Wear. Mainte nance. coating of a well-maintained road the proportion of stones of various sizes varies, but generally from one-third to one-half is found to consist of detritus under three-eighths of an inch in diameter, and there is a very constant proportion of about one-fifth of mud and detritus under one-thirtieth of an inch in diameter. This appears to be the amount necessary to fill the voids between the fragments of stone when compacted together. In an ill-kept road, from which the mud is not removed, the proportion of detritus is much higher, and mud may constitute nearly one-half of the coating. In proportion as the detritus and mud are kept down to the minimum by constant removal from the surface, so will the road be able to resist the action of wet and frost and the wear of the traffic. The wear of materials, resulting in their gradual reduction to detritus, is due to the joint action of the traffic and the weather, which cause surface wear, wear arising from cross breaking, and from rubbing of the stones together. When there is no movement in the body of the road, and the wear is confined to the crushing and grinding at the surface, it is the least possible; but, when a road is weak from insufficient thickness or solidity on a yielding foundation, bending and cross breaking of the coating take place under passing loads in addition to the surface wear, and the effects are aggravated by the softening action of water finding its way into the road through cracks formed in the surface and by the disintegrating action of frost. The wear and waste are thus far larger than on roads of sufficient strength, properly maintained. The destructive effect of wheels is greater as the diameter is less, and to a much greater degree as the tire is narrower. On hard and strong roads no greater width of wheel than 4 inches is useful, as a wider tire does not bear evenly, but on yielding roads a greater width is of some advantage, though it does not prevent damage from bending and cross breaking of the whole coating under excessive loads. A good deal of attention has been given by French engineers to the measurement of traffic, wear, and the consumption of road materials. Without a knowledge of the amount of traffic accurate comparisons of wear are impossible, and an account of the traffic on the roads of France is taken periodically in "collars" or horses drawing loads, and in the weight drawn. Traffic as measured by weight drawn has of late been observed in some of the streets of London and Liverpool, and has been reduced for comparison to the weight per foot or yard of width of the carriage-way. Wear may be measured by loss of thickness in the coating; but the loss of stone in proportion to detritus must also be ascertained before all the effects of wear can be determined. The accurate measurement of wear as practised by the French engineers is a complicated process, and it must suffice here to state that measured by thickness the wear is seldom found to exceed half an inch, or on the most frequented roads of France one inch, of consolidated surface per year, and that about 100 cubic yards of good materials per mile per year are considered as the average consumption under 100 collars of traffic per day. Observations in the United Kingdom on roads well and systematically maintained have confirmed these results. The new materials may be added to the road either in thin coats and small patches year by year or in a thick coat consolidated by rolling. The first method, by which the wear is replaced annually and the traffic is depended on to work the materials into the road, can be followed with excellent results, and at no great inconvenience to the public under proper management when the traffic is not excessive. Considerable care in the use of materials is required that none may be unnecessarily applied. The annual employment of one-fifth or one-sixth the quantity which it would take to cover the whole surface one stone in thickness is often sufficient to replace wear, and it will then take five or six years to coat every part of the road if it is covered regularly. It is therefore important to apply the new materials only where they are needed, and not to use them where the road is already sufficiently thick. The irregularity of wear and of thickness enables a good roadman to judge where new materials must be applied, and he will apply them in small quantities wherever weak places appear. To facilitate this the materials should be placed in heaps by the roadside in the summer, and they should be carefully spread in the autumn and attended to afterwards to ensure consolidation without waste. By good management a large quantity of materials may be incorporated in a road before the middle of the winter without harassing the traffic, and the strength may not only be maintained but increased. On a hard strong road consolidation may be aided by loosening the surface with a pick; generally only the margin of a patch need be picked up. But if the road is soft or weak it is better not to disturb the surface at all. Binding may sometimes be used to aid consolidation, but it is seldom necessary if the materials are properly laid and attended to, as the coating already contains detritus enough. In the second method a coating of materials is laid on at once sufficient to endure the wear of several years with such slight repairs as may be necessary to keep a good surface, and, when the wear has gone as far as it can be safely allowed to go, the process is repeated. Unless the wear is very considerable there is no economy in this method, though the convenience to the public, especially in towns, is undeniable. Consolidation by rolling (after the manner already described) is essential, and it is generally desirable to loosen the old surface to ensure the incorporation of the new coating with it. Scraping and attention are required between one coating and another and also slight repairs to the surface, as, however well the materials may be laid and rolled, the wear of the ordinary traffic will search out places which have escaped the full pressure of the roller and produce inequalities. Besides a regular application of new materials to replace wear, there must be in road maintenance on proper principles a systematic removal of the detritus by scraping or sweeping, which must be regarded as keeping the whole coating in proper condition, and not as mere surface cleansing. The wear should also be reduced as far as possible by providing sufficient thickness to