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rapidly away, and the expense of maintaining it in order by scraping and laying on materials will be very much increased.
The great fault of most roads in hilly countries is that after ascending a considerable height, they constantly descend again before they gain the summit of the country which they have to traverse. In this way the number of feet actually ascended is made many times more than would be the case were no height, once gained, lost again.
An instance of this defect is observable in the line of old road across the island of Anglesea, on which a horse was obliged to ascend and descend 1283 perpendicular feet more than was found necessary by Mr. Telford, when he laid out the present new line, as shown by the annexed table: —
Another instance may be observed in the road from South Mimms to Barnet. The old road ascends three rather steep and long hills, while the new road avoids, almost entirely, two of these hills, at the same time that it is shorter by 638 yards.
In tracing a road across a deep valley between two hills, it should be earned in a direction opposite to the fall of the valley, as by so carrying it, that is, by crossing the valley at the highest practicable point, the descent and ascent are diminished.
Thus, in going from A to B, across a valley, if it be found by levelling that in a straight line the valley is too deep to make an embankment at a reasonable expense, the surveyor should try a line, A C B, higher up the valley, rather than in the direction A D B, where he would get into a lower level (see Plate I. Fig. 2.). Although this ought to be the general principle, instances may occur, where a valley may be crossed with more advantage down stream; as, for instance, if the sides of a valley contract considerably, it may require much less embankment to raise the road to the same height, than if it were carried higher up the valley; see Plate I. Fig. 3., by which it appears that it would be more advisable to take the line A D B, than either the straight line A B, or the line A C B, higher up the valley.
Another instance where a valley may be crossed with more advantage down stream, is where detached or insulated hills are situated in the valley below the straight line of direction, as represented in Plate I. Fig. 4. Here it would be proper to pass the valley lower down, to take advantage of the intervening high ground, as will be seen by the section, in which it is evident that much less embankment will be required in the line A D B, than in either the direct line A b B, or the line A c B, higher up the valley. Lately, when it was proposed by the Parliamentary Commissioners of the Holyhead Road, to improve the valley of the Geese Bridge, between Towcester and Daventry, on the road from London to Birmingham, six different surveys and plans of doing so were made. The report on these surveys is given in note A, more fully to explain the rules for crossing valleys. In many situations, particularly in mountainous countries, it will be found necessary to pass valleys or deep ravines by means of high arches of masonry, as in some parts of Scotland, where Mr. Telford has erected several great works of this description; of these, the most remarkable are the bridges over the Mouse Water, at Cartland Craigs, on the Lanark Road, represented in Plate I. Fig. 5. The bridge over Birkwood Burn, near Lesmahago, on the Glasgow Road, represented in Plate I. Fig. 6., and the Fidlor Burn Bridge, on the Lanark Road, represented in Plate I. Fig. 7.*
The suspension bridge over the Menai Straits, in North Wales, is of a similar character, for, besides its use in passing these straits, it has improved the road by its being no longer necessary
* To this list may be added the Dean Bridge over the Water of Leith at Edinburgh, which is above 100 feet high, and consists of four arches of 90 feet span; and a bridge at Pathhead, on the Coldstream road, of five arches of 80 feet span.
to descend to the level of the water. See Plate I. Fig. 8.
In most cases, however, valleys may be crossed by high embankments of earth, such as the chalk hill embankment near Dunstable, and that near Chirk, in North Wales.
In some situations it may be advisable to pass through a hill by means of a tunnel, instead of by deep cutting.
There are three works of this kind on the Simplon Road. One of them, "la grande galerie de Gondo" is 240 yards in length, 8£ in breadth, and the same in height. There is a similar work at Puzzuoli, near Naples, which is nearly half a league long; it is fifteen feet broad and as many high.
The peculiar circumstances of a river may render it necessary to deviate from a direct line in laying out a road.
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.
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 t ircuitous 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 in annual repairs and horse labour that will be the result 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 impossible 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