Book II. ROADS OF ENGINEERS. 525 



tlie communication of fields by gates ought to be carefully managed, so as not to injure 

 the public road, the footpath, the water table, or the inner drain. All gates should open 

 inwards to the fields, and not to the road. 



3371. That plantations (f trees should not be made close to roads all are agreed. What 

 the distance ought to be, must depend on the elevation of the country, the soil and sub- 

 soil, the breadth of the road, its direction, whether the plantation is to be made on the 

 north or south side of the road, its thickness, kind of tree, &c. An elevated situation is 

 always more exposed to the wind than a level or hollow ; and a dry soil and subsoil will 

 always, other circumstances being the same, have a favorable effect on the roads which 

 pass over them. A broad road, and one winding in its direction, has chances of the 

 direct influence of the sun and wind, according to the obliquity of its angles : a road 

 running north and south, though planted closely on both sides, will enjoy the sun during 

 a part of every day in the year ; one running east and west, planted on the south side, 

 with trees forty feet high, will enjoy no sun but through the interstices of the branches 

 during the three winter months. Supposing the average height of the sun from ten to 

 two o'clock during these three months to be 20 degrees, then a tree forty feet high will 

 throw a shadow every day during that period, upwards of 100 feet long, which may shew 

 that no plantation should be made nearer the south sides of roads than 80 or 100 

 feet. On the north-east and west sides, they may be nearer, according to the elevation and 

 natural tendency to dryness of the site, and also taking it into consideration whether the 

 trees are evergreens, and with or without underwood. The least injurious trees are 

 single rows trained to high stfems, properly pruned in, or foreshortened, 



3372. The preparation of the base of a road, for the reception of the metals or hard ma- 

 terials, is a matter of primary importance. Marshal, Edgeworth, and some other writers, 

 with almost all practical men, seem to have entertained much less enlightened notions 

 on this subject than M' Adam. 



3373. Marshal's preparation consists in striking off tiie protuberances, and filling up 

 the hollow parts : the footpath and the higher side of the soft road being raised with the 

 earth which is required to be taken off the bed of the hard road ; whose base or founda- 

 tion ought to be formed with peculiar care. Every part is required to be firm and sound : 

 dry earth, or hard materials, being rammed into every hollow and yielding part. In a 

 dry situation, as across a gravelly or stoney height, little more, he says, is required than 

 to remove the surface mould, and lay bare the rock, or bed of gravel, beneath it : and, 

 then, to give the indurate base a round or a shelving form, as the lying of the ground 

 may require. In this way, a travelable road may be made, and kept up, at one-tenth of 

 the expense incurred by the ordinary practice in this case ; which is to gather up the sur- 

 face-soil into a ridge, and, on this soft spongy bed, to lay, coat after coat, some hard ma- 

 terials, fetched perhaps from a distance. 



3374. A soft bed is now found by far the best, and M'Adam has proved in the case of 

 part of the road between Bridgewater and Cross, that a stratum of hard materials covering 

 a morass will last longer than a similar stratum laid on rock . indeed it may be questioned 

 whether a properly made road on a bog, which yields by its elasticity, will not last longer 

 than one on a firm surface. We have been told by a gentleman of some experience in 

 road-making, that in Ireland this is actually found to be the case. " Precisely," as Fry 

 observes, * for the same cause that a stone placed upon a wool-pack would bear a greater 

 pressure before it would be broken, than it would if placed on an anvil. " [Essay on Wheel 

 Carriages, ^c App. 129.) 



3375. Covering the base if an unsound road with faggots, branches, furze or heath, is 

 recommended by Edgeworth. Flat stones, he adds, if they can be had, should then be 

 laid over the faggots, and upon them stones of six or seven pounds' weight, and, lastly, a 

 coat of eight or ten inches of pounded stone. If the practicability of consolidating a mass 

 of stones of six or eight ounces weight and under each, so as to act as one plate or floor- 

 ing, be admitted, then the faggots and flat stones must at least be useless, and the stones of 

 six or seven pounds weight injurious ; because whenever the upper stratum had worn down 

 a few inches, some of these stones, and eventually the greater number, Avould be worked 

 up to the surface, and the road destroyed or put in a state to require lifting, breaking, and 

 relaying. 



3376. A basement of trees, bavins, or bushes, is made use of by Walker when the ground 

 is very soft. They carry off the water previous to the materials of the road, being so 

 consolidated as to form a solid body, and to be impervious to water. Bushes are, how- 

 ever, not advisable to be used, unless they are so low as always to be completely moist. 

 When they are dry and excluded from the air they decay in a very few years, and produce 

 a sinking in place of preserving the road ; a thickness of chalk is useful for the same 

 purpose in cases where bushes are improper, the chalk mixing with the gravel or stones 

 becomes concreted, and presents a larger surface to the pressure. 



3377. The base of the road is constructed by Telford of an elliptical form; if it is upon 

 clay or other elastic substance, which would ictain water, he would recommend to 