carry the traffic, by keeping an even surface on which water can never stand and soak, and by good drainage both of surface and subsoil. An adequate amount of skilled manual labour is necessary for economy of maintenance, and this and the constant attention which is required to keep a road in good order are best secured by putting a man in permanent charge of a defined length. In the autumn and winter, when more labour is wanted, extra men should work under the directions of the permanent road labourer, whose knowledge of his length of road will enable him to employ them to the best advantage. Concrete macadam, formed by grouting with lime or cement Concrete mortar a coat of broken stone laid over a bed of stone previously and tar well rolled, has been tried as an improvement on an ordinary macmacadamized surface, but not hitherto with much success. When adam. cleanliness is of importance, and great durability is not required, tar macadam or bituminous concrete may be usefully employed. It is sometimes made by first spreading a coating of broken stone and consolidating it by a roller, and then pouring over it a mixture of coal-tar, pitch, and creasote oil, upon which a layer of smaller stone is spread and rolled in, and the surface finished with stone chippings rolled in. More usually the broken stone and bituminous mixture are well incorporated together before they are spread, the stone sometimes being previously heated. The lower layer, about 4 inches thick, may be of stone broken to 24 inches gauge, and the next layer, about 2 inches thick, may be of smaller stone. Each layer must be well rolled, and when perfectly solid a thin coating of fine stone or granite chippings is spread over the surface and rolled in. Hard limestone is found to be more suitable than silicious or igneous rocks for this material. A road surface well made in this manner will last several years under light traffic without any repairs, and it can easily be patched when necessary. Stone Pavements.-Early pitched roadways consisted of pebbles STONE or rounded boulders, bedded in the natural surface or in sand or PAVEgravel. The next step in advance was to employ roughly-squared MENTS. blocks; but the wide and irregular joints admitted the water to the subsoil, and the mud worked up and the stones sank irregu larly under the traffic. Telford, who was called upon to report on the street pavements of the parish of Hanover Square in 1824, saw the necessity of cutting off all connexion between the subsoil and the paving stones. He recommended a bed of about 6 inches of Founda clean river ballast, rendered compact by being travelled upon for tion. some time before the paving was laid, but he subsequently considered that nothing short of 12 inches of broken stone, put on in layers 4 inches thick and completely consolidated by carriages passing over them, would answer the purpose. He recommended paving stones of considerable depth and of from 4 to 6 or 7 inches in breadth for the greatest thoroughfares, and he pointed out the importance of working the stones flat on the face and square on all sides, so as to joint close and preserve the bed or base as nearly as possible of the same size as the face, and of carefully placing together in the same course stones of equal breadth. Many pavements thus laid with stones of considerable breadth still remain, but experience proved that it was a mistake to suppose that broad stones having a larger base would support better the weight and shocks of heavy traffic; on the contrary, a wide stone has a tendency to rock on its bed, and also to wear round on the top and become slippery. To obtain an evener surface and a better foot hold for the horses the stones were reduced in width, and in 1840 a granite pavement was laid by Walker on Blackfriars Bridge, which may be considered the first of modern set pavements. The stones were 3 inches broad and 9 deep; they were laid on a bed of concrete 1 foot thick and were jointed with mortar. The reduc tion of breadth to about 3 inches was generally followed, but it is only of late years that a concrete foundation has been employed to any great extent, the frequent breaking up to which streets are subject having prevented it. In London a foundation of broken stone has been continued in the chief thoroughfares, the sets being evenly bedded in gravel upon it and rammed with a heavy wooden rammer. Hard core-a mixture of broken stone, clinker, brick rubbish, and old building materials-has also been largely used to form a foundation. In the northern towns of England cinders have been employed, and where the traffic is exceptionally heavy a pitched foundation of stones on edge has been laid when the sets were not paved upon an old macadamized surface. The concrete for a foundation to a paved street should be made with the Materials. Jointing. best Portland cement, thoroughly mixed in proper proportions with the sand and gravel or other materials used, water being added as sparingly as possible. A thickness of 6 inches of wellmade cement concrete is sufficient for the heaviest traffic, and it can be cut out in slabs for pipe-laying or repairs and can be relaid and cemented in its place. To obtain the best result a new foundation should not be paved upon for a week. A foundation of bituminous concrete is sometimes used where only a thin bed can be laid, in consequence of there being an old foundation which it is undesirable to disturb. It is made by pouring a composition of coal-tar, pitch, and creasote oil while hot over broken stone levelled and rolled to the proper form, and then spreading a thin layer of smaller broken stone over the surface and rolling it in. It has the advantage that it can be paved upon a few hours after it has been laid. The best materials for pavement sets are the hard igneous and metamorphic rocks, though millstone grit and other hard sedimentary rocks of the same nature are used when the traffic is comparatively light. Excessively hard stone which wears smooth and slippery is objectionable in spite of its durability. Penmaen-Mawr stone, which is much used in many of the large Lancashire towns, is of this character, and its use was discontinued in London in consequence of its slipperiness and noise. Guernsey granite (syenite) and Mount Sorrel granite (syenite) have the same nature in a less degree, and in London Aberdeen blue granite is preferred, as, though it wears faster, it keeps a rough surface. Walker's observations on the wear of tram stones showed that Aberdeen