3311. All crossings, intersections, and abuttings of roads, should be made at right angles, for the obvious reason of facilitating the turning from one road to the other, or the more speedily crossing. Where roads cross each other obliquely, or where one road abuts on another at an acute angle, turning in, or crossing, can only be conveniently performed in one direction.

3312. In laying out a road over a hill or mountain of angular figure and considerable height, much practical skill as well as science are requisite. In order to preserve a moderate inclination, or such a one as will admit of the descent of carriages without locking their wheels, a much longer line will be required than the arc of the mountain. In reaching the summit or highest part to be passed over, the line must be extended by winding or zig-zagging it along the sides, so as never to exceed the maximum degree of steepness. This may occasion a very awkward appearance in a ground plan, but it is unavoidable in immense works. If a hill, 50 feet in perpendicular height (fig. 454.),

has an arc (a, b, c), or would require 150 feet of road (a, b, c) to go over its summit in a straight line, then to pass over the same hill on a road rising at the rate of two inches in six feet (the slope of the Simplon road), would require a length of 600 feet. If this length were extended in a straight line (d, b, e) on each side, it would require an enormous mound, and an immense expense; but by being conducted in a winding direction (b), up the hill on one side, and down the other, the same end is gained at a moderate cost. Such works shew the wonderful power and ingenuity of man; and perhaps no example exists where this is so displayed in road-making as in the case of the Simplon.

3313. In laying out a road towards a river, stream, ravine, or any place requiring a bridge or embankment, an obvious advantage results from approaching them at right angles; and the same will apply in regard to any part requiring tunnelling or crossing by an aqueduct, &c.

3314. In tracing out winding railroads, or such carriage roads as are only to be metalled in the horse track and paths of the wheels, some management is necessary in the case of quick bends. Where the line is straight, the horse path ought to be exactly in the middle between the wheel tracks. But, where the road winds, and most especially at a quick bend, the horse track ought ever to incline toward the outer side of the curve; by which the wheels will be uniformly kept on the middles of the supports prepared for them. Hence, it is advisable to dig the trench for the horse path (fig. 433 a.), first; and to draw a carriage for which the road is intended, with the horses walking in this middle trench: thus marking out, by the impressions of the wheels, the precise middle lines of the outer trenches, in every part of the road, from end to end.

3315. The directions of roads through an extensive estate, cannot be determined on without having in contemplation the other fundamental improvements, such as the situations of villages, farmeries, mills, or other objects; and these artificial improvements must be taken in connection with the natural surface, soil, materials, waters, &c.; the probable system of agriculture that will be pursued, and the external intercourse. A hilly country under aration, will evidently require more roads than if chiefly under pasture; and, indeed, other circumstances the same, a country abounding in hills and valleys, requires many more roads than one of a more even surface. The roads in such a country are also more expensive, on account of the bridges, and extra work at their abutments. On an estate composed of gentle hills chiefly intended for arable or convertible husbandry, the best situation for the roads will generally be found about half way between the bottoms and highest surfaces. By this means the labor of carting up the produce from the fields below the road, and carting up the dung to the fields above it, is evidently much less than if the road were either entirely on the highest ground or the lowest. Bridges over the brooks or open ditches necessary for drainage in valleys, are also rendered less frequent.

3316. Accurate sections of the rises and falls of the natural surface on which a road is to be formed should always be taken before the line is finally determined on. As the figure of an exact section of this sort on any ordinary scale, would convey no data sufficiently accurate for execution, it is usual to adopt one scale for the length, and another for the rises and falls of the road, and to mark the latter with the dimensions as taken on the survey.

SECT. III. Of the Form and Materials of Roads.

3317. On the structure and composition of roads, men of science and practical roadmakers are much more divided than on their laying out. The subject is of itself of greater importance in old countries, because it more frequently occurs that a road is to be enlarged or renewed, than that a new line is to be devised. We shall first lay down the fundamental principles of the formation, and wear of roads, and next treat of forming them, and of the different kinds of road materials.

