The American Cyclopædia (1879)/Bridge
BRIDGE, a structure, with one or more transverse apertures, for passing a river, canal, or valley, and formed of various materials, as timber, stone, or iron. The construction of perfect bridges is a complex operation, and among ancient nations, even of the highest civilization, did not always keep pace with the progress of the other arts. The type of the primitive bridges of earlier ages is to be found at the present day among rude and uncultivated nations, and consists simply of lintels of wood stretching from bank to bank, or, when the span renders this impracticable, resting on piers or posts fixed in the bed of the river. The inevitable frequency of these in a rapid stream, and consequent contraction of the waterway, would result in a torrent injurious to navigation, and destructive to the piers themselves; hence it would be found essential to the stability of such structures that the openings should be sufficiently wide to allow every facility for the passage of the water, and as this could only be effected by arches or trusses, it is evident that these inventions were perfected before bridges of any magnitude became common. One of the most extraordinary bridges of ancient times was that which, according to Herodotus, Queen Nitocris constructed over the Euphrates at Babylon, and the length of which is given by Diodorus Siculus as five furlongs; the construction of this bridge is supposed to have been of the kind just alluded to, viz., with lintels or architraves extended from pier to pier. The bridges of Darius upon the Bosporus, Xerxes upon the Hellespont, Cæsar upon the Rhine, and Trajan upon the Danube, are celebrated in history, but were all constructed for purposes of war. The first examples of stone bridges that we can find are those constructed by the Romans. An exception may perhaps be made in regard to the Chinese, as we are not positively acquainted with the date of many of their structures; but in Egypt and India, the birthplaces of so many of the arts and sciences, arched bridges were entirely unknown; neither do we meet with them in the ancient remains of Persia or Phœnicia; and even in Greece, at the period when her architecture was the finest in the world, and when Pericles had adorned Athens with splendid edifices, her people were unprovided with a bridge over the Cephissus, notwithstanding it crossed the most frequented thoroughfare to the city. The principal bridges of Rome were the following: 1. The Pons Sublicius, the first built over the Tiber, and memorable from its defence by Horatius Codes against Porsena; it was twice rebuilt, and the ruins of the last structure are still visible. 2. The Pons Triumphalis, sometimes termed Pons Vaticanus, from its proximity to the Vatican; it derived its former name from being the bridge over which those to whom the senate decreed a triumph passed on their way to the capitol. 3. The Pons Fabricius, named from its founder, L. Fabricius, curator viarum, who erected it during the period of Catiline's conspiracy. 4. The Pons Cestius, built in the reign of Tiberius, and named from Cestius Gallus. 5. The Pons Janiculi, which led from the Campus Martius to the Janiculum. 6. The Pons Ælius, erected in the reign of the emperor Ælius Hadrianus; it is said this bridge had originally a roof of bronze, supported by forty columns, but was despoiled during an incursion of the barbarians. Clement IX., who restored the bridge, placed on it ten colossal statues of angels, carved in white marble, whence is derived its present name, Ponte San Angelo. 7. The Pons Milvius, built in the time of Sulla, on the ancient Via Flaminia, at a short distance from the city; on this bridge Cicero arrested the Allobrogian ambassadors who were the bearers of letters to Catiline, and here also occurred the celebrated victory of Constantine over Maxentius, when Constantine had the miraculous vision of the cross.
Fig. 1.—Pons Senatorius.
