INDEX AND GLOSSARY

Abutment Piers, these are so strong that they act as abutments, and hence the loss of one arch does not overthrow another by withdrawing a counterbalancing thrust from one side of a pier. Perronet says: “The piers of bridges ought to be considered either as performing the duty of abutments, or as relieved of this duty by the counteraction of the collateral arches, through which the thrust is carried from abutment to abutment of the bridge. In the first case, piers should resist lateral pressure as capably as the abutments themselves, that they may withstand the side thrust of the arch-stones which tends to overturn them, and which increases by so much the more as the arches are flatter and the piers loftier. In the second case, the piers must have substance enough to carry the weight of the two half arches raised upon the two sides of each pier respectively,” together with those parts of the upper works that lie over each pier. Roman piers are abutments also, as a rule, their thickness ranging from a half to a third of the spaces between them; the effect of this great bulk both on the current of rivers and on Roman bridge-building is described on page [284]. A great many bridges of the Middle Ages had abutment piers, but in many cases they were dams rather than bridges; the piers occupied far too much space in the waterways and caused terrible floods like those that happened at Lyons in the winter of 1839-40. Old London Bridge was a perforated dam ([p. 220]); and after her removal in 1831-2, an improvement was noted in the drainage, and consequently in the healthiness, of all the lower parts of London above bridge. So abutment piers, when they are either too thick or too numerous, are social evils. This fact was recognised by bridgemen at the beginning of the sixteenth century, when some diminution took place in the relative proportion of the piers of bridges to the spans of arches; and little by little a new routine came into vogue and displaced the abutment pier from all service. Here was another social evil, for long arched bridges with no abutment piers were unmilitary, and therefore at odds with the strategy of national defence. Not an arch could be cut without endangering its neighbouring arches. Gabriel and Perronet, after considering this fact, wished abutment piers to be revived in a discreetly effective manner ([footnote p. 338]), but their excellent advice was not followed. Defenceless bridges became fashionable everywhere, though they added innumerable anxieties to the perils of military war. The Valentré Bridge at Cahors should be studied as the best example of a mediæval battle-bridge, but the abutment piers might have been improved, [283-4]. To-day a new era in bridge-building is heralded by rapid improvements in airships and aeroplanes; there should be a congress of architects and engineers to discuss the urgent questions of national defence that the piers and footways of bridges bring before our common sense, [335], [358].

Arabian Arches, their shapes are of three sorts, the horseshoe, the semicircular, and the pointed. Often they are enriched by a sort of feathering or foliation around the arch, and this ornament is closely akin to Gothic work, which it preceded by a considerable time. The Arabian style, known also as Saracenic and Moorish, is a fanciful composition in which details from Egypt and Greece and Rome are alembicated with “the light fantastic lattice-work of the Persians.” To-day we find its graceful influence in the greatest bridges at Isfahan, [213], and also in much Spanish work, [28-9], [285-6], [288]. Some writers believe that pointed arches were invented by the Arabs, yet they were built in Egypt during the Fourth Dynasty, [155-6], and also by the Babylonians, [275 footnote]. The Saracenic pointed arch was a forerunner of the Gothic pointed style, and it became familiar to the Crusaders, [86-93]; but we must draw a wide distinction between the pointed arch and the pointed Gothic style. Arabian architects did not achieve an upward flight and rhythm akin to the vertical principle of inspired Gothic; their buildings preserved the horizontal line which gave and gives character to classical traditions, [152], [153], [336]. If, then, the pointed arch in Europe was borrowed from Arabian architects, as many antiquaries believe, [88], it passed through a great transformation in technical sentiment, and became an original inspiration.

Boats ought to be added to the remarks on [page 58], or to the first section of the second chapter ([pp. 109-12]), for primitive man got his first boats from Nature. The earliest were floating branches and trees on which men sat astride, drifting with the current of rivers; the later were trees hollowed out by decay, which became models for dug-outs. “Between the primitive dug-out and a modern man-of-war there is, apparently, an impassable gulf; but yet the two are connected by an unbroken chain of successive improvements all registering greater efficiency in mechanical skill. Each of those intermediate increments constitutes a numbered milestone in the history and development of navigation.”—Dr. Robert Munro.

