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thus, by destroying the momentum of it, add greatly to the labour of the horses in drawing it*
For this reason it will generally be prudent to deviate from the direct line in laying out a new road, if by doing so this sort of subsoil can be avoided, without adding much to the length of the road. But when the additional length of the road would be considerable, it will then be necessary to incur the expense of a proper drainage, and of forming a high embankment, as to compress and harden by its weight the moist and porous subsoil. Such an embankment, of 1,740 yards in length, having this object in view, was made over Maldreath Marsh, in the Island of Anglesea, on the new line of the Holyhead Road.
It will sometimes happen that road materials can be better obtained by carrying a line of road in one direction than in another. This will be a good reason for making a road deviate from the direct line, because the expense of making and repairing it will much depend on the distance which materials have to be carried.
It is necessary, in making a road through a hilly country, to take particular care to give it a proper
* The reference which will be made to the laws of motion in a subsequent chapter will show how extremely injurious elasticity is in increasing the labour of horses.
aspect. It is a great advantage to have a road on the north side of a valley fully exposed to the sun. For the same reason, all woods, high banks, high walls, and old fences ought to be avoided, in order that the united action of the sun and wind may have full power to produce the most rapid evaporation of all moisture. Too much attention cannot be bestowed on this object, in consequence of the effect of water in contributing to cut and wear down the hardest substances. It is for this reason that road materials, when they are wet or damp, wear rapidly away under the weight and pressure of heavy carriages. The hardest limestones wear away very quickly when wet, and all stones of an aluminous character, and also gravel that consists of flint, sandstone, or other weak pebbles.
The great advantage of having a road perfectly exposed to the action of the sun and wind, will be more accurately conceived, by reference to writers of science on evaporation. Dr. Halley states, that one tenth of an inch of the surface of the sea is raised per diem in vapour. He also says, that the winds lick up the water somewhat faster than it exhales by the heat of the sun. Other writers say, the dissipation of moisture is much accelerated by the agency of sweeping winds, the effects being sometimes augmented five to ten times.
Trees are particularly injurious, by not allowing the sun and wind to have free action on the surface of roads in producing evaporation. Besides the benefit which a road receives from drying rapidly, by an open exposure to the atmosphere, there is another of great importance, namely, that of affording to horses the advantage of free respiration; for it is well known that the power of a horse to perform work with ease, particularly when .moving rapidly, depends upon the quantity of cool and fresh air that he can pass through his lungs. If the cause of horses tiring or becoming ill under their work be carefully examined into, it will often be found that it is not that their muscles or limbs fail them, but their wind; and therefore it is particularly important to have a road so circumstanced, that a horse may, on all parts of it, have the benefit of a free current of air.
It may sometimes be proper to make a road deviate from a straight line, in order to go through a town ; but the expediency of such a deviation must wholly depend on the general object of the road. If it be intended to expedite the communication between two places of great trade, or otherwise of great importance, then nothing can be more erroneous than allowing the general line of road to be taken from the best and shortest direction in order to pass through a town. It is for this reason that little attention should be paid to the opposition of inhabitants of towns to new roads, to be made for the advantage of the general communication between distant and important parts of the kingdom.
Some persons may be disposed to say, that a road should be made to deviate from a direct line in order to avoid crossing parks, or demesnes, and, to a certain extent, no doubt it should; but this motive ought not to be allowed to have much weight, where the consequence is to force the road over an inconvenient ascent, or to add very materially to its length. It should be recollected, that, by judicious management, a road may be made, if not ornamental, at least not injurious or detrimental to the appearance or privacy of a park, by carrying it in hollow ground, or between sunk fences.
The principle of protection of private property is itself founded on the same principle that should govern the line of a road, and that principle is the public advantage; and therefore it should be laid down and acted upon as a general maxim, that private considerations ought in all cases to be made to give way, with respect to roads, to public convenience. "For let it be remembered that society is formed for the mutual and general benefit of the whole; and it would be a very unjust measure to incommode the whole merely for the convenience, or perhaps the gratifying of the whim or caprice of an individual."i
After fixing upon the general line of a road with respect to its direction, the precise line of it must be marked out, according to the smaller acclivities and declivities of the natural surface of the country it is to pass over, in such a manner that the cuttings shall furnish sufficient earth for the embankments. As moderate curves add but little to the length of a road, they will not be objectionable, if they assist the inclinations and save expense.
* Bateman on Roads, p. 122.
PRINCIPLES OF ROAD-MAKING.
In this chapter, the general principles will be examined, according to which the art of constructing a road should be practised; and the particular methods will be explained, by which various kinds of roads should be constructed. The art of roadmaking, like every other art, must essentially depend on its being exercised in conformity with certain general principles, and the justness of these principles should be rendered so clear and selfevident as not to admit of any controversy.*
It has been stated in the introduction, that the business of road-making has hitherto been almost entirely under the management of persons ignorant of the scientific principles by which it ought to be conducted. It is for this reason particularly necessary to show that it is a business of science. Until it shall be so considered, the numerous de
* "A knowledge of true principles is indispensably necessary in every art, and in that of making roads as much as in any other. Some preliminary species of knowledge is very necessary in every superintendant or surveyor. A beaten track of knowledge is but a bad guide in cases which very frequently occur, when, amongst several ways, the best is to be preferred."— M. S. Haldimand, Secretary to the Bailiwick of YverduD, on the Construction of Highways.