8. The Pons Senatorius or Palatinus, still remaining, though in ruins, near the Palatine hill.—From the fall of the Roman empire, we have for several centuries no account of any bridges worthy of note, except those built by the Moors in Spain, one of the finest of which was the bridge of Cordova, over the Guadalquivir, built by Hasbem or Issem, the son and successor of Abderrahman, the first of the Moorish kings of Spain. One of the most ancient bridges of modern Europe is that over the Rhône at Avignon. It was built by a religious society called the “Brethren of the Bridge,” which, according to Gautier, “was established upon the decline of the second and commencement of the third race of kings, when the state fell into anarchy, and there was little security for travellers, particularly in passing rivers, on which they were subject to the exactions and rapacities of banditti.” This society was founded with a view to remedy these evils, by forming fraternities for the object of building bridges, and establishing ferries and caravansaries on the banks of the most frequently crossed rivers; their first establishment was on the Durance, at Maupas, which name was afterward changed to Bonpas, in acknowledgment of their services. The bridge at Avignon was commenced in 1176, and completed in 1188. The association soon after built the bridge of Lyons, composed of 20 arches, and that of Saint Esprit, over the Rhône, of 19 arches, besides many other structures of less note. One of the oldest bridges in England is that of Croyland in Lincolnshire; it is formed by three semicircles which succeed each other, and are based upon a central arch; the ascent is so steep that only foot passengers can accomplish it. The bridge at Burton over the Trent, now partly removed, was the longest in England, and was built in the 12th century; it had 36 arches of squared freestone, and was 1,545 ft. in length.
Fig. 2.—Old London Bridge in 1616.
The first stone bridge over the Thames, known as the old London bridge, was commenced in 1176 by Peter of Colechurch, who is supposed to have belonged to the Brethren of the Bridge; Peter died before the completion of his work, and was buried in the crypt of the chapel erected on the centre pier; this was in accordance with a custom of the society, that when any member died during the superintendence of an important work, his remains should be entombed within the structure; the work was completed in 1209, during the reign of King John, and was chiefly remarkable for its massiveness, and enormous surplus of material, having 20 arches in a span of 940 ft., with piers varying in solidity from 25 to 34 ft., so that two thirds of the stream was occupied by the piers, and at low water a still greater proportion, leaving at that time less than one fourth of the whole span for waterway, and causing a most dangerous fall. The bridge of the Holy Trinity at Florence, over the Arno, was built in 1569; it has a total length of 322 ft., is composed of three elliptical arches, and stands unrivalled as a work of art; the material used in its construction is white marble. The Rialto at Venice, designed by Michel Angelo, and erected in 1588-'91, has a single span of 98½ ft., with 23 ft. rise. The total number of bridges in that city was estimated by Gautier at 340.
Fig. 3.—Bridge of Pont-y-Prydd.
The bridge of Pont-y-Prydd, over the Taff in Wales, is considered one of the most extraordinary in Britain; it was built in 1755, by an uneducated mason named Edwards, after the failure of two structures which he had previously erected at the same spot; the first was carried away by a freshet after standing two and a half years, and the second failed in consequence of the weight on the haunches forcing out the keystone before the parapet was finished; the present structure consists of a single circular arch, with a span of 140 ft. and a rise of 35 ft. The bridge of Mantes, over the Seine, was erected by Perronet and Husseau in 1765, and consists of three elliptical arches, the centre one having a chord of 128 ft.
Fig. 4.—Bridge of Neuilly.
The famous bridge of Neuilly was constructed by Perronet in 1774; its total length is 766 ft., with a clear waterway of 639 ft.; there are five arches of equal width, the curves being false ellipses, with chords of 128 ft. and versed sines of 32 ft. The bridge of St. Maixence, over the Oise, was also built by Perronet between 1774 and 1785; it is chiefly remarkable for the flatness of its arches, the chords being 76 ft. 8 in., while the versed sines are only 6 ft. 3 in., and the thickness of the voussoirs at the vertex of the arch 4 ft. 8 in. Waterloo bridge, over the Thames in London, built by Rennie in 1811-'17, is 1,240 ft. in length, and composed of nine elliptical granite arches, each of 120 ft. span, and a versed sine of 32 ft.; the piers are fronted with coupled Doric columns, producing an elaborate effect; another peculiarity is that the roadway is level, differing in this respect from the other bridges across the Thames. Westminster bridge, completed in 1750, by Labelye, is remarkable as inaugurating a new era in bridge architecture; the novelty consisted in the manner of laying the foundations, which was effected by means of caissons, the depth of water and rapidity of the current rendering the expense of coffer-damming undesirably great; the bridge is 1 ,220 ft. in length, and has in all 15 semicircular arches, two of which, however, are quite small.