granite wore three and a quarter times as fast as Guernsey granite, and in the set pavement of Blackfriars Bridge it was found that after thirteen and a half years' wear the Aberdeen stone had worn 14 inches, while the Guernsey granite had only worn one-fourth of an inch (equal to 11 and 019 inch per year respectively), or that the former had worn six times as fast as the latter. Observations made by Mr Haywood showed the general rate of wear of Aberdeen granite under heavy traffic in the City of London pavements to have been from 14 to 23 inch per year. The rate of wear of PenmaenMawr and Carnarvonshire sets in Liverpool under the greatest traffic is stated to be seldom more than 02 inch per year. A certain proportion between the depth and the length and breadth of sets is required for stability. A shallow stone is more easily tilted up by a heavy weight coming on one edge, and a narrow stone has a tendency to turn over sideways. The length, measured across the street, must be sufficient to break joint properly, as two or more joints nearly in a line lead to the formation of grooves. For the softer stones a breadth of 4 or 5 inches may be adopted, but for sets of granite or other hard material, with which the joints must be depended on for foothold, the breadth should not much exceed 3 inches. The depth should not be less than twice the breadth, and, as deeper sets weigh more and cost more than shallower ones and the loss by wear is but slight, there is some reason for not exceeding the minimum depth. Where, however, the speedy relaying of a street pavement is of more importance than a saving in first cost, deeper sets are used, and when they have become so worn as to be uneven the street is relaid with new sets and the old ones are removed to be redressed for use in other streets, the sets being used again and again in less important streets as their depth is reduced. In London sets 3 inches wide, 10 to 15 long, and originally 9 deep are used in this manner. Liverpool sets 4 to 14 inches wide, to average with the joints, 5 to 7 inches long, and 6 to 74 deep according to the traffic are used. In Manchester the sets are 3 to 3 inches wide, 5 to 7 long, and 5 to 6 deep, or 7 in exceptional situations. Sets should be well squared and not taper from the face downwards; both joints and face should be free from irregular projections. On a concrete foundation sets are generally bedded on a thin layer of sand or fine gravel; sometimes they are laid in a bed of fine cement concrete, enough of which is spread over the concrete foundation to be covered while fresh by the sets, which are put in place and smartly tapped, and the joints aro grouted at once with cement grout. To allow the cement to become thoroughly set it is desirable that traffic over the pavement should not be allowed for a fortnight, if that can be arranged. The courses of sets are laid square across the street, no advantage arising from a slanting direction, which makes the wear more irregular. At junctions of streets the courses are laid meeting at an angle at the centre line of the narrower street, so that the courses may not run in the direction of the traffic. On steep inclines the sets are sometimes slightly tilted on their beds, forming a serrated surface to give foothold, and slate has been inserted in the joints for the same purpose. The water channels are formed by two or three courses of sets laid parallel to the kerb. In Joints simply filled in with gravel are of course pervious to water, and a grout of lime or cement does not make a permanently watertight joint, as it becomes disintegrated under the vibration of the traffic. Grouted joints, however, make a good pavement when there is a foundation of concrete or broken stone or hard core. Where there is not a regular foundation imperviousness in the joints is of great importance. In some of the Lancashire towns the joints have for many years past been made by first filling them with clean gravel, well shaken in by ramming, and then pouring in a composition of coal-tar, pitch, and creasote oil. The Manchester pavements are good examples of this system of trusting to impervious jointing to prevent unequal settlement. The foundation, where there is not already an old road surface, is a bed of cinders about 1 foot thick, over which are laid 3 inches of gravel, which are thoroughly consolidated by allowing the traffic to pass over them. The sets are evenly bedded and well rammed after the joints have been filled with clean gravel, ramming and gravelling being repeated till the joints are full of gravel. The mixture of coal-tar, pitch, and creasote oil, well boiled, is then poured over the surface and allowed to percolate and fill up all interstices in the joints, and the pavement is finished by covering it with small gravel. Joints so formed are impervious to wet and have a certain amount of elasticity; the foundation is kept dry; and the pavement keeps its form well for many years. The objection is made that in hot weather the composition runs from the joints and makes the streets unpleasant for foot-passengers. This sort of jointing is used in Liverpool and some other large towns, where the sets are laid on a concrete foundation. The elasticity diminishes vibration and noise, and pavements so jointed are said to wear better than others. A curve like that before described, flattening gradually towards Cross the sides, and having a rise equal to one-sixtieth of the width of section. the carriage-way, is a common cross section for a paved street. Sometimes the rise is even less. A pavement consisting of broad, smooth, well-jointed blocks of Granite granite for the wheel tracks, and pitching between for the horse tramway. track, was laid by Walker in Commercial Road (London) for the heavy traffic to the West India Docks in 1825, and similar pavements have been successfully used elsewhere, principally for heavy traffic, in streets only wide enough for one vehicle. In Milan, Turin, and other towns of northern Italy tramways of the same sort are extensively used for the ordinary street traffic. The tractive force required is small, while the foothold on the horse track is good; but the tram-stones are slippery for horses to pass over. The rigidity of the roadway renders it more suitable for slow heavy traffic than for light quick vehicles, and the improvement in other pavements has limited the application of this one in ordinary streets. Wood Paving-Wood pavements were introduced in England in WOOD 1839. Hexagonal blocks of