SUBSECT. 1. Of the Formation of Roads, and of their Wear or Injury.

3318. A road may be defined a path of transit on the earth's surface, for men, animals, and machines; of sufficient width for the given traffick; - of sufficient strength and solidity for the given weight; —of sufficient smoothness to permit no impediment; and of as great durability as possible.

3319. The width is obviously determinable by the nature and extent of the traffick: every road should be made sufficiently broad to admit two of the largest sized carriages which are in use in the country or district, to pass each other; and highways and roads near towns should be made wider in proportion to their use. The maximum and minimum can only be determined by experience: sixty feet is the common and legal width of a turnpike-road in Britain, and this includes the footpath.

3320. The strength of a road depends on the nature of the material of which it is formed, and of the basis on which it is placed. A plate of iron or stone of the road's width placed on a compact dry soil would comprise every thing in point of strength; but as it is impracticable to employ plates of iron or stone of such a size to any extent, recourse is had to a stratum of small stones or gravel. The great art, therefore, is so to prepare this stratum, and place it on the basis of the road, as that the effect may come as near as possible to a solid plate of material. To accomplish this, the stones or gravel should be broken into small angular fragments, and after being laid down of such a thickness as experience has determined to be of sufficient strength and durability, the whole should be so powerfully compressed by a roller as to render it one compact body, capable of resisting the impression of the feet of animals, and the wheels of carriages in a great degree, and imperveable by surface water. But the base of the road may not always be firm and compact; in this case it is to be rendered so by drainage, artificial pressure, and perhaps in some cases by other means.

3321. The durability of a road as far as it depends on the original formation, will be in proportion to the solidity of its basis; the hardness of the material of which the surface stratum is formed; - its thickness;- and the size and form of the stones which compose it. The form and size of the stones which compose the surface-stratum have a powerful influence on a road's durability. If their form is roundish, it is evident they will not bind into a compact stratum; if it is large, whether the form be round or angular, the stratum cannot be solid; and if they are of mixed sizes and shapes, though a very strong and solid stratum may be formed at first, yet the wheels of carriages and the feet of animals operating with unequal effect on the small and large stones, would soon derange the solidity of the stratum to a certain depth, and consequently, by admitting rain and frost to penetrate into it, accelerate its decay. A constant state of moisture, even without any derangement of surface, contributes to the wearing of roads by friction, and hence one requisite to durability is a free exposure to the sun and air by keeping low the side fences; and another is keeping a road clear of mud or dust, the first of which acts as a spunge in retaining water, and the second increases the draught of animals, and of course their action on the road. Both the strength and the durability of a road will be greater when the plate or surface-stratum of metals is flat or nearly so, than when it is rounded on the upper surface: first, because no animal can stand upright on such a road with a regular bearing on the soles of its feet; and, secondly, because no wheeled carriage can have a regular bearing, excepting on the middle or crown of the road. The consequence of both these states is the breaking the surface of the plate into holes with the edges of horses' feet, or ruts by the ploughlike effect of wheels on the lower side of the road, or the reiterated operation of those which pass along the centre.

3322. The smoothness of a road depends on the size of the stones and on their compression either by original rolling or the continued pressure of wheels. The continued smoothness of a road during its wear depends on small stones being used in every part of the stratum; for if the lower part of it, as is generally the case in the old style of forming roads, consists of larger stones, as soon as wheels or water penetrates from above, these stones will work up and produce a road full of holes and covered with loose stones.

3323. The wear or decay of roads takes place in consequence of the friction, leverage, pressure, grinding and incision of animals and machines, and the various effects of water and the weather.