Fig. 5.—New London Bridge.
The new London bridge is an imposing structure of granite, and was completed by Rennie in 1831; it has a total length of 928 ft., with five elliptical arches, the span of the centre arch being 152 ft., and its versed sine 29 ft. 6 in. In the United States there are comparatively few stone bridges of great size. Perhaps the finest example we have is the High bridge of the Croton aqueduct, over the Harlem river at New York; its total length is 1,460 ft., and the top of the parapet is 116 ft. above high water; there are in all 15 semicircular arches, 8 of which are of 80 ft. span, and 7 of 50 ft. (See Aqueduct.)—The arches of bridges are of three principal kinds: 1. The semicircular; these were anciently most in use, and have the advantage of being easy to construct, and forming a solid structure; their span is, however, restricted, on account of the great relative height of this form of arch, and as they are usually made of moderate size, they have the inconvenience of considerably obstructing the passage of the water. 2. Arches of a flat vault, either forming portions of an ellipse, or else described by several arcs of circles of different radii. Elliptical arches are pleasing to the eye, but troublesome to construct, on account of the continual change in the form of the successive voussoirs; hence it is usual to employ curves composed of a certain number of arcs of circles, varying generally from three to eleven. The use of flat-vaulted arches was introduced into France about the close of the 17th century, and their adoption was due to the necessity of affording a wider discharge without considerably augmenting the height of the arches. This form not only answers this object effectually, but, when the two diameters are not very unequal, presents as much solidity and facility in construction as the semicircular arch. 3. Arches formed from an arc of a circle, which are of two principal kinds: first, those in which the springing planes are under water, examples of which are seen in the bridge of Saint Esprit, and the ancient bridge of Avignon; this form has the disadvantage of greatly reducing the discharge. In the second kind the springing planes are on a level with the highest water of the river, as in the bridge de la Concorde at Paris. In this case the arc is necessarily very low, and the lateral pressure of the voussoirs so considerable as to require great care in the construction. Besides the three of which we have spoken, there is the Gothic arch, composed of two arcs of a circle, sometimes though rarely employed; it has the fault of greatly reducing the outlet. Though the size of the arches is usually determined by local circumstances, yet there are a few general principles to be considered. Small arches are best adapted to quiet rivers, whose waters do not rise to any considerable height, while large arches are best suited to torrents, where it is difficult to lay the foundations, and where the piers are exposed to damage by obstacles brought down against them by the current. Arches of stone cannot be applied, however, to the wide rivers of Europe or America. In fixing the width of arches two plans are pursued: in one, the apertures are all equal, giving the tops of the vaults the same elevation above the water, and enabling the constructor to use the same centering for all the arches. The economy of this arrangement may, however, be counterbalanced by the necessity of forming considerable embankments at the termini of the structure. In the other plan, the diameters of the arches are unequal, allowing a reduction of the embankments, thus diminishing the obstacles to the approaches. The advantages of both systems are sometimes combined by forming the arches of the same width, and placing the springing planes at heights decreasing from the centre to either extremity of the bridge. The breadth of the bridge depends wholly on the locality, and should be proportioned to the importance of the road on which it is built. For country roads a width of 14 to 16 ft. will be sufficient, particularly if the bridge be a short one. On what are termed roads of the second class, 20 to 25 ft. should be allowed, which will afford sufficient room for two carriages to pass at once, besides a space for foot passengers. On roads of the first class, 30 to 35 ft. is considered a fair allowance, while in the interior of cities from 80 to 60 ft. will be required. The Pont Neuf at Paris, which is perhaps one of the greatest thoroughfares in the world, has a width of about 70 ft. between the parapets.