fir, 6 to 8 inches across and 4 to 6 PAVING. deep, were bedded in gravel laid on a foundation previously levelled and beaten. The blocks were either bevelled off at the edges or grooved across the face to afford foothold. Other wood pavements were tried in London about the same time, but they soon got out of order from unequal settlement of the blocks, and most of them lasted but a few years. The best of these was Carey's, which consisted of blocks 6 to 7 inches wide, 13 to 15 long, and 8 or 9 deep, the sides and ends having projecting and re-entering angles locking the blocks together with the view of preventing unequal settlement. Pavements on this system were laid in Minc ing Lane in 1841 and in Gracechurch Street in 1842. In the latter street the blocks appear to have been relaid every three or four years and to have been entirely removed about every eleven years, until the pavement was removed in 1871, to be superseded by asphalt. Experience led to a reduction in the width of the blocks to 4 inches and in the depth to 5 or 6, and the salient and re-entering angles disappeared from the sides. With these modifications Carey's pavement remained in use from 1841 until after the introduction of more modern systems in recent years. The "improved wood pavement" was first used in London in 1871. After the foundation was formed to the proper cross section a bed of sand 4 inches deep was laid, upon which came two layers of inch deal boards saturated with boiling tar, one layer across the other. The wooden blocks were 3 inches wide, 5 deep, and 9 long; they were dipped in tar and laid on the boards with the ends close together, but transversely the courses were spaced by fillets of wood three-fourths of an inch wide nailed to the floor and to the blocks. The joints were filled up with clean pebbles rammed in and were run with a composition of pitch and tar, the surface being dressed with boiling tar and strewed with small sharp gravel and sand. In this pavement a somewhat elastic foundation was provided in the boards, which were also intended to prevent unequal settlement of the blocks; but the solidity of the pavement depended upon its water-tightness, for, when the surface water reached the sand, as it did sooner or later, settlement and dislocation of the blocks under the traffic arose. Pavements on this system were laid between 1872 and 1876, and were kept in repair and relaid from time to time, but about 1877 the plank foundation was abandoned for a foundation of cement concrete. A concrete foundation for a wood pavement appears to have been Founda first employed in a pavement laid in 1872 in Gracechurch Street by tion. the Ligno-Mineral Company. The concrete was of blue lias lime 4 inches thick formed to the curve of the road. The blocks were of beech, mineralized by a special process, 3 inches wide, 4 deep, and 7 long, with the ends cut to an angle of 60°, so that each block might derive support from the next one. They were laid with the XX. 74 The ends inclining in opposite directions in alternate courses. upper edges of the blocks were chamfered, and there was a chamfered groove near the bottom. In a few years this form of block was abandoned for rectangular blocks, and mineralized fir was substituted for beech. The blocks were bedded in Portland cement and laid with joints one-fourth of an inch wide, partly filled with asphalt and then grouted with mortar. The adoption of a bed of concrete as the weight-bearing foundation of the road marks a new departure, and in all the more recent systems of wood pavement a substantial foundation of concrete is an essential feature. In Norwich, however, a large quantity of wood pavement has been laid on the old street foundation, the blocks being bedded in gravel and sand and rammed, and the joints grouted with lime and sand. The experience of from four to seven years has proved the pavements to be successful, but the foundation is exceptionally dry and hard and the traffic not very heavy. With a concrete foundation there is no reason for complicated shapes and contrivances for locking the blocks together; and wood pavements in their modern form consist of rectangular blocks (obtained by cutting off the end of a deal plank), bedded on the concrete with the fibres of the wood vertical, thus constituting a slightly elastic wearing surface on a rigid foundation, by which the weight of the traffic is borne. There is, however, considerable variation in the method of bedding and jointing the blocks. The Asphaltic Wood Pavement Company laid half an inch of asphalt upon the concrete, and formed the lower part of the joint of asphalt and the upper part of a grout of Portland cement and gravel, the advantage claimed being a slightly elastic bed for the blocks and water-tight joints. The blocks have been laid in unset cement over the concrete and rammed to an even surface; but the ramming is liable to split the blocks, and the indentations formed in the cement surface of the foundation have to be removed when the time comes for renewing the blocks. It is now more usual to bed the blocks directly on the concrete, a smooth surface being formed either with the concrete itself or by a floating of cement, and to fill the joints with a grout of cement and gravel. A cement joint adheres to the blocks, resists wet, and does not wear down too much below the surface of the wood, Henson and so form a receptacle for mud. In Henson's system, which pave- has been largely used, the blocks are bedded and jointed with ment. ordinary roofing felt, a strip of which, cut to a width equal to the depth of the blocks, is placed between every two courses. The joint is made as close as possible by driving up the blocks as every eight or ten courses are laid with heavy mallets,—a plank being laid along the face of the work. A perfectly close and slightly elastic joint is thus formed. A continuous layer of felt is likewise laid over the concrete foundation to give a slightly elastic bed to the blocks. A V-shaped groove along the centre of every fourth block was at first considered necessary for foothold, but its use has been discontinued except on gradients steeper than 1 in 30. The surface of the pavement is dressed over with a hot bituminous compound, and covered with fine clean grit. This method of laying a wood pavement, although somewhat more expensive, is probably the best that has hitherto been devised for smoothness and durability. The blocks are laid in courses across the streets, any change in the direction of the latter