435

3324. Friction will in time wear down the most durable and smooth material. Its effects are more rapid when aided by water, which insinuates itself among the particles of the surfaces of earthy bodies, and being then compressed by the weight of feet or wheels, ruptures or wears them. Even when not compressed by wheels or other weights, the action of frost, by expanding water, produces the same effect. This any one may prove by soaking a soft brick in water and exposing it to a severe frost. 3325. The leverage of the feet of animals has a tendency to depress one part of the surface and raise up another. The line which forms the sole of every animal's foot may be considered as a lever of the second kind, in which the fulcrum is at the one extremity (fig. 435 a), the power at the other (b), and the weight between them (c). Hence the injury done to the road, even if formed on the best construction, will be as the

pressure on the fulcrum: this amounts to the half of the weight of bipeds and their loads, and a fourth of that of quadrupeds. But if the stones of the road are large, that is, if they are more than two inches in breadth, the horse's foot acts as a compound lever, and by depressing one end of the stones and raising the other, deranges the surface of the stratum, and renders it a receptacle for water, mud, or dust.

3326. The leverage of wheels is of a nature to be less injurious to roads than that of the feet of animals, because the fulcrum (fig. 436 a), is continually changing its position. But if the stones of the road are large, then the wheel acts as a compound lever, and raises up the one end (b), and presses down the other (a), of every stone it passes over, and in this case becomes more injurious on a bad road than the feet of loaded animals. The reiterated operation of this effect

436

by wheels following in the same track, soon destroys badly constructed roads. 3327. Such being the effect of leverage, and especially of compound leverage, in wearing roads, it becomes of the first importance to ascertain that size and shape of stone on which its effects will be least; that is to say, how short a compound lever may be made use of consistently with other advantages. This must in general be a matter of experience, and chiefly depends on the hardness of the stone. The size must always be sufficiently large, and the shape sufficiently angular to form, when embedded, a compact, hard, and immoveable stratum, and the smaller the size the better, provided that object be obtained. Two inches in diameter may be considered the medium size.

3328. The mere pressure of objects on a smooth road does little mischief, and hence the advantage of perfectly cylindrical wheels, and a road as nearly level as practicable. But if the surface of the road is rough, the pressure both of cylindrical wheels, and the feet of animals, may do mischief by forcing down a loose stone among others of different sizes, and thus loosening the latter and raising the largest to the surface. Where a road, however, is composed of materials of small size, the pressure of cylindrical wheels, when the surface is clean and dry, will probably always be of greater service by acting as a roller, than of injury by the friction of the pressure.

3329. Grinding is produced by the twisting motion of the feet of horses or other animals when pulling hard or carrying a heavy weight, and by the twisting, dragging, or sliding of wheels from whatever cause. The grinding of wheels, Fry observes, "may in every case be defined to be the effect produced on any substance interposed between two bodies, one of which has a sliding motion, yet so firmly confined or pressed between them, that the moving body cannot slide over the interposed substance; but, in consequence of the pressure, the interposed substance, adhering firmly both to the fixed and to the moving body, is necessarily lacerated or torn asunder, and reduced to atoms. This is the process in corn-mills, in drug-mills, and in every other mill, properly so called. I remember," he adds, " frequently when a boy, to have trodden with one heel on a piece of soft brick, or of dry old mortar, which was firm enough to bear the weight of my body, uninjured; but, on giving my body a swing round with my other foot, I have instantly reduced it to powder. The action in this case is very obvious: the weight of my body confined the piece of brick firmly to the ground; my heel was also

pressed by the same weight firmly upon the brick; one part of the brick therefore remaining confined to the ground, and the other part being carried round by my heel, the brick of course was torn asunder and reduced to powder. This I conceive is a simple elucidation of the difference between pressing and grinding, and this is the difference of the effects on the materials of our roads, produced by the use of upright cylindrical wheels, which act only by pressure, by the use of conical wheels, which by their constant twist, act also by grinding, and by very convex roads, by which means the wheels of all carriages, excepting such as occupy the crown of the road, whether cylindrical or otherwise, act in the same twisting, sliding, and grinding manner." (Obs. on Roads, &c. 1819.)