—Timber, as a material for bridges, is much less costly and more easily worked than stone; but all such structures lack the advantage of durability, and are more troublesome to keep in repair. The oldest wooden bridge of which we have any account is the Pons Sublicius already mentioned; it is supposed that no iron whatever was used in its construction. Cæsar's bridge was also of wood, and so was Trajan's across the Danube, though it is probable that the piers of the latter were of stone. One of the most famous wooden bridges on record is that of Johann Ulrich Grubenmann, an uneducated carpenter of Switzerland; it was built at Schaffhausen in 1757, and was composed of two wooden arches with the respective spans of 193 and 172 ft., supported at either terminus by abutments, and at their junction by a stone pier. After Grubenmann's death the bridge began to settle, as the oak beams, which had been placed too low, and not sufficiently exposed to the air, rotted at their points of contact with the stone abutments. Owing to the peculiar arrangement of the structure, by which the principal supports were so intimately connected together, it became necessary to support the whole bridge before a single part could be removed; this was performed by means of screw jacks, and the decayed timbers replaced. No other repairs were ever required, and the bridge excited much attention as a remarkable specimen of carpentry. It was burnt by the French in 1799, having lasted 42 years. In modern times, the wooden bridges of Germany and France have taken high rank from their scientific combination in arrangement, but during the last few years the United States have justly claimed the precedence for simplicity and boldness of design.
Fig. 6.—Schuylkill Bridge.
The upper Schuylkill bridge at Philadelphia, which was burned in 1838, had the remarkable span of 340 ft. It was designed and built by L. Wernwag, and consists of five ribs, each formed of a curved, solid-built beam, connected with an upper single beam by radial pieces, diagonal braces, and inclined iron stays. In the Trenton bridge, the roadway bearers are suspended from curved, solid-built beams, by iron bar chains and suspension rods; it consists of five spans, the central and widest being 200 ft. Burr's plan, which at one time received considerable favor on railroads and aqueducts, particularly in Pennsylvania, consists essentially of open-built beams of straight timber, connected with curved, solid-built beams, termed arch timbers, and which are formed of several thicknesses of scantling, between which the framework of the open-built beam is clamped. Town's plan, commonly known as the lattice truss, consists of two main strings, each formed by two or three parallel beams of two thicknesses, breaking joints with a series of diagonal pieces, crossing each other and inserted between the parallel beams, being connected with the strings and with each other by treenails. As the timbers are of a uniform cross section and length, the construction is simple and economical, though the plan is not well adapted to the resistance of variable strains and jars. Long's truss consists in forming the upper and lower strings of three parallel beams, between which are inserted the cross pieces, or posts, which are formed of beams in pairs, placed at regular intervals along the strings, and connected with them by wedge blocks; between each series of posts are placed braces and counterbraces, suitably connected by treenails, and in long spans arch braces are also introduced. In Howe's truss, the upper and lower strings are each formed of several thicknesses, breaking joints, while on the upper side of the lower string, and the lower side of the upper, are placed blocks of hard wood or cast iron, inserted in notches, and bevelled on each side to form a support for the braces and counterbraces; through the blocks are passed bolts of iron to connect the two strings, and by means of a nut and screw any desired amount of tension can be given to the truss. The preceding combinations are those which are in general use in the United States.
Fig. 7.—Bridge over the Susquehanna at Havre de Grace.
One of the most remarkable wooden bridges is that at Havre de Grace, over the Susquehanna river; this is 3,271 ft. long, divided into 12 spans, resting upon granite piers. It is constructed upon Howe's plan, and combines great lightness and strength. There are many works of this character throughout the country, where, owing to the cheapness of timber, they can be built with great economy. The practice of American engineers in this class of structures has been taken as a guide in similar works throughout the world. The modern tendency is, however, toward the substitution of iron and steel for timber.—Suspension bridges are of very remote origin