In arranging the works necessary for obtaining the required level, the preference should be given to embankments; and, wherever it is practicable, the bed of the road should lw elevated two feet above the natural level, for the sake of efficient drainage. Tunnels are very rarely introduced on common roads, being very costly, and. when of considerable length, inconvenient from their darkness. When the road is in an excavation, the aide-slopes should never be steeper than two horizontal to one vertical, and it is desirable to have those on the south aide three to one; because, though many materials will stand at eteeper inclinations, it ie essential to the preservation of the road, and the comfort of horses travelling upon it, that the sun and air should have free access to its surface. Where stone can be readily procured, the erection of walla at the bottom of the slopes gives a neat and finished appearance to the road, and prevents earth, which may be loosened from the sides, from falling into the side channels or drains. The Archway road affords an example of the great difficulties that oftaitorsally attend a deep etc ivation, owing to the accumulation el water; the remedy for which is described hereafter. Where em lenteente are required. strong fencing is especially necessary to guard against the occurrence of accidents. Some of the roads formed by 'relied aro conducted across deep valleys by bridges or viaducts of ;neat magnitude, in order to maintain the desired level without the incouremence and expense of large earth embankments'.
In old roads' the bridges erected for the passege of rivers are frequently made much smaller than is advisable, so that the level of the read is made too low, and the water is impeded by the contracted arches to such a degree as to occasion much damage during floods. Modern engineers, by adopting bolder dimension for the bridges, and forming raised approaches, avoid these inconveniences, and secure their nude from the risk of obstruction by floods. The raising of the road wherever it 1431101 through marshy or low land is a very necessary measure. Many old reach+ still in WO are sunk several foot below the surface of the ground, because they hare originally been exposed to the destructive action of water, and the materials thus softened have been ground into mud and cleared away, until, by the repetition of these operations, the roads have been converted into deep trenches, ehich are frequently flooded in winter. Of the extent to which this process has been occasionally suffered to go on, an idea may bo formed from the statement of Edgeworth, that " the stag, the hounds, and the horsemen have been known to leap over a loaded waggon, in a hollow way, without any obstruction from the vehicle." In conducting it road through a mountainous district, in addition to numerous bridges for the purpose of crossing ravines (for which purpose suspension bridges have been occasionally applied, as in the passage of the Metuti strait), embankments between retaining walls of stone, sad walls to support the road along the face of a precipice, aro frequently necessary. Some works of the latter character have excited much admiration. If the slope of a precipice be only six inchea horizontal to a foot vertical, such a road may be formed by building a wall thirty feet high, hexed on steps cut into the rock, and cutting into the rock to the depth of ten feet on a level with the top of the wall, the apace between which and the face of the precipice is filled in with earth or stone. By this means a platform twenty-five feet wide is
obtained. Many works of this character have been executed by Telford and other engineers, in various parts of Scotland, iu the High Iand made, and those forming the communication between Edinburgh and London : and others, the boldness of which commands universal admiration, occur in the great mountain-passes of the Simplon and Mont Canis, which form imperishable monuments of the talent and energy of the engineers of Napoleon, by whom they were executed.
\Then the works are completed to the proper level for receiving the bed materials that form the surface of the road, the earth should be famed into the intended width and a nearly level surface, the foot path or paths being elevated a few inches above the bed of the carriage-way. Thirty feet is the ordinary width of the carriage-way, exduaive of footpaths, of the Holyhead road ; but the propriety of reducing the width, in most places to twenty-four or twenty-five feet, his been suggested. This width may be more or less exceeded in the vicinity of large towns, according to the amount of traffic, but should be exactly adhered to in other situations, as uuiformity in this particular greatly improves the appearance of a road, and also contributes to economy, both as to the land and materials, and the cost of maintenance. Some engineers recommend that the bed should be made convex, in the same degree as the finished surface of the mad; but it is quite flat in the 1 lolyhead road, by which means a greater depth of materials is allowed in the centre than at the sides of the road. Much has been said on the subject of the best form for the transverse section of a road. Formerly it was common to make it very convex, often to a degree that was highly dangerous, with the Ides of throwing off water; but this notion is very fallacious, bemuse if a road be allowed to wear into ruts, no degree of convexity that can be given is sufficient to keep it dry ; while, if the surface be drool, a very moderate slope is sufficient to carry off water, and a steep inclination will cause it to run with such velocity as to wear away the road materials. Another disadvantage of too great an inclination is, that, by throwing the weight of a carriage on one side, the vehicle inself is injured, and the overloaded wheels cut up the road more than necessary. Some have gone so far in opposition to this practice as to advocate perfectly flat or even concave roads, in favour of both of which much may be said ; but the general practice of modern road makers is to make the surface slightly convex. In Telford's roads the convexity is elliptical, the fall being ludf an inch at four feet from the centre, two inches at nine feet, and six inches at fifteen feet. It has been recommended to form the cross section into three flat planes, that in the centre being horizontal, and the others slightly inclined from it Very narrow roads are often sloped in one direction only, like one-half of a convex road ; and roads on the face of a steep hill are occasionally treated in this manner, the surface water being con ducted towards the hill, and carried off by drains under the road. This plats has the advantage of checking any tendency in carriages to roll or turn over towards the least protected side of the road.