The inefficiency of the system of maintenance by parish and statute labour was proved before the panning of the first Turnpike Act in 1G53; yet the necessity of improvement, and the obvious justice of maintaining roads by the produce of tolls, did not lead to the extensive adoption of the turnpike system for about a century after that time.
During the last sixty years the attention of government has been repeatedly directed to the importance of this class of public works, and the Highland and Holyhead roads, formed by Telford and others, did much in improving and extending the science of road-making. The Highland roads alluded to were made under the commission of 1S03, and originated in the military roads formed in consequence of the rebellions of 1715 and 1745, which had been found very beneficial to the districts to which they afforded the means of access. The made made and improved under the management of the Highland road Commissioners extend to more than I?00 miles, the whole being in a mountainous district, but so well laid out that their inclinations are always moderate. The works executed in the formation of these roads are very extensive, and comprise npwarde of 1100 bridges. The Holy road improvementa were commenced in ISIS, and in these Telford and his able ansintants had the opportunity of carrying into effect, under a government commission, a plan of reed-making suitable to a greet traffic, on principles generally considered to be nearly perfect. The principles on which these important works were executed are very fully detiiled by the late Lord Congleten, then Sir Henry Parnell, in his valuable ' Treatise on Roads; to which work the writer of this article is indebted for much of the following information. The name of McAdam must not be lamed over without notice In this place, as his exertions have done much towards attracting public attention to the Improvement of road; even where his peculiar prin ciples have not been acted upon.
Though much remains to be accomplished, and the philosophy of road-making is yet very imperfectly understood by a Large proportion of those to whom the care of the highways is committed, it is im psible to compare the post and prevent state of ramie without feeling grateful for their improvement, and observing in how great a degree that improvement has benefited the agricultural, commercial, and moral interests of the community.
Deriesasig a Line of Road ; Lard-works, d.e.—Thongh formerly little attended to, the design of the plan of a line of road is a subject which requires extensive knowledge and mature deliberation. It is often advisable to survey several different lines, in order to the @election of the one which, on careful comparison, appears to have the prepon 4- ranee of desirable qualities. To be theoretically perfect, a road should combine the qualities of straightness and level, and its surface should be smooth and hand; and the best road, practically, will be that which makes the best compromise between unavoidable deviations from this theoretical perfection. It may be observed, however, that although some writers speak of the absolute perfection of each of these qualities as essential to the idea of a good road, it may be questioned whether it be desirable of any, excepting of the first. Of these qualifications the first two belong to the design or laying out of the line, and the drat two to the execution of the road and the materials made use of.
The qualities of straightness and level, or the lice of direction and line of drowyni, should be very carefully adjusted to each other. Some remarks on this subject will bo found in the article Iloltwar, which apply equally to the laying out of common coals, though the proportionate retardation due to a given ascent is very different, owing to the great comparative resistance of a common road. Among the
circumstances that may anthorise a deviation from the straight line, are the power of obtaining imitable materials for the road, avoiding valuable property or difficult ground, and including towns or villages in the route.
It perms to be a prevailing opinion with modern engineers, that the line of direction bee not generally been mode as subordinate as it ahould be to the line of draught ; and it will be well to remember, in laying out a new road, that while the effect of gravity must ever remain the lame, the resistance occasioned by imperfection,' in the road and carriages will be reduced by every prospective improvement in their construction ; thereby Increasing the proportionate effect 01 gravity, and making the line of direction still inure enborclinnts to that of draught, or, in other wools, increasing the length of level road that may be traversed with the same expenditure of power as would raise the load up a given elevation. Curves increase the resistance to the motion of carriages, and add to the risk of accident ; but If slight, the increase the length of the road much less than might be supposed Edgeworth, In his c Essay on the Construction of Ronda and Carriages, says," A road ten miles long, and perfectly straight, can scarcely Is However desirable a perfect level may be in theory, a road with moderate inclinations, as of 1 in 100, is found to be preferable in practice, because without such a slope it is difficult to get rid of water fast enough, unless the road be raised a few feet above the surrounding land, and thereby exposed to the free action of sun and wind. Slight undulations are also considered, by most authors, to be desirable in all cases where animal power is employed; frequent changes in the nature of exertion being considered favourable to the horses. On this principle it is recommended that where an undulating road is reduced to a uniform gradient, occasional levels should be introduced to ease the draught. Any inclination exceeding the angle of repose, or that beyond which a carriage would roll clown by its own gravity, occaaions a loss of power ; but all below it are attended with a compensating effect when the traffic In both directions is taken into account; the advantage gained by descending carriages being equal to the additional labour required in the ascent. This angle has been stated by Lordlier to be about 1 in 40, with a good carriage on a broken stone road of the beat quality ; but the inclination allowed on the Holyhead road is 1 in 35, a slope which may be ascended at a good rate of speed, and descended at twelve miles an hour without risk. A greater slope not only occasions much additional resistance in the ascent, but, by ren dering it unsafe to drivo down at full speed, causes a loss of time in the descent also. Modern engineers consider it unadvisable in any case to exceed an inclination of 1 in 24, though there are hills at least twice as steep on some turnpike roads. The following table shows the effect of varium, inclinations in increasing the draught of it stage-coach at different velocities on the same description of road, as indicated by a dynamometer contrived by Mr. Macneill for experiments on the draught of carriages :—• It should always be borne in mind that the occurrence of one steep hill on a line of road affects the working of the whole stage, as the number of horsee required for ascending it must be need, although a portion of their power may be unemployed, oar the greater part of the road. The inconvenience of a steep inclination may be diminished by Laying a stone tramway for the use of ascending vehicles ; a ineaeure adopted with success on the iliolyhead road, where, on a slope of about 1 in 20, the power required to draw a ton was reduced by this means from 294 lb*. to 132 the.