being accommodated by shorter courses ending with wedge-shaped blocks. At street junctions the courses are laid diagonally, or meeting at right angles. Two or three courses are laid parallel with the kerb to form a water channel. The blocks may be laid close end to end across the street if some allowance be made for expansion by wet, without which the kerbstones and footways will be displaced, or the courses will be bent in reversed curves. To afford relief the joints of the courses parallel to the kerb may be left open, or the course next the kerb may be left out until expansion has ceased, the space being temporarily filled in with sand. In the direction of the traffic joints more or less wide are generally thought necessary for foothold. A wide joint allows the fibres of the wood to spread and give way at the upper corner of the blocks for want of lateral support, and it also forms a receptacle for mud and wet. Experience has shown that the space of three-fourths of an inch or one inch, once thought necessary for foothold, may safely be reduced to one-fourth or three-eighths of an inch. For spacing the courses to form the joints strips of wood of the proper thickness may be laid in and removed before the joints are filled, or they may be nailed to the lower part of the blocks. Two fillets have been nailed on, or three cast-iron studs fixed in the sides of each block to keep them steady in place until the joints are filled and thoroughly set. The latter method secures more uniformity in the width of the joints. Materials. There is some difference of opinion as to the best material for a wood pavement. Pitch pine and the harder red and yellow deals are the most durable, but they are less elastic than the softer woods, and are apt to wear slippery. Soft white woods have been recommended for the sake of a more elastic surface; but on the whole either Memel or Swedish yellow deal is generally considered the best material. Whatever wood is used, it should be sound, close-grained, even in quality, free from knots and sap, and from the blue tinge which is a sign of incipient decay. After the blocks are cut, all those that are unsound, knotty, or badly shaped should be carefully rejected, as defective blocks soon cause holes in the surface and must be replaced, or the adjoining blocks will suffer undue wear and the surface become irregular. The breadth of the blocks never now exceeds 4 inches, and it is generally 3, the length being determined by the breadth of the deal or batten from which they are cut. The depth is usually 5 or 6 inches; 5 inches are considered by many to be enough to give sufficient depth for as long as the pavement will retain a sufficiently good surface without renewing the wood, and blocks of that depth have been laid in many London streets. It is doubtful if any advantage is derived from creasoting or from dipping the blocks in creasote oil or coal tar. Dipping affords a cover for the use of defective or inferior wood, and thorough creasoting, though it preserves the wood from decay, has little or no influence on the wear, which in almost all cases determines the life of the blocks. With a curved cross section like that already described a rise Cross from the mean level of the channels to the crown of the road equal section to one-sixtieth or one-seventieth of the width of the carriage-way is enough. The necessary profile must be accurately given to the concrete foundation when wet. Wooden moulds or templates are fixed across the street 10 or 12 feet apart, over which a straight batten is worked to give the concrete the required form and a smooth surface. The moulds are removed when the concrete is partially set and the spaces are made good with cement mortar. In a level street provision should be made in the foundation for a Founda fall in the side channels towards the gullies of not less than 1 in tion. 150, and the necessary modifications of cross section at the intersection of streets must also be provided for. Every care should be taken to ensure a good homogeneous concrete for the foundation, as upon that the strength of the road depends. With a well-made Portland cement concrete a thickness of 6 inches is sufficient. It should be allowed to set thoroughly before the blocks are laid, and traffic should not be allowed to pass over it for a week. The finished pavement should be covered with a thin layer of sharp grit, which is forced into the wood by the traffic and forms a hard face. Several applications of grit are desirable at first, and from time to time afterwards, both as a protection to the wood and to prevent slipperiness. Systematic cleansing is required to prevent slipperiness and foul smells, and to preserve the pavement. Cleans ing may be aided by washing, and when it is thoroughly carried out but little watering is required to keep down the dust. A wood pavement is the quietest for the residents, pleasant to travel over, and favourable to the wear of vehicles. Traction on it is easy and foothold good, so that it may be laid on gradients as steep as 1 in 20. The wear of wood pavements in London is stated by Mr Stayton to be from 065 inch per year in Sloane Street, with a traffic of 279 tons per yard of width per day, to 456 inch per year in Fleet Street, with a traffic of 1360 tons per yard of width per day. Reduced to a standard of traffic of 750 tons per yard per day, the comparative annual wear becomes 175 in the former and 251 in the latter street. In Parliament Street, Westminster, blocks removed after four years in places where patching was required had lost 1 to 1 inches in thickness, equal to one-third of an inch per year under traffic stated to be 1106 tons per yard of width per day. From information afforded by Mr Haywood it appears that in the City of London under traffic of from 300 to 660 vehicles per yard of width per day of 12 hours the wear is from 2 to 3 inch per year, and that in King William Street, London Bridge, under a traffic of about 1200 vehicles per yard of width in 12 hours the wear was found to be 2 inches in 34 years in the middle of the road, or '81 inch per year. This is the heaviest traffic to which wood pavement has been subjected. The wear is generally considered to be as much due to the horses' feet as to the wheels, and the action of the former is more destructive on steep gradients. Towards the end of the life of the blocks the wear is more rapid than at first. Few wood pavements retain a sufficiently good surface after about six years' wear without extensive repairs, and it is probably not advantageous to lay blocks of a greater depth than will provide for a duration of seven years; 5 inches are almost always sufficient for this. Wood pavements of plain blocks on a cement concrete bed are Cost. now (1885) laid at from 10s. 6d. to 12s. 6d. per square yard, a considerable reduction on the prices paid for patented systems a few years ago. Of the above prices 2s. 3d. to 3s. 9d. is the cost of the foundation, which does not require renewal like the blocks. As suming the average life of the latter to be seven years, Mr Stayton estimates the annual cost of wood paving in Chelsea with a traffic of 500 to 750 tons per yard of width per day to be 1s. 9d. per square yard, which includes the cost of original construction, repairs and renewals, and interest, spread over fifteen years. Cleansing and sanding are estimated to cost 5d. per square yard in addition. Asphalt Paving.