Boats, Bridge of, at Cologne, [1]. It will be remembered that Julius Cæsar frequently made use of boat-bridges, and that Xerxes, four hundred and eighty years before the Birth of Christ, made a bridge of boats across the narrowest part of the Hellespont, between the ancient cities of Sestus and Abydus. So the boat-bridge at Cologne, like the wooden pontoon, has an old and fascinating lineage, yet a modern bridge was going to displace it when the present Great War began. “Kultur” cancels history.

Brackets, below the parapet of the Pont Neuf at Paris, [321]. Brackets are ornamental projections from the face of a wall, to support statues and other objects. Some are adorned only with mouldings, while many are carved into angels, or foliage, or heads, or animals. Parker says: “It is not always easy to distinguish a bracket from a corbel; in some cases, indeed, one name is as correct as the other.” See Brangwyn’s drawing of the Pont Neuf facing [page 320].

Bradford-on-Avon, Wiltshire, the bridge there has a tiny oratory, [231-2], which was profaned after the Reformation, becoming a “lock-up,” and then a powder magazine, [232]. The bridge has nine arches; the two pointed ones uniting the oratory to the bankside have ribbed vaults, and the others are round-headed arches with double rings of voussoirs, [305 footnote]. Originally the bridge was a narrow one for packhorses, but it was widened in 1645, or thereabouts. A hospital used to stand at one end of the bridge, and doles of charity for it may have been collected in the little place of prayer. Leland admired this bridge, and noted its nine fair arches of stone, and a fair large parish church standing beneath the bridge on Avon ripe.

Bridge built with Arches, its anatomy. Professor Fleeming Jenkin says: “An arch may be of stone, brick, wood, or metal. The oldest arches are of stone or brick. They differ from metal and from wooden arches, inasmuch as the compressed arc of materials called the ring is built of a number of separate pieces having little or no cohesion. Each separate stone used in building the ring has received the name of voussoir, or archstone. The lower surface of the ring is called the soffit of the arch. The joints, or bed-joints, are the surfaces separating the voussoirs, and are normal to the soffit. A brick arch is usually built in numerous rings, so that it cannot be conceived as built of voussoirs with plane joints passing straight through the ring. The bed-joints of a brick arch may be considered as stepped and interlocked. This interlocking will affect the stability of the arch only in those cases where one voussoir tends to slip along its neighbour. The ring springs from a course of stones in the abutments, called quoins. The plane of demarcation between the ring and the abutment is called the springing of the arch. The crown of an arch is the summit of the ring. The voussoirs at the crown are called keystones. The haunches of an arch are the parts midway between the springing and the crown. The upper surface of the ring is sometimes improperly called the extrados, and the lower surface is more properly called the intrados. These terms, when properly employed, have reference to a mathematical theory of the arch little used by engineers. The walls which rest upon the ring along the arch, and rise either to the parapet or to the roadway, are called spandrils. There are necessarily two outer spandrils forming the faces of a bridge; there may be one or more inner spandrils. The backing of an arch is the masonry above the haunches of the ring; it is carried back between the spandrils to the pier or to the abutment. If the backing is not carried up to the roadway, as is seldom the case, the rough material employed between the backing and the roadway is called the filling. The parapet rests on the outer spandrils.”

Bridge-Wreckers, [352], [355]. It is worth noting that the King of the Belgians in the present Great War has used a cyclist corps of bridge-wreckers, whose work is described in the Daily Mail, December 14, 1914, page 4. “The cyclists led the way. The explosives followed in a car. The charge was fixed to the girders under the bridges, an electric wire affixed, you touched a button and the near span of the bridge was in a moment no more than a gap. Their greatest achievement ... was a railway bridge between Courtrai and Audenarde. It needed two charges.” The cyclists regarded their work as “fun,” because no bridge was at all difficult to destroy.

