The almost incredibly bad state of the roads in England
towards the latter part of the 17th century appears from the
Engnsh accounts cited by Macaulay (Hist. c. iii.). It was due
roads, chiefly to the state of the law, which compelled each
1°3.“' parish to maintain its own roads by statute labour, but
°e"t'"7' the establishment of turnpike trusts and the maintenance
of roads by tolls do not appear to have 'effected
any great improvement. At the time of Arthur Y0ung's six
months' tour in 1770 the roads would seem to have been almost
as bad as ever, and it is doubtful if there was much improvement
up to .the beginning of the 19th century. The turnpike roads
-were generally managed by ignorant and incompetent men until
Telford and McAdam brought scientific principles and regular
system to their construction and repair. The name of Telford
is associated with a pitched foundation, which he didnot always
use, but which closely resembled that which had been long in use
in France, and the name of .McAdam often characterizes roads
on which all his precepts are disregarded. Both insisted on
thorough drainage and on the use of carefully prepared materials,
and adopted a. uniform cross section pf moderate curvature
instead of the exaggerated roundness given before; but, while
Telford paid' particular attention to a foundation for the broken
stone, McAdam disregarded it, contending that the subsoil,
however bad, would carry any weight if made dry by drainage
and kept dry by an impervious covering. McAdam was engaged
more with the repair of old roads than with the construction of
new ones, and, though it is not possible to agree with all his
doctrines, the improvement which he effected in road management
and maintenance was great and lasting.
Construction of Roads.—A road should be as short as possible between two points to. be connected, but straightness must often be sacrificed to avoid difficulties and expense and to secure good gradients. The latter should be as easy as practicable, having regard. to the country-to be traversed, and it is desirable that there should be a ruling gradient than which none should be steeper. On the level macadamized road in ordinary repair the force which thehorse has to put forth to draw a load may be taken as one-thirtiet»h> of the load. But in going uphill the horse has also to lift the load, 'and the additional force to be put forth on this account is very nearly equal to the load drawn, divided by the rate of gradient. -Thus on a 'gradient of 1 in 30 the force spent in lifting is one-thirtieth of the load, and in ascending a horse has to exert twice the .force required to draw the load onaflevel. In descending, on the other hand, , on such a gradient, the vehicle, when once started, would just move of itself without pressing on thehorse. A horse can without difficulty exert twice his usual force for a time, and. can therefore ascend gradients of 1 in 30 on a macadamized surface without sensible diminution of speed, and can trot freely down them.' i1These considerations have led to 1 in 30 being generally considered as the ruling gradient to be aimed at on, first-class roads, though 1 in 40 has been advocated. Telford adopted 1 in 30 as the ruling gradient on the Holyhead road through North Wales, and there are only two gradients steeper, in-places where they were unavoidable. All unnecessary rises and falls should be avoided, but a dead level is unfavourable for drainage, and on this account 1 in 100 to 1 in 150 is the fiattest gradient that is desirable. Such slight rises and falls are probably rather favourable than otherwise to ease of draught by horses. In transverse section, roads in the United Kingdom generally consist of a carriage-way, with spaces on each side, on one or Cross both of which there may be a footpath, and fences and ditches. The width of the carriage-way may be from 1 5 ft., which allows of the easy passage of two vehicles, to 30 or 50 ft. for roads of importance near towns. The side spaces may be-from 4 or 5 to-8 or IO ft. wide; wide sides give the sun and air access to the road, and tend to keep it dry, and also afford space for the deposit of road materials and scrapings; Incuttings or on embankments the transverse section has of course to be modified. The road surface should have just enough convexity to throw the wet off freely, and a very moderGradient. section.
ate amount is sufficient when a good surface is maintained; On a too convex road the traffic keeps to the middle, and wears ruts which retain the water, so that the surface is not so dry as with ai flatter section which allows the traffic to distribute itself over the whole width. Telford used' a cross section differing slightly from an arc of a circle in being more convex in the .middle than at the sides. I. Walkerrecommended two straight lines joined in the middle of the road by a curve, and inclined about 1 i n 24 towards the sides, the objection' to which is that theviiat sides are liable to wear hollow. On the whole a curve of the form of a flat ellipse is the best, ” the rise in the curve from the sides to the centre need not exceed one-fortieth of the width, and one-sixtieth is generally enough on Well-kept roads. It is generally best to obtain the requisite convexity by rounding the formation surface or seat of the road and giving a uniform thickness to the coating of stone, but often, especially in country roads where the traffic is not very heavy and keeps mainly to the centre, the formation is made level and the convexity is obtained by using more road material at the centre than the sides. . When there is not a kerb there should be a -F' shouldering ” of sods and earth on each side to keep the road materials in place, and to form with the finished surface the water tables or side channels in which the surface-drainage is collected, to be conveyed by outlets at frequent intervals to the side ditches. The outlets are open cuts through the sides or drains beneath the footpaths.. The side ditches should be deep enough thoroughly to drain the foundation of the road, and cross or mitre drains under the road communicating with the side ditches may be required in wet soil. A thorough drainage of the subsoil is of the greatest importance, and it is economical in the end to go to considerable expense to secure it. Ina cutting, or where there are no side ditches, the surface water may be taken off by gratings and under drains beneath the side channels.; ~ <
r M zwadam Roads.-The thickness to be givento a road made altogether of broken stone will depend on the traffic it is intended for.' On a good well-drained soil a thickness of 6 in. will make an excellent road for ordinary traffic, and McAdam's opinion that 'IO in. of well-consolidated material was sufficient to carry the heaviest 'traffic on any substratum if properly drained has proved to be generally correct. In a new road the loss of thickness during consolidation- must be allowed for, and the materials should be laid about one-half thicker than the coating is intended to be. When the materials are not rolled, a thickness of 3 to 6 in. should be laid first, and when that has partly consolidated under the traffic othericoats may be added to make up the full thickness. There is great wear and waste of the materials in consolidating if they are laid too thickly at once. Inferior material is sometimes used in the lower part of the road coating, especially when the surface is to be of granite or other hard expensive stone. Thus flints or gravel may be used for the lower 5 or 6 in. of a road to be coated with 3 or 4 in. off granite. Telford covered the broken stone of new roads with 1% in. 'of gravel to act as a binding material. McAdam absolutely interdicted the use of any binding material, leaving the broken stone to work in and unite by its own angles under the traffic.-If
the ideas of the inventor are strictly followed, macadam, when the fine network of joints is thinly masked with hardened mud worn from the stone, comes near to a perfect surface. But stones that will pass through a ring of a given size may be twice as much in length, and unless their form is about that of a cube not exceeding ri in. on its longest side, they cannot be rammed or rolled into the regular mosaic characteristic of the 'true macadam. The best' modern roads are of hand broken stone dressed slightly on the surface with stone chips, while the mass of the road-metal is kept free from any kind of binding. Some road makers, however, have found /the large irregularly shaped stones from the machine so difficult to consolidate that they have had to reconsider the question of binding. The engineer of Central Park, New York, found that, with- the greatest care and attention to rolling, such stones would not
