Book II. ROADS OF VARIOUS ENGINEERS. 583 



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. 



3613. 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 

 under, 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 ; 

 if the stones are square and heavy, those small conduits may be built without mortar, 

 except at the ends. 



3614. In building tunnels or arches across a road in a flow-bog, great pains must be taken 

 with the foundation, or the whole structure will inevitably sink : the building of those 

 should be deferred as long as possible, till the peat has subsided, and has obtained a 

 tolerable consistence ; then make an opening equal to the whole work, and sink it eigh- 

 teen inches below the intended bottom of the arch or gullet ; collect a quantity of black- 

 thorn bushes, and tie them in faggots of the same size ; place these in regular courses in 

 the direction of the road, and lay across them a platform of strong plank three inches 

 thick, the whole length and width of the intended mason work; on this build your arch, 

 and make an allowance in the height of the abutments for sinking. Wherever walls are 

 necessary to support banks, and prevent their crumbling down upon the road, if large 

 even stones can be procured, they will not require any mortar ; when mortar is used, there 

 ought to be a great many apertures in the work to give vent to the water, otherwise the 

 pent-up moisture from behind will push out the wall. In many cases, where embank- 

 ments can be made of earth and sods, they are to be preferred to masonry, which is ex- 

 tremely expensive at the commencement, and very perishable ; for mortar soon loses its 

 cementing quality, when exposed alternately to frost and damp. 



3615. Draining the site of a road on ajlow-bog, according to Clarke, is a tedious oper- 

 ation, and often requires some years. A single drain at each side will not be sufficient, 

 as the water from the adjacent moss would fill it up as fast as it was made. Lay out the 

 road here sixty feet wide, which will allow for the banks when the whole shall be finished ; 

 make a drain at each side six feet wide, and at a distance of fifteen or twenty feet more, 

 parallel drains of the same width. If the interval between the parallel drains be after- 

 wards cut away regularly for fuel, it will tend still to the condensation of the moss. 



361 6. Open, drains, in the case of ground liable to sink or to moulder down by frost, ought 

 to be made very much sloped on the sides, especially the side next the road, otherwise, 

 after repeated scouring out, the road will be found to have sunk at the sides; — a very 

 common case, and highly injurious in the case of narrow roads. Whenever this tendency 

 to sink is observed, it should be made up by the scrapings of the road, or by other mate- 

 rials. Roads made over bogs and artificial mounds are particularly liable to sink at the 

 sides, which should be immediately counteracted to prevent the bad consequences. 



3617. Fences along the sides of roads are essential in all enclosed countries; and all 

 engineers and road-makers agree that they should never be allowed to rise of a greater 

 height than what is necessary for a fence. To give free admission to the sun and air by 

 keeping the fences low, Marshal considers as providing an unexpensive, yet most accurate, 

 method of cleaning roads — incomparably more so than washing or scraping. The legis- 

 lature, Edgeworth observes, has limited, in several instances, the height of hedges to five 

 feet ; but this limitation is neglected or evaded. Even were it strictly adhered to, it 

 would not be sufficient for narrow roads : the hedges would be still too high ; for it is 

 the sweeping power of the wind which carries off dust in dry weather, and which takes 

 up moisture in wet. In fact, roads become dry by evaporation ; and when they are ex- 

 posed to the sun and wind, the effect of heat and ventilation is more powerful than any 

 surface drainage that could be accomplished. 



3618. Walker observes, that the advantage of having the hedge next the road consists in its 

 greater safety to the traveller, particularly if a ditch of any considerable depth is necessary, 

 and in the hedge being supported in its growth from the ground under the road, without 

 drawing upon the farmer's side of the ditch. 



3619. The fences, Telford observes, form a very material and important subject, with 

 regard to the perfection of roads; they should in no instance be more than five feet in 

 height above the centre of the road, and all trees which stand within twenty yards from 

 the centre of it ought to be removed. I am sure that twenty per cent, of the expense of 

 improving and repairing roads is incurred by the improper state of the fences and trees 

 along the sides of it, on the sunny side more particularly : this must be evident to any 

 person who will notice the state of a road which is much shaded by high fences and trees, 

 compared to the other parts of the road which are exposed to the sun and air. My 

 observations with regard to fences and trees apply when the road is on the same level as 

 the adjacent fields: but in many cases, on the most frequented roads of England, more 

 stuff has been removed from time to time than was put on ; the surface of the road is 

 consequently sunk into a trough or channel from three to six feet below the surface of 



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