Centres or Centring, the curved scaffolding upon which arches are built. The voussoirs rest on the centres while the ring is in process of being constructed. When the centres are not rigid enough, arches sink a good deal while the masons are at work and after the scaffolding is carefully struck. In Perronet’s bridge at Neuilly-sur-Seine, for example, the sinking amounted to twenty-three inches, [338]; thirteen inches while the centre was in its place, and ten inches after the centre was removed. On the other hand, when the centres of Waterloo Bridge were taken down, no arch sank more than 1½ inches. There is reason to believe that modern centres are more complicated than were the mediæval. See page [264] and page [286].

Cutwaters, [262], [316]. The French words for cutwaters, avant-bec and arrière-bec, would be very useful to us if we translated them as “forebeak” and “aftbeak.” British pontists need a good many technical terms.

Devil’s Bridges, [66,] [67], [70,] [170], [184], [296]. Many other bridges have been attributed to the devil. In plate 58 of the treatise by Hann and Hosking, you will find the Devil’s Bridge over the Serchio near Lucca; there is also a very interesting account of it, p. cxxxv. It is a gabled bridge with one big arch and four smaller ones. The span of the big arch is 120 feet, and its height above low-water level is more than 60 feet. The roadway is very narrow, being only 9 feet wide, and it turns abruptly at the wings, as if to close the entrances against wheeled traffic. The quoins of the smaller arches and all the voussoirs of the wide arch are of dressed stone. Every other part of the bridge is rubble masonry bound together with most excellent mortar. The courses of stone in the wide arch vary from 8 inches to 21 inches deep, but only a few have the latter depth. Yet this slight bridge, which is nothing more than a broad arcaded wall, has withstood many centuries of floods. On October 2nd, 1836, for example, a head of water more than 30 feet deep swept roaring through the five round arches and against the four piers at the rate of 8 miles an hour; yet no harm was done. If this bridge was built about the year 1000 A.D., as Hann and Hosking say, it is somewhat older than the controversial date of Albi Bridge.

Girders, there are three types or classes of bridge: the girder, the arched, and the suspended. Girders may be of various materials; wrought iron, cast iron, and wood are chiefly used. Professor Fleeming Jenkin describes with apt brevity the essential difference between the three classes of bridge. “In all forms of the suspension bridge the supporting structure is extended by the stress due to the load; in all forms of the arch the supporting structure (i.e. the ring of voussoirs) is compressed by the stress due to the load; and in all forms of the beam or girder the material is partly extended and partly compressed by the flexure which it undergoes as it bends under the load. Thus when a beam of wood carrying a load bends, the upper side of the beam is thereby shortened and the fibres compressed, while the lower side of the beam is lengthened and the fibres extended.” So, too, in a girder of metal. In some bridges, as in the High Level Bridge at Newcastle, the girder principle is united to bowstring arches of metal, but a true girder is less expensive and lighter, [80].

Hosking has many good remarks on the subject of cramps and joggles. He says (p. 208): “It is very desirable that all the archstones of a large and flat arch should be dowel-joggled in the beds; but as the usual dowel-joggle cannot be introduced with the key-course, plugs of proportionate size must be used instead, and the stones may, besides, be cramped together. In arches of small size, or in large ones of quick sweep, joggling may not be so desirable as in those of large size and flat sweep; though it is to be understood that in any case both joggles and cramps should be considered as surplusage, and as precautions merely, to counteract the effect of any imperfections in the work from want of fulness in any of the stones in an arch, or otherwise. In building London Bridge iron bars were let into the back ends or tails of the archstones, and run with lead as cramps or transverse ties in several courses, and they do not appear to have produced any injurious effect, though it may be questioned how far they are of any use. They ought not to be of any use.” Viollet-le-Duc went further than this; he regarded iron cramps in a stone bridge as likely to be injurious.

Kircher, Athanasius, German traveller and philosopher, b. 1602—d. 1680, his book on China, translated into French by Dalquié, [314], [345], and [footnote].

Lacer, Caius Julius, Roman architect, and builder of Trajan’s Bridge over the Tagus at Alcántara, [121], [184], [190], [344]. He was buried on the left bank quite close to his bridge, [184], a romantic circumstance, like the burial of Bénézet and Colechurch in their bridge chapels.

