Book II. ROADS OF VARIOUS ENGINEERS. 595 



retain water, which, by the traffic of the road, would render the substratum, in al! such 

 places, a mass of mud ; and the whole would become bad in proportion to the traffic, the 

 subsoil, and the climate. Marshal is equally wrong in his directions for forming farm- 

 roads, by filling the wheel-tracks with hard materials. In depositing these, he says, the 

 largest and roughest are to be thrown to the bottoms of the wheel-trenches, as found- 

 ations for the hardest, which ought to receive the immediate pressure of the wheels, the 

 softest and finest being disposed of in the horse-track. It is evident the continual action 

 of the wheels in the same rut, aided by the water which must infallibly lodge there, 

 would soon work up the larger and rougher stones, and render the traction more oppres- 

 sive than if no metals had ever been laid there. 



3685. Telford's mode of disposing of the materials of roads is as follows : — Where a 

 road has no solid and dry foundation, it must be constructed anew. Upon the eighteen 

 centre feet of it stones must be put, forming a layer seven inches deep. Soft stones will 

 answer, or cinders, particularly where sand is prevalent. These bottoming stones must 

 be carefully set by hand, with the broadest end down, in the form of a close neat pave- 

 ment ; the cavities should be filled with stone chips, to make all level and firm, and 

 no stone should be more than five inches broad on its face. Over its bottoming of stones 

 or cinders, six inches of stones, of a proper quality, broken of a size that will, in their 

 largest dimensions, pass through a ring of two and a half inches' diameter, must be laid. 

 The six feet of the road, on each side of the eighteen centre feet (making thirty feet), 

 when formed of a proper shape, may be covered with six inches of good clean gravel, or 

 small stone chips. 



3686. Xo covering or mixture of any sort is added to the material by Edgeivorth, except 

 clean angular gravel, that may insert itself between the interstices of the stones ; but no 

 more should be used than what will thus sink to a level with the surface. If the whole 

 were covered with gravel, it would be impossible to discover the defects of the road, till 

 it might be too late. No stones larger than an inch and a half in diameter should be suf- 

 fered to remain on the road ; where much inaccuracy in this respect is suspected, an iron 

 ring may be employed as a gauge. In all cases, after the road has been covered with 

 stones, it should be carefully examined, and every stone that is too large should be picked 

 off to be broken smaller. 



3687. The preference generally given to gravel, Paterson considers to be greater than it 

 deserves, and that the earth obtained from the sides of the road, free of expense, will not 

 only barely answer the purpose, but in most cases equally well ; and that on a perfectly 

 dry bottom, it is questionable whether it should not even be preferred to gravel. It is in 

 winter only, and on wet ground, that I consider gravel entitled to any preference what- 

 ever. (Treatise, fyc. p. 43.) 



3688. The mode of laying on gravel, according to Walker, "is to lay it on as it comes 

 from the pit, except the upper foot, or eighteen inches or so, which is screened : 

 but in all cases, whether the material is gravel or hard stone, the interstices between the 

 pieces should be filled up solid with smaller pieces, and the finishing made by a thin 

 covering of very small pieces, or road-sand or rubbish ; for those interstices must be filled 

 up before the "road becomes solid, either in this way or by a portion of the materials of 

 the road being ground down, which last mode occasions a waste of the material, and 

 keeps the road unnecessarily heavy and loose. In the original making or effectually 

 repairing of a road, it is, I think, best that the whole of the proposed thickness be laid on 

 at once, for the sake of the road as well as of the traveller ; the materials of the road then 

 form a more solid compact mass than when they are laid in thin strata at different times, 

 for the same reason that a deep arch of uniform materials is preferable to a number of 

 separate rings." Laying on a stratum of unsifted gravel, under a sifted stratum, is 

 rather at variance with the doctrine of " a deep arch of uniform materials ; " and 

 it seems to us, that when a stratum of properly broken stones are to be powerful lv 

 rolled, the previous filling up of their interstices with very small matters might 

 counteract the effect of rolling, in squeezing the angular stones into the angular 

 interstices. 



3689. The mode of laying on gravel by M'Adam is that of scattering with a shovel, 

 and never emptying down cart or barrow-loads on the middle of the roadway, as is 

 generally practised. He completes the stratum by three separate layers ; leaving the 

 first to be consolidated by wheels, and in some cases a heavy roller, before he lays on 

 the second ; and the second, in like manner, before he lays on the last. 



3690- A covering from four to five inches thick, according to Fry, forms a bed or mass, 

 which is proof against the severe crush of heavy wheels ; while in the case of a very thin 

 covering, the stones lying bare upon a hard road, and receiving in this unprotected 

 state the stroke of every wheel that passes over them, like the thin covering on a mill- 

 bed, they are quickly reduced to powder, and disappear. Stones in a thick bed are 

 protected from the immediate destructive grind ; while stones that are thinly laid on are 

 instantlv reduced to powder, either by pressure or grinding. 



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