-Asphalt was first used for street paving in ASPHALT Paris in 1854. It was introduced in London in 1869, when Thread- PAVING. needle Street was paved by the Val de Travers Asphalt Company, and since then it has been extensively used for paving both streets and footways. The material is a hard limestone impregnated with bitumen in the proportion of from 6 to 8 per cent. in the Seyssel Com cost. rock, and from 10 to 12 in that from Val de Travers. Asphalts containing less than the former proportion have not sufficient coherence for street pavements, and those containing more than the latter proportion soften from heat in the summer. Asphalt is employed either as a mastic or compressed. The mastic is previously prepared in cakes and is melted for use in caldrons with a small quantity of bitumen, and for a street pavement is thoroughly mixed with sand or grit. It is spread in one thickness on a concrete foundation, covered with sand, and beaten to an even surface. This material has not proved so successful for street surfaces as compressed asphalt. To produce this the rock asphalt, previously reduced to a fine powder by mechanical means, is heated in revolving ovens to from 220° to 250°, spread while still hot, and compressed into a solid mass by hot disk-shaped rammers, and afterwards smoothed with irons heated to a dull redness. The original rock is thus as it were reconstructed by taking advantage of the power of coherence of the molecules under pressure when hot. In heating the powder the moisture combined in the limestone must be driven off without reducing the proportion of the bitumen more than is unavoidable. The powder cools very slowly and may be conveyed long distances from the ovens; it may even be kept till the next day before use. When laid it should still retain a temperature of from 150° to 200°. It is spread evenly with a rake by skilled workmen for the whole width of the street to a thickness about two-fifths greater than the finished coating is intended to be. Ramming is commenced with light blows to ensure equality of compression throughout and is continued with increased force until the whole is solidified. The ramming follows up the spreading, so that a joint is required only when the work is interrupted at the end of a day, or from some other cause. In a few hours after it has been laid an asphalt pavement may be used for traffic. When finished, its thickness may be from 1 to 24 inches, according to the traffic; a greater thickness than the latter cannot be evenly compressed with certainty. The asphalt loses thickness by compression under the traffic for a long time and to the extent, it is said, of one-fifth or one-fourth, but the wear appears to be very small. A pavement in Paris which had lost more than one-fourth of its thickness was found to have lost only 5 per cent. of its weight after sixteen years' wear. The pavement in Cheapside, after fourteen years under exceptionally heavy traffic, has been reduced, where not repaired, from its original thickness of 2 to about 18 inches. The wear-resisting power of the asphalt is due to its elasticity; tracks are made by the wheels at first, but when thoroughly compressed by the traffic the surface retains little or no trace of the heaviest loads. Repairs are casily and quickly made by cutting out defective places and ramming in fresh heated powder, which can be done in the early morning without stopping the traffic. An unyielding foundation is indispensable; it should be of the best Portland cement concrete, 6 inches in thickness, which must be well set and perfectly dry throughout before the asphalt is laid, or the steam generated on the application of the hot powder will prevent coherence and lead to cracks and holes in the asphalt, which quickly enlarge under the traffic. For the same reason the asphalt should be laid in dry weather. The concrete foundation must be carefully formed to the proper profile, with an inclination towards the sides of not more than 1 in 50, which is sufficient with so smooth a surface. About 1 in 50 is the steepest gradient at which an asphalt pavement can be safely laid. When either dry or wet it affords good foothold for horses, but when beginning to get wet, or drying, it is often extremely slippery. This is said to be due to dirt on the surface, and not to the nature of the material. Sand is strewed over the surface to remedy the slipperiness; it tends, however, to wear out the asphalt, and great cleanliness is the best preventive. An asphalt pavement can be kept cleaner than any other, is impervious to moisture, and dries quickly. It is noiseless, except from the clatter of horses' feet on it; it is the pleasantest pavement to travel upon, but it has the drawback of imperfect foothold and slipperiness at times. The cost of a compressed asphalt pavement 2 to 24 inches thick on a Portland cement concrete foundation 6 inches thick is from 13s. to 16s. a square yard, and the maintenance is usually undertaken for a period of seventeen years by the company laying the pave ment, the first two years free and at 3d. to 1s. 6d. per square yard, according to the traffic, in succeeding years. Comparison of Street Surfaces.