Law, Modern, in Great Britain law prescribes minimum dimensions for the over and under bridges of railways; but it takes no notice at all of the military considerations which can never be wisely disconnected from the circulation of traffic along roads and over bridges. An over bridge is one in which a road goes over a railway; an under bridge is one in which a road goes under a railway. Both are exceedingly vulnerable, yet the law centres all its attention on details that concern their size, not on details that concern their protection from violence. Over Bridges.—Width: turnpike road, 35 feet; other public carriage road, 25 feet; private road, 12 feet. Span over two lines (narrow gauge), generally about 26 feet; head room, 14 feet 6 inches above outer rail. Under Bridges.—Spans: turnpike road, 35 feet; other public road, 25 feet; private road, 12 feet. Head room: turnpike road, 12 feet at springing of arch, and 16 feet throughout a breadth of 12 feet in the middle; for public road, 12 feet, 15 feet, and 10 feet in the same places; private road, 14 feet for 9 feet in the middle; for exceptions the Acts must be studied.

Machicolations, openings between the corbels that support a projecting parapet, or in the floor of a gallery or the roof of a portal, for shooting or dropping missiles and boiling liquids upon assailants attacking the base of the walls. They were used in the defence of old bastille bridges, and silly modern engineers have copied them as dummy ornaments with which to decorate trumpery defenceless gateways and towers, [275], [323].

Orthez, Vieux Pont, mediæval war-bridge, [278-9]. There are two conflicting accounts of the part played by this bridge in the battle of Orthez, February 27th, 1814. One of them says that the bridge was neutralised by agreement in order to spare it from destruction; the other account declares that the solidity of the stonework baffled the French attempts to break it down. Anyhow, the bridge was not used in the action. Hill crossed well above it, and Picton and Beresford below. Napier says: “Hill, who had remained with 12,000 combatants, cavalry and infantry, before the bridge of Orthez, received orders, when Wellington changed his plan of attack, to force the passage of the Gave, partly in the view of preventing Harispe from falling upon the flank of the sixth division, partly in the hope of a successful issue to the attempt: and so it happened. Hill, though unable to force the bridge, forded the river above at Souars, and driving back the troops posted there, seized the heights above, cut off the French from the road to Pau, and turned the town of Orthez.”

Parapets, low walls or railings serving to protect the edge of a bridge; they rest on the outer spandrils; sometimes they project beyond and need brackets or corbels, like the Pont Neuf at Paris, [321-2], and plate facing page [320]. Often in the Middle Ages some parts of the parapets were crenellated, as they are above the angular piers of the Valentré at Cahors, see the colour plate facing page [264]; even some modern defenceless bridges have battlemented parapets, for the imitative silliness of industrial engineers delights in foolish make-believe. Parapets cannot be studied with too much care, so there are frequent references to them throughout this monograph. Some Roman bridges were built without parapets; there is an example near Colne, [162], [164]; and many of the gabled bridges in Spain repeat in a giddy manner this dangerous defect, [27].

Paris and her Bridges, [225], [321-2]. Here is a fine subject for a book. There is a good reference to the Paris bridges of the year 1517-18 in the “Revue des Deux Mondes,” xlvii., Sep., 1908, p. 467. Five bridges existed then, three stone structures, and two of wood; and all of them had houses from one end to the other. Tolls were charged and they belonged to the King. Several illustrations of Paris bridges will be found in Lacroix, “Manners, Customs and Dress during the Middle Ages.” On page 321 there is one of the year 1500; see also in the same book pp. 302, 316, and 471.

Parliament of Taste, a, necessary in all large towns for the discussion of art in all matters that concern the public intimately, [324-5].

Peace, considered in her relation to the varied strife circulated by roads and bridges. She is an illusion of the mind and belongs to a routine of idle sentiment, [vii], because every phase of human enterprise claims a battle-toll of killed and wounded and maimed, [vii], [3], [4], [33-6]; see also section ii, Chapter I, [14-52], and [333], [351], [360-1].

