588 PRACTICE OF AGRICULTURE. Paet III. 



3643. Artificial materials for roads are sometimes had recourse to, when stone or 

 gravel is not to be procured, and sometimes used because unfit for any thing else. 

 They are chiefly the scoria of founderies, dross, cinders, &c., to which may be added 

 burnt clay ; the last a very perishable material. It is burned in clamps like bricks, 

 and differs from them in being in irregular masses, and in not having been previously 

 worked. 



3644. Chambers's substitute for road metals, or for gravel in gardening, is nothing more than vitrified 

 clay, loam, marl, or any other earth that will not fall to powder or burn to lime. The material is intended 

 to be burnt in a temporary kiln, to be erected by the side of the road about to be made or repaired ; the 

 earth may be taken from the side drains. The kilns are to be of about six yards in width, and of any 

 length : a stratum of dried earth is to be laid about two feet in thickness, between two layers of com- 

 bustibles, so as to turn to a vitrified state the greater portion of the earth so enclosed. The principal part 

 of the contents of the kiln will then be in lumps, which are to be separated from the dust or powder ; and 

 such vegetable matter as produces alkali may be burnt with the other materials, to assist the vitrification : 

 salt, barilla, potash, or soap-ashes also, if they can be procured at a small expense, maybe employed for the 

 same purpose. The dust unavoidably produced, or remaining from the above described burning of clay, 

 &c having been separated from the vitrified matter, is first employed to damp or extinguish the fire, and 

 afterwards, though not applicable to road-making, becomes a valuable material, and may be appropriated 

 for dressing land. {Newton's Journal, voL i. p. 354.) 



3645. The preparation of materials relates chiefly to their proper size or weight, and 

 cleaning from earthy matters. 



3646. Breaking the materials evenly is a point, Marshal observes, on which very much 

 depends ; for by doing this, the wear of the road becomes regular. Where the heads 

 of large stones rise above the general surface, they become obstacles to carriages, and 

 stumbling-blocks to horses : beside their tending, by the jolting motion which they give 

 to carriages, to indent the surface on either side of them ; and thus to increase the rough- 

 ness, and hasten the decay of the road. 



3647. The proper size of road stones requires much latitude. Not only the intended 

 use of the road, but the nature of the material, is to be considered. A road for broad- 

 wheeled carriages of burthen only, may be made of larger stones than one for narrow 

 wheels ; and hard stones require to be broken- smaller than those which more readily 

 wear down and form a travelable surface. For when once the surface of the materials 

 becomes united and cemented together, and its rock-like texture established, the stones 

 that are crushed, and the smaller fragments which are splintered off", in wear, serve to 

 encrust and bind together the stratum of stones which lie next in succession beneath : 

 especially if proper attention be paid to the irregularities of wear, and to bring back the 

 surface, wherever it is requisite, to its original evenness of convexity ; so that it may, in 

 every part, act as an arch, and may be able to resist, with the greatest firmness, the 

 weight with which it may be impressed. 



3G48. In forming and repairing roads with stones of large size, a considerable share of 

 the expense arises fi'om the labour of reducing the materials ; and, in consequence, the 

 smaller they are broken, the greater becomes the expense. This, on ordinary occasions, 

 is a serious consideration. Hence, in constructing and repairing common roads, it is 

 advisable, instead of reducing the surface stones to small fragments, with the hammer, 

 at a great cost, to cover them with materials that are already reduced; as the rubbish 

 of stone quarries, soft stones or gravel, or the scrapings of the road to be repaired. 

 Such cementing materials being washed and worked down, by rains, and the action of 

 carriages and the feet of travelling animals, among the surface stones, assist much in 

 binding and fixing them in a firm crust, and in making the road immediately passable by 

 horses and light carriages ; most particularly, if the whole be compressed and united 

 together, by a heavy roller (suitable to the purpose) repeatedly passed over the surface. 

 Such is Marshal's opinion ; how much it differs from M'Adam's and Paterson's cannot 

 but be remarked by the reader. 



3649. The size of stones^preferred by Edgeworth is not specifically mentioned ; but on 

 bogs he would lay stones of six or seven pounds' weight : he elsewhere observes that no 

 stones larger than an inch and a half in diameter should be left on the surface of the road. 



3650. The size which Walker approves of he has not given in very definite terms ; 

 and his observation as to the foundation acting by an arch is, in our opinion, erroneous. 

 He says, " Where whin or other stone is to be used, the size of the pieces into which 

 it is broken should decrease as we approach the surface the superficial coating not ex- 

 ceeding a cube from one inch to one inch and a half. If the foundation is bad, breaking 

 the bottom stone into small pieces is expensive and injurious, upon the principle I have 

 above described, and also for the same reason that an arch formed of whole bricks, or of 

 deep stones, is to be preferred to one of the same materials broken into smaller pieces ; 

 for in some counties the materials will admit of the foundation of the road being con- 

 sidered as of the nature of a flat arch, as well as of being supported by the strata directly 

 under it." 



3651. The size of metals, according to Paterson, should be diflferent for the upper and 

 under surfaces of roads ; and both should be regulated according to the situation of the 

 road, and the nature of the ground over which it is formed, " Such small broken 