-The comparative cost of various parative street surfaces in Liverpool, including interest on first cost, sinking fund, maintenance, and scavenging, when reduced to a uniform standard traffic of 100,000 tons per annum for each yard in width of the carriage-way, is given by Mr Deacon as follows:- Asphalt paving may be placed between wood and bituminous con- The comparative ease of draught on various surfaces is largely Draught. doubted if dynamometer experiments, however carefully made, are influenced by the amount of foothold afforded, and it may be altogether conclusive. The tractive force is influenced by the gradient, the diameter of the wheels, the friction of the wheel axles, and the speed, as well as by the resistance of the road surface, and these must be all taken into account to obtain accurate results. Some recent experiments made, under the direction of Sir J. W. Bazalgette, with Easton and Anderson's horse dynamometer on London street surfaces gave the following mean results : Macadamized surface Granite sets : The gross load was 4 tons, drawn at a speed of from 2 to 6 miles an hour. It is remarkable that the tractive force on asphalt is so high; but the other results are consistent with former experiments by Morin, Macneill and others. The comparative safety of granite, wood, and asphalt pavements Safety. in the City of London was the subject of careful observations, which were fully reported on by Mr W. Haywood in 1873. The pavements selected were granite sets 3 inches wide, ligno-mineral pavement of beech blocks 3 inches wide, improved wood pavement of fir blocks 3 inches wide, and Val de Travers compressed asphalt pavement. On known lengths of these the traffic, the accidents to horses, the weather, and other circumstances were observed for fifty days, and when the distance traversed was taken into account it was found that as a mean result a horse might be expected to travel 132 miles on granite without falling, 191 on asphalt, and 446 on the improved wood pavement. The condition of the weather had considerable effect: on the granite when dry a horse might be expected to travel 78 miles without falling, when damp 168, and when wet 537; on wood when damp 193 miles, when wet 432, and when dry 646; on asphalt when damp 125 miles, when wet 192, and when dry 223. It thus appeared that wood pavement was less slippery than either granite or asphalt in a marked degree, it being only more slippery than granite when both pavements were wet. About 85 per cent. of the falls on the wood pavement were falls on the knees, which are less likely to injure the horses and are less inconvenient to the traffic than other falls. On the granite the falls were falls on the knees or complete falls in about equal proportions, with about 7 per cent. of falls on the haunches. On the asphalt 43 per cent. were complete falls and 24 per cent. falls on the haunches. Watering.-On macadamized roads in Great Britain watering is Wateronly good for the road itself when the materials are of a very sili- ing. cious nature and in dry weather. With other materials the effect is to soften the road and increase wear. In and near towns watering is required for the comfort of the inhabitants, but it should not be more than enough to lay the dust without softening the road, and the amount required for this may be greatly reduced by keeping the surface free from mud, and by sweeping off the dust when slightly wetted. Pavements are watered to cleanse them as well as to lay the dust, but it must be remembered that both wood and asphalt are more slippery when wet, and that therefore watering should be obviated as far as possible by thorough cleansing. Hydrostatic vans, by improvements in the distributing pipes and regulating valves, water a wide track uniformly with an amount of water which can be regulated at pleasure. Where hydrants exist in connexion with a water supply at high pressure, street watering can be effected by a movable hose and jet, a method much more effective in cleansing the surface, but using a much larger quantity of water. Another method which has been tried, but not much used, is to lay perforated pipes at the back of the kerb on each side of the road, from which jets are thrown upon the surface, The first cost is considerable, and the openings for the jets are liable to choke and get out of order. Deliquescent salts have been used for street watering, by which the surface is kept moist, but at the expense of the moisture in the air. Sea water has the same effect in a less degree. Cleans ing. Footways. Kerbing. Cleansing. The principal streets of a town are generally cleansed daily, either by hand-sweeping and hand-scraping or by machines. Whitworth's machine consists of a series of revolving brooms on an endless chain, whereby the mud or dust is swept up an incline into the cart. A less costly and cumbersome machine consists of a revolving brush mounted obliquely, which sweeps a track 6 feet wide and leaves the dust or mud on one side to be gathered up by hand. A horse scraping-machine which delivers the mud at the side is also used, the blades of the scrapers being mounted obliquely and covering a width of 6 feet. For general use, more especially in the country, scraping machines worked by a man from side to side of the road, and scraping a width of about 4 feet, are more convenient. All street surfaces suffer from the constant breaking up and disturbance to which they are subjected for the purpose of laying and repairing gas and water pipes. Subways, either under the middle of the road or near the kerbs, in which the pipes may be laid and be always accessible, have often been advocated, and in a few instances have been constructed; but they have not hitherto found general favour. Footways.-Gravel is the most suitable material for country or suburban footways; it should be bottomed with a coarser material, well drained, and should be laid with a roller. An inclination towards the kerb of about half an inch in a foot may be given, or the surface may be rounded, to throw off the wet. Where greater cleanliness is desirable and the traffic is not too great a coal-tar concrete similar to that already described, but of smaller materials, makes a good and economical footway. The coating should be 21 or 3 inches thick, composed of two or three layers each well rolled, the lower layer of materials of about 14 inches gauge, and the upper of a half or a quarter of an inch gauge, with Derbyshire spar, or fine granite chippings over all. Concrete footways require to be carefully made and must be allowed to set thoroughly before they are used. Concrete has a tendency to crack from contraction, especially when in a thin layer, and it is better to lay a footway in sections, with joints at intervals of about 2 yards. Concrete slabs, especially when silicated and constituting artificial stone, make an excellent footway. The material is composed of crushed granite, gravel, or