Perforated Towers on bridges; modern engineers have passed suspension cables through towers instead of passing them over the summits, [346], [354].

Piers of Bridges, [114], [200], [264], [316], [338], [341], [342], [353], [354]. There are other references also, but the reader will be able to follow the history of piers from the natural bridge of stepping-stones through the many changes and defects mentioned in the text. To-day, with the rapid improvements in airships and aeroplanes, new armoured piers will have to be designed, strong enough to bear the great weight of a roofed superstructure of armour-plate steel, yet not thick enough to obstruct rivers. Now that bridges are as vulnerable as Zeppelin sheds, engineers have an excellent chance to serve their countries well by inventing new and powerful bridges. How to protect piers—at least as much as possible—from direct artillery fire is one very difficult problem; how to protect them from falling shells and bombs is another. When London is fitted adequately with new defensive bridges her river will be as impressive as a fleet of super-Dreadnoughts. See also [“Abutment Piers.”]

Pisa, her chapelled bridge, [209]. The late Mr. S. Wayland Kershaw wrote as follows in 1882: “The most remarkable bridge chapel abroad is the one dedicated to Santa Maria del’ Epina on the side of the bridge over the Arno at Pisa, erected about 1230. Built of the rich stone and marble of the district, it is ornamented with niches and figures, and, though renovated and repaired, still presents a graceful appearance.”

Pontist Brothers or Friars, or Frères Pontifes, [83], [90], [91], [92], [296], [297], [342]. St. Bénézet was one of the leaders in this religious brotherhood of good craftsmen.

Many railway bridges over strategical rivers can be displaced by tunnels, but many others must be armoured with cone-shaped roofs as a protection against overhead wars from airships and aeroplanes, [358]. See Albi Railway Bridge, the plate facing page [8], and Cannon Street Railway Bridge, the plate facing page [48].

Rameses II, Temple of, at Abydos, has a primitive vault built with horizontal courses of stone, showing its descent from the rock archways made by Nature, [155].

Relief Bays for Flood Water, they were introduced by the Romans, 284, and were copied by mediæval bridgemen; witness the Pont des Consuls at Montauban, [255], [256], and the Pont St. Esprit, [293], [297]. Pontists should note both the difference of shape in flood-water bays and the variation of their position in the architecture. At Mérida, for example, in the great squat Roman bridge, they are long and round-headed, and rise from the low and bold cutwaters, which are overgrown with grey-green mosses and grass. On the other hand, a Moorish bridge of four arches near Tangier has much smaller relief bays with round heads, and they are pierced high up through the spandrils. They look like three little windows that give light and air to a work of sun-bleached antiquity. Moreover, their shape is repeated in about a dozen little holes cut through the base of the parapet, perhaps to help in the drainage of the roadway, perhaps to be useful in military defence. This Moorish bridge has semicircular arches, and the road is inclined over each abutment, just like the Roman bridge at Rimini. But the technical sentiment is less virile than the Roman.

Religious Emblems or Symbols on Historic Bridges, such as the Phallus on the Pont du Gard, [174]; the Janus heads on the Pons Fabricius, [196]; the idol or image on the Chinese bridge at Shih-Chuan, [247]; and the cross and crucifix on Gothic bridges of the Middle Ages, [96], [230], [246]. The symbolic lion and tortoise on the Chinese bridge of Pulisangan were borrowed from the singa and Kûrma of Hindu mythology, [311 footnote]. I should like the cross to be raised again on all bridges in unfortified towns, as a protest against a Teutonic misuse of flying warfare.

Rennie, John, b. 1761—d. 1821, his poor bridge over the Thames at Southwark was financed by a Company, not by the City, as if London were a trivial village with some new industries that needed encouragement, [326-7].

Rennie, Sir John, son of John Rennie and brother of George Rennie, was the acting engineer during the building of New London Bridge, according to Professor Fleeming Jenkin.

Rhône, the River, his two famous old bridges, the Pont St. Bénézet and the Pont St. Esprit, both constructed by the Frères Pontifes, or Pontist Brothers. See Brangwyn’s pictures and the text.

