Book II. ROADS OF ENGINEERS. 523 



swelling up, become quite loose and open. This is a natural consequence, where 

 the material is not thick, and where the soil under the road is not perfectly dry. But 

 where a road is dried in the way described, it will be uniformly seen, that the water, 

 instead of spewing out on the return of a thaw, is sucked in by the drains, so leaving 

 the surface of the road quite dry. It may be observed, at such times, that the 

 places of the road where a few roods of such drain had been introduced, presented to the 

 eye, at a quarter of a mile distance, quite a contrast to the other parts of the road, the 

 one opaque and dry, from the moisture being sucked in the other all wet and glister- 

 ingjjrum its being thrown out to the surface. {Patersons Letters, &c. 44. 48. 84.) 



3359. IVie surface-drains, or water-tables, should be made a few inches lower than the 

 side of the road, and of the common width of a spade at the bottom, and they should have 

 frequent cross drains under the path and fence, back into the outer side drain. 



3360. irater-tables across the road become requisite in some cases, as in flat roads on 

 a steep slope. Tliese should always be made at right angles to the road, with their sides 

 gently sloping, to occasion as little obstruction to carriages as possible. In some few 

 cases, where roads are liable to floods, or are deficient in drainage, these surface-tables 

 may require to be made of a considerable breadth and paved ; in this case Greig {App. to 

 Strictures on Road Police, p. 219.) directs to Tay six feet of the bottom of it flat, and 

 twelve feet on each side, to rise at the rate of one inch in the foot, which will make the 

 depth one foot ; and from the size, no carriage will feel any jerk or shake in passing it. 

 The pavement should be made of hammered stones, of nearly equal depth, each stone 

 from nine to twelve inches long on the surface,' and four to eight inches broad, and nine 

 inches to a foot deep ; the under-side to be flat in the under-face, and not of an irregular 

 or angular under-surface, as in that case it would not be solid. (^Appendix to Greig's 

 Strictures, p. 219.) 



3361. Bridges and embankments of different degrees of magnitude, are required in all 

 lines of road of any length or variety of surface. The subject of large bridges we leave 

 to the engineers ; no department of their art having attained higher perfection, of which 

 the wonderful erections by Telford, in almost every mountainous district in Britain 

 may be referred to as proofs. We confine ourselves entirely to such stone arches as 

 may be designed by road-surveyors, and built by country masons. In many cases, cast- 

 iron might be substituted for stone with economy and advantage as to water-way ; but 

 though the principle of constructing both cast and wrought iron bridges is perfectly 

 simple, the execution, and especially the putting up, requires more skill, and is attended 

 with much more risk than the erection of either stone or timber bridges. 



3362. One loio arch is in general the most desirable description of common road- 

 bridge. But most of the country bridges, as Clarke observes, consist of several small, 

 high, semicircular arches : where there is a single arch, the stream passes without inter- 

 ruption ; if there are two or three in the same situation, the space through which the 

 water is to pass is necessarily contracted by the width of the piers. Ice, and large bodies 

 carried down by floods, frequently stop up the small arches, and the accumulated water 

 carries away the bridge,^; but if such accidents should not happen, the constant currents 

 rushing against those piers wash out the mortar, loosen the stones, and very soon under- 

 mine the work, if it is not extremely well put together, which is seldom the case. Unless 

 the river or stream is narrow, or the banks very high, a semicircle is an inconvenient shape 

 for an arch ; it has been adopted on account of the insufficiency of the abutinents, and 

 because the pressure is more perpendicular ; but scientific engineers in all countries, now 

 construct their bridges with wide openings, and make the arches either semi-ellipses, or 

 segments of large circles so that the space above the highest floods is comparatively 

 little, and the ascent over the bridge inconsiderable. In country bridges in Ireland, 

 Clarke continues, the foundations are invariably, and often intentionably, defective : 

 the mason considers himself an honest man, if his bridge lasts seven years ; whereas, 

 from the durability of materials in this country, it ought to endure for ages. Whatever 

 is under water is out of sight, and is generally composed of loose stones, thrown 

 promiscuously together, on which the masonry is erected, and all the pains and ex- 

 pense are bestowed on the cut-ivaters and wings, when the heaviest stones, and those 

 accurately jointed, ought to be laid in the foundations. The greatest attention should 

 be paid to the quality of the materials : the stones should be large, and laid in level 

 courses, in the best mortar, composed of sharp sand, free from loam, and quicklime, 

 accurately mixed together ; the coping of the parapet is generally so slight, that it is 

 broken down as soon as built, and the entire parapet quickly follows; it ought to be 

 of large heavy stones, roughly hammered, and there should be substantial quoins at the 

 ends of the parapets with an immovable stone over them. 



3363. Arches not exceeding eight feet span may be semicircular; tunnels not exceeding 

 eighteen inches wide, may be covered with strong flags, and either flagged or paved 

 imder, and there ought to be across either end a deep long stone, sunk below the surface 

 of the current, and under the walls, to prevent the water from undermining the work j 



