Book II. FORM AND MATERIALS OF ROADS. 529 



8396. Breaking the materials evenly is a point. Marshal observes, on which very miich 

 depends. For by doing tliis, 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 roughness, and hasten the decay of the road. 



3397. The proper sizes of road stones requires much latitude. Not only the in- 

 tended use of the road, but the nature of the material, is to be considered. A road 

 for broad- wheeled carriages of burden, 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 succes- 

 sion, beneath : especially if proper attention be paid to the irregularities of wear, and 

 to bring back tlie 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. 



3398. Informing and repairing roads, with stones of size, a considerable share of the 

 expense arises from the labor 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 ad- 

 visable, 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, with 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. ' 



3399. The size of stones preferred bi/ 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 inch and a half diameter should be left on the surface of the road. 



3400. The size ivhich 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 coat- 

 ing not exceeding 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 considered as of the nature of a flat arch, as well 

 as of being supported by the strata directly under it. 



3401. The size of metals, according to Patcrson, should be different 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 metals as are most proper for a road formed on a sloping bank, or on a very 

 dry bottom, would be quite improper for a road that is perfectly level, and is much 

 subjected to dampness. In the former case, even six or eight inches deep of such 

 metals will make a good road ; but in the latter case, twelve or fourteen inches will 

 be found inadequate. In the former case, too, the metals should be of such a size 

 as may fill and pass through a ring from two to two inches and a half in diameter ; 

 and in the latter cases, they should not be under three inches ; as under that size I 

 have never found them to make a durable road in such situations. Every road that 

 has more than eight inches (deep of metals, should have the half of these in the bottom 

 broken considerably larger than those on the top. If the road, however, has a dry 

 hard bottom, there is not so much need for this ; but if the bottom is soft and wet, it 

 is of the greatest service in making a firm road, and preventing the raetids from sink- 

 ing: and the softer the bottom, the larger, of course, they should be." But it is to 

 be remarked, that the same author in his Letters, ^c. published three years afterwards, 

 says, <' In my former treatise I proposed, where the bottom was soft, to have the 

 under course of stones a little larger than those at top. This I have seen of service, in 

 several cases : but my mode of draining, which should never be neglected, supersedes this 

 entirely. 



M m 