Ribbed Arches, like those in the Monnow Bridge at Monmouth, [281], and the Pont de Vernay at Airvault, Deux-Sèvres, plate facing page [96]. The introduction of ribbed vaulting into English churches and bridges, [93-100]. Professor Moseley’s remarks on groined or ribbed arches may be quoted here from Hann and Hosking’s profuse volumes. “The groin ... is nothing more than an arch whose voussoirs vary as well in breadth as in depth. The centres of gravity of the different elementary voussoirs of this mass lie all in its plane of symmetry. Its line of resistance is therefore in that plane.... Four groins commonly spring from one abutment; each opposite pair being addossed, and each adjacent pair uniting their margins. Thus they lend one another mutual support, partake in the properties of a dome, and form a continued covering. The groined arch is of all arches the most stable; and could materials be found of sufficient strength to form its abutments and the parts about its springing, I am inclined to think that it might be built safely of any required degree of flatness, and that spaces of enormous dimensions might readily be covered by it.” Yet “modern builders, whilst they have erected the common arch on a scale of magnitude nearly approaching perhaps the limits to which it can be safely carried, have been remarkably timid in the use of the groin.” Progress may be compared to a dilatory army that ever fails to march forward with all its needed units.

Ring of an Arch, the compressed arc of voussoirs, [264]; the lower surface of a ring is called the soffit of an arch. In some bridges the voussoirs form a double or a triple ring, [305], and [footnote]. Two very fine bridges of this sort, in my collection of photographs, are the Pont de Vernay at Airvault, 12th century, and the Pont Saint-Généroux over the Thouët, also in Deux-Sèvres, 13th and 14th centuries. Another monument to be studied is the reputed Roman bridge at Viviers over the Rhône, built mainly with small materials. Whether Roman or Romanesque, the structure of the arches has great interest, and a large photograph is sold by Neurdein, 52 Avenue de Breteuil, Paris.

Roads, ancient British, [22]; Roman, [139], and [footnote]; they and bridges circulate all the strife in the overland enterprise of mankind, [4], [14-52]; types of society are as old as their systems of circulation, just as women and men are as old as their arteries, [13]; mediæval roads in England, [51], [52]. Many of them were a survival of the Roman empire, in which the construction of highways was a military and political necessity. The genuinely mediæval roads connected new towns with the main or ancient thoroughfares, which had traversed Roman Britain from her principal colonies, London and York, to the other settlements. “The roads of England,” says Thorold Rogers, “are roughly exhibited in a fourteenth century map still preserved in the Bodleian Library, and are identical with many of the highways which we know familiarly. In time these highways fell out of repair, and were put in the eighteenth century under the Turnpike Acts, when they were repaired. But comparatively little of the mileage of English roads is modern. What has been constructed has generally been some shorter and easier routes, for in the days of the stage-coaches it was highly expedient to equalize the stages.”

Ross-on-Wye, Wilton Bridge, an Elizabethan structure with ribbed arches and angular recesses for pedestrians, [94], [182], and [footnote]. Recently, I regret to say, this beautiful old bridge has been attacked by the highwaymen called road officials; and now she is horribly scarred all over with “pointing,” just like the mishandled Roman bridge at Alcántara. A new bridge of ferro-concrete, suitable for motor lorries and the like, would have cost the county less than this uneducated trifling with a genuine masterpiece.

Springing of an Arch, the plane of demarcation between the ring and the abutment is called the springing. In other words, the springing marks the place where a ring of voussoirs starts out on its upward curve from a pier or from an abutment.

Verona. The fine Veronese bridges are not mentioned in this monograph; they passed from the text in a revision; but pontists know them well, and set great store by the charming Ponte di Pietra, and by the old sloping bridge with forked battlements that swaggers picturesquely across the Adige from the Castel Vecchio. The Ponte di Pietra rises from ancient foundations and she still retains two Roman arches, certainly often restored; the other spans are gracefully architectural. A circular bay for the relief of floods tunnels the spandrils above the cutwater of the middle pier.