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A difficulty may arise from the breadth of the river requiring a bridge of extraordinary dimensions, or from the land for a considerable distance on the sides of the river being subject to be covered with water to the depth of several feet in floods.
directions given to him. I was, however, deceived in this expectation.
The plan prescribed by the council did not permit, in the ascent from the bridge of Ganther to the summit of the Simplon, any inclination exceeding five inches in seventytwo. M. C, however, caused to be constructed three pieces of road, in a zig-zag form, which, while they increased the length of the road to the extent of 2169 yards, far from moderating its acclivity, occasionally increased it, a serious error in works of this nature. He acted in this case on his own responsibility; relying, perhaps, on that of his chief of brigade. As a consequence, it became necessary to mount and traverse some difficult ground, to form enormous excavations, worse than useless, because dangerous from the snow which would collect in them, a circumstance most to be avoided, and to carry the level of the road under the glaciers; all consequences of the original error committed at the base, in having ascended too rapidly, instead of having ascended by one regular inclined plane, rising five inches in seventytwo, as had been decided on in the original plan, and thus reaching the summit of the Simplon and the Hospice, by the course at once the most gentle in its ascent, and the noblest in its length. This piece of work, on which about 300,000 francs had been squandered, is likewise due to M. C—, who in two leagues and a half of road has committed, as we have seen, several serious blunders, in spite of all my efforts to oppose him.
In these cases it may appear, upon accurately calculating and balancing the relative inconvenience and expense of endeavouring to keep a straight line and of taking a circuitous route, that upon principles of security, convenience, and expense, the circuitous course will be the best.
In general, rivers have been allowed to divert the direct line of a road too readily. There has been too much timidity about incurring the expense of new bridges, and about making embankments over flat land to raise the roads above the level of high floods.
These apprehensions would frequently be laid aside, if proper opinions were formed of the advantages that arise from making roads in the first instance, in the shortest directions, and in the most perfect manner. If a mile, half a mile, or even a quarter of a mile of road be saved, by expending even several thousand pounds, the good done extends to posterity, and the saving that will be the result in annual repairs and horse labour, will, before long, pay off the original cost of the improvement.
BOGS AND MARSH GROUND.
The elastic nature of all bogs and marshes, and of all boggy and bottom land, makes it im
possible to form a road of perfect hardness over a soil of this kind, unless a great deal of labour and expense is applied in draining the soil, and afterwards compressing it, by loading it with large quantities of earth embanked upon it, in order to destroy the elasticity of the subsoil.
Although the surface coating of a road over such a subsoil may be made with a great abundance of the hardest materials, and be perfectly smooth, the porous and moist texture of the subsoil will cause the road to yield to a carriage passing over it; and 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 it. 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 so high an embankment, as to compress and harden by its weight the moist and porous subsoil. Such an embankment, of 1740 yards in length, having this object in view, was
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.
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 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 mate
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 referring 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 its 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 powers 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