3330. By the incision of objects passing along roads, we allude to the dividing operation of wheels, which, independently of their effect as moving levers, act also as moving wedges, or perhaps, more properly, as endless saws in forming ruts or deepening such as are already made. Flat roads, so as to produce less temptation to follow in the middle track, watchful repair, and broad wheels, are the mitigators of this description of

wear.

3331. Water is one of the most serious causes of the wear of roads. As we have already observed (3324.), it acts, aided by pressure, like gunpowder in rending the surface of bodies. Frozen it acts exactly in the same manner; and when it has penetrated deeply into a stratum of materials, a thaw produces their entire derangement. Mud is formed in consequence of the presence of water and dust, or earth, and acts as a sponge to retain it, and perpetuate its bad effects. A well-composed and thoroughly compressed substratum will not imbibe water unless it rests in ruts or other hollows. To form such a stratum, therefore, and obliterate all hollows as soon as they appear, and to remove mud and dust, are the palliatives of this mode of wear. On such a road heavy showers may do good by washing away the earthy particles, dung, and other injurious earthy or vegetable matters.

3332. Wind is mostly a favorable agent to roads by drying them and blowing off the lighter dust; but in some cases, in very exposed situations, it has been known to blow the dust into heaps, and sometimes to carry off larger particles than could be spared. The last evil is fortunately rare; the other only requires the removal of the accumulated heaps of dust.

SUBSECT. 2. Of M'Adam's Theory and Practice of Road-making.

3333. M'Adam agrees with other engineers, that a good road may be considered as an artificial flooring, forming a strong, solid, smooth, surfaced stratum, sufficiently flat to admit of carriages standing upright on any part of it, capable of carrying a great weight, and presenting no impediment to the animals or machines which pass along it. In forming this flooring, M'Adam has gone one step beyond his predecessors in breaking the stone to a smaller size than was before practised, and in forming the entire stratum of this small-sized stone. By the former practice a basement of large stones are first laid, then stones a degree smaller, and, lastly, the least size on the surface. It is in this point of making use of one small size of stones throughout the stratum, that the originality of M'Adam's plan consists, unless we add also his assertion, "that all the roads in the kingdom may be made smooth and solid in an equal degree, and to continue so at all seasons of the year." It is doubted by some, whether this would be the case in the northern districts at the breaking up of frosts, and especially in the case of roads not much in use, and consequently consisting of a stratum less consolidated, and more penetrable by water. M'Adam, probably, has much frequented public roads in view. The durability of these," he says, "will of course depend on the strength of the materials of which they may be composed, but they will all be good while they last, and the only question that can arise respecting the kind of materials, is one of duration and expense, but never of the immediate condition of the roads." (Remarks on Roads, &c. p. 11.) The following observation of Marshal is worthy of remark, as tending to confirm to a certain extent the doctrine of M'Adam. "It may seem needless to repeat, that the surface of a road which is formed of well-broken stones, binding gravel, or other firmly cohesive materials, and which is much used, presently becomes repellent of the water which falls upon it: no matter as to the basis on which they are deposited; provided it is sound and firm enough to support them."

3334. M'Adam's theory of road-making may be comprised in the following quotation from his Report to the Board of Agriculture (vol. vi. p. 46.). "Roads can never be rendered perfectly secure until the following principles be fully understood, admitted, and acted upon: namely, that it is the native soil which really supports the weight of traffick; that while it is preserved in a dry state it will carry any weight without sinking, and that it does, in fact, carry the road and the carriages also; that this native soil must previously be made quite dry, and a covering impenetrable to rain, must then be placed over it to preserve it in that dry state; that the thickness of a road should only be regu

lated by the quantity of material necessary to form such impervious covering, and never by any reference to its own power of carrying weight.