other suitable material, mixed with Portland cement and cast in moulds, and when set saturated with silicate of soda. This paving has proved more durable than York stone flagging, but it is more slippery, especially when made with granite. York stone makes a good and pleasant foot pavement, but is somewhat expensive considering its durability; it is apt to wear unevenly and to scale off when the stone is not of the best quality. It should not be laid of a less thickness than 2 inches; 2 or 3 inches are more usual. The flags should be square jointed, not under-cut at the edges, and should be well bedded and jointed with mortar. Caithness flag is much more durable than York stone and wears more evenly; it is impervious to wet and dries quickly by evaporation. The edges are sawn, and the hardness of the stone renders it difficult to cut it to irregular shapes or to fit openings. Staffordshire blue bricks and bricks made of scoria from iron furnaces are both very durable, though somewhat brittle. Asphalt either laid as mastic or compressed is extensively used for footways; the former is considered inferior in durability to York stone and the latter superior to it. Asphalt should not be laid less than three-fourths of an inch thick on 4 inches of cement concrete, and 1 inch of asphalt is desirable where there is great traffic. Footways in a street must be retained by a kerbing of granite, York stone, Purbeck, or other stone sufficiently strong to stand the blows from wheels to which it is subjected. It should be at least 4 inches wide and 9 deep and in lengths of not less than 3 feet. A granite kerb is usually about 12 by 6 inches, either placed on edge or laid on the flat. When set on edge a kerb is generally bedded on gravel with a mall; when laid on the flat a concrete bed is desirable. Side In a macadamized street pitched or paved water channels are channels. required, to prevent the wash of the surface water from undermining the kerb. The pitching consists of cubical blocks of hard stone about 4 inches deep, bedded on sand or mortar, or preferably on a bed of concrete. A paved channel consists of flat stones about 1 foot wide inclining slightly towards the kerb. Moulded bricks and artificial stone are also used both for side channelling and for kerbing. Such an inclination must be given to the channel as will bring the surface water to gullies placed at proper intervals, and the level of the kerbing and consequently of the footway will depend to some extent on the surface drainage as well as on the levels of adjacent houses. To lay out a street satisfactorily the longitudinal and transverse sections must be considered in relation to these matters as well as to the levels of intersecting streets. For fuller information on the subject see Sir Henry Parnell, A Treatise on Roads; Thomas Codrington, The Maintenance of Macadamized Roads; Debauve, Manuel de l'Inégnieur des Ponts et Chaussées; Annales des Ponts et Chaussées; Minutes of Proc. Inst. Civ. Eng., "Street Pavements," vol. Iviii. p. 1, and "Wood Pavements," vol. lxxviii. p. 240; Reports by W. Haywood, engineer to the commissioners of sewers of the City of London. (T. C.) ROANNE, a town of France, at the head of an arrondissement in the department of the Loire, lies on the left bank of the Loire in 46° 2′ 26′′ N. lat. at a height of 912 feet above the sea. It is now the point of junction for the railway from Paris (262 miles north-north-west) to Lyons (50 miles south-east), via Tarare, with the line from Paris to St Étienne (50 miles south-south-east), and a branch connecting Roanne with Paray le Monial; and as the terminus of the Roanne-Digoin Canal (1832-38) the town is the real starting-point of the Loire navigation. Besides the modern town-house (1868-73), it is enough to mention the ruins of a castle with a tower dating from the 11th century, and a fine bridge of seven arches connecting Roanne with the industrial suburb of Le Côteau on the right bank of the river. Cotton is the staple manufacture, employing 1200 hands. Hosiery, hats, woollen yarn, weaving looms, chemicals, and paper are also produced; and, as the town stands in the centre of the Loire and Rhone coal-field (output 4224 tons in 1884) and in the neighbourhood of the St Etienne coal-field, it has a considerable trade in coal and coke. In 1881 Roanne had a population of 24,992. Roanne (Rodomna, Ptolemy; Roidomna, Tab. Peut.) was an ancient city of the Segusiani and a station on the great Roman road from Lyons to the ocean. The absence of coins later than the time of Constantius II. among the numerous local relics of the Roman period seems to show that the town was sacked by the barbarians in the 4th century. In 1447 the lordship of Roanne became the property of the celebrated banker Jacques Coenr. A favourite scheme of his was to make the town a great industrial centre by regulating the course of the Renaison, an affluent from the Monts de la Madeleine which joins the river a little higher up; his death prevented its execution, but the subject has since been frequently revived. ROBBERY. See THEFT. ROBBIA, DELLA, the name of a family of great distinction in the annals of Florentine art. Its members are enumerated in chronological order below.1 I. LUCA DELLA ROBBIA (1399 or 14002-1482) was the son of a Florenţine named Simone di Marco della Robbia. According to Vasari, whose account of Luca's early life is little to be trusted, he was apprenticed to the silversmith Leonardo di Ser Giovanni, who from 1355 to 1371 was working on the grand silver altar frontal for the cathedral at PISTOIA (q.v.); this, however, appears doubtful from the great age which it would give to Leonardo, and it is more probable that Luca was a pupil of Ghiberti. During the early part of his life Luca executed many important and exceedingly beautiful pieces of sculpture in marble and bronze. In technical skill he was quite the equal of Ghiberti, and, while possessing all Donatello's vigour, dramatic power, and originality, he very frequently excelled him in grace of attitude and soft beauty of expression. No sculptured work of the great 15th century ever surpassed the singing gallery which Luca made for the cathedral at Florence between 1431 and 1440, with its ten magnificent panels of singing angels and dancing boys, far exceeding in beauty those which Donatello in 1433 sculptured for the opposite gallery in the same choir. This magnificent work now lies scattered in various parts of the 1 Genealogical tree of Della Robbia sculptors : |