoded by the dirt of the metalled road.
In Holland, pavements of brick, which are also probably derived from the practice of Roman engineers, are used, not only for footpaths, but also for the peerage of heavy carriages, which run on them with great facility. The bricks used for this purpose are thin, and well bedded in lime.
Common stone pavements are, by most writers, divided into two classes: rubble causeway, in which the stones are of irregular shape, and very imperfectly dressed with the hammer ; and dressed causeway, which is formed of stoues of larger size accurately squared and dressed. In both kinds the excellence of the pavement depends greatly on the firmness and evenness of the bed, and the careful fitting of the stones to each other, which may be accomplished with very irregular stones by judicious selection. If one stone be left a little higher or lower than these adjoining it, or if it become so in consequence of defective bedding, the jolting of carriages in isissing over the defective place will quickly damage the pavement ; the wheels acting like a rammer in driving the depressed stones deeper into the earth, while the derange ment of the lateral support that each stone should receive from those adjoining it, occasions the dislocation of the pavement to a consider able distance, and the consequent working up of the earth through the disturbed joints. Defective joints form another fruitful source of injury and inconvenience both to the pavement itself and to the vehicles jolted over it. If, as is often the case in inferior pavements, the edges of two adjoining stones do not meet with accuracy, narrow wheels will have a tendency to slip into the joint, and by doing so, to wear the edges of the stones, till, as may be frequently seen, the surface of each stone is worn into a convex form that renders the footing of horses insecure, and causes the motion of vehicles drawn rapidly over them to consist of a series of bounds or leaps from one stone to another, accompanied by a degree of lateral slipping highly injurious to the carriage, while the irregular percussion produced tends greatly to the destruction of the pavement, In order to procure a firm foundation, and to prevent earth from working up between the stones, it is advisable in the first instance to form a good carriage-way of gravel or broken stone, and to allow it to be used by carriages till consolidated, before laying the pavement. This plan is stated by Edgeworth, in his' Essay on the Construction of Roads and Carriages,' 1817, to have been practised successfully by Major Taylor, of the Paving Board, in some pavements in Dublin, and it is strongly advocated by more recent road-makers. Where broken stone is laid to a considerable depth, it should, as in the case of metalled roads, be applied in thin layers, each being separately worked into a compact state. In streets of very great traffic, it is a good plan to lay a sub-pavement of old or inferior atones, bedded on broken stone, as a foundation for the surface pavement, a measure which has been practised with advantage in Paris ; but of late years it has been the custom in London to form the bed of paved roads by means of a layer of gravel, perfectly clean, which is rudely converted into a species of concrete by floating the surface with lime-water. The bed of
the pavement should be formed into a slight convexity, the elopes being about 2 inches in 10 feet. A thin coat of gravel or sand laid immediately under the paving blocks is of use in filling up slight irregularities in their shape, and enabling them to form a com pact bed.
For the paving stone. hard rectangular blocks of granite are pre ferred, though whinstone, limestone, and even freestone, may be used. Guernsey granite, as shown by the table in a previous column, appears to be the most durable, kut it is more liable to become inconveniently smooth than some stones of inferior hardness, such as the Mount Sorrel or Aberdeen ',frank°. The stones may vary, according to the traffic, from 6 to 10 inches deep,6 to 18 inches long, and 4 to 18 inches wide; but It is very essential that the depth of all the blocks in one piece of pavement should be alike, and that where the width is unequal, the stones be so sorted that all used in one course should be uniform in this particular. The accurate dressing of the stones is a point often too little attended to; and an injudicious mode of forming contracts for paving, in which the payment has been by the square yard of paving laid, has, in connection with the effect of competition in bring ing prices below the remunerating point, led to the use of stones in which the base is smaller than the upper surface, and which, when laid, scarcely come in contact with each other except at their upper edges. In some pavements the stones are made smaller at the top than the bottom, the joints being filled up with stone-chips, concrete, or an aephaitie composition; and in those of the more common construction the aides of the stones are occasionally hollowed, so as to receive a small quantity of gravel or mortar, which serves as a kind of dowelling. Itamnung the stones with a heavy wooden hammer is a practice that has been much recommended, and it is considered that a more efficient application of the process, by means of a ramming-machine, or portable monkey, would remove some of the defects arising from imperfect bedding ; but when the stones are well laid, and bedded in strong mortar, as time best recent pavements are, a few blows with a wooden maul of about 14 pounds weight are sufficient to fix them firmly in their place. Grouting with lime-water poured all over the pavement facilitates the binding of the whole together, and fills up the joints, so as to effectually prevent the working up of the substratum. The blocks are commonly laid in rows across the road, the joints in each row being different from those of the adjoining ones ; but pavements of superior smoothness have been laid in courses stretching diagonally across the street, by which means all the joints are leased over by carriages with greater ease, This arrangement is particularly desirable at the Intersection of streets. as it ilimiribffies the risk of horses slipping. Longitudinal courses are objectionable on account of the tendency of narrow wheels to enter the joints. In paving steep inclinations, it is well to use narrow stones, on account of the number of cross.joints ; or, if large stones be used, to cut deep furrows across their surface, to afford secure footing. A plan of loving for such situations, which has been foiled very effectual, is represented in the annexed diagram, in which the stones are so inclined as to present a series of steps. The chief objection to this plan seems to be the jolting caused to carriages, which produces so deafening a noise that, in one instance, such a pavement was taken up at the request of the inhabitants of the street. Many patents have been procured for plans of forming stone pavements in wluch the pressure of carriages might be simultaneously distributed over several stones, by various contrivances for dovetailing and other wise fitting the stones together ; but such pima are generally too com plicated, requiring an accuracy of formation that would be ve7 expensive, owing to the hardness of the stone. Thin blocks of atone, bedded in asphalte, have been tried, and appear to make a good lavement.