3335. The erroneous opinion so long acted upon, and so tenaciously adhered to, that by placing a large quantity of stone under the roads, a remedy will be found for the sinking into wet clay, or other soft soils, or in other words, that a road may be made sufficiently strong, artificially, to carry heavy carriages, though the sub-soil be in a wet state, and by such means to avert the inconveniences of the natural soil receiving water from rain, or other causes, has produced most of the defects of the roads of Great Britain. At one time M'Adam had formed the opinion that this practice was only a useless expense; but experience has convinced him that it is likewise positively injurious.

3336. If strata of stone of various sizes be placed as a road, it is well-known to every skilful and observant road-maker, that the largest stones will constantly work up by the shaking and pressure of the traffick; and that the only mode of keeping the stones of a road from motion, is to use materials of a uniform size from the bottom. In roads made upon large stones as a foundation, the perpetual motion, or change of the position of the materials, keeps open many apertures, through which the water passes.

3337. Roads placed upon a hard bottom, it has also been found, wear away more quickly than those which are placed upon a soft soil. This has been apparent upon roads where motives of economy, or other causes, have prevented the road being lifted to the bottom at once; the wear has always been found to diminish, as soon as it was possible to remove the hard foundation. It is a known fact, that a road lasts much longer over a morass than when made over rock. The evidence produced before the committee of the house of commons, showed the comparison on the road between Bristol and Bridgewater, to be as five to seven in favor of the wearing on the morass, where the road is laid on the naked surface of the soil, against a part of the same road made over rocky ground.

3338. The common practice, on the formation of a new road is, to dig a trench below the surface of the ground adjoining, and in this trench to deposit a quantity of large. stones; after this, a second quantity of stone, broken smaller, generally to about seven or eight pounds weight; these previous beds of stone are called the bottoming of the road, and are of various thickness, according to the caprice of the maker, and generally in proportion to the sum of money placed at his disposal. On some new roads, made in Scotland in the summer of 1819, the thickness exceeded three feet. That which is properly called the road is then placed on the bottoming, by putting large quantities of broken stone or gravel, generally a foot or eighteen inches thick, at once upon it. Were the materials of which the road itself is composed properly selected, prepared, and laid, some of the inconveniences of this system might be avoided; but in the careless way in which this service is generally performed, the road is as open as a sieve to receive water; which penetrates through the whole mass, is received and retained in the trench, whence the road is liable to give way in all changes of weather. A road formed on such principles has never effectually answered the purpose which the road-maker should constantly have in view; namely, to make a secure, level flooring, over which carriages may pass with safety, and equal expedition, at all seasons of the year.

3339. An artificial road in Britain is only required to obviate the inconvenience of a very unsettled climate. Water, with alternate frost and thaw, are the evils to be guarded against; consequently nothing can be more erroneous than providing a reservoir for water under the road, and giving facility to the water to pass through the road into this trench, where it is acted upon by frost to the destruction of the road. As no artificial road can ever be made so good and so useful as the natural soil in a dry state, it is only necessary to procure and preserve this dry state of so much ground as is intended to be occupied by a road.

3340. The first operation in making a road should be the reverse of digging a trench. The road should not be sunk below, but rather raised above, the ordinary level of the adjacent ground; care should at any rate be taken, that there be a sufficient fall to take off the water, so that it should always be some inches below the level of the ground upon which the road is intended to be placed: this must be done, either by making drains to lower ground, or if that be not practicable, from the nature of the country, then the soil upon which the road is proposed to be laid, must be raised by addition, so as to be some inches above the level of the water.

3341. Having secured the soil from under-water, the road-maker is next to secure it from rain-water, by a solid road made of clean dry stone or flint, so selected, prepared, and laid, as to be perfectly impervious to water; and this cannot be effected unless the greatest care be taken that no earth, clay, chalk, or other matter, that will hold or conduct water, be mixed with the broken stone; which must be so prepared and laid, as to unite with its own angles into a firm, compact, impenetrable body.

3342. The thickness of such road is immaterial, as to its strength for carrying weight; this object is already obtained by providing a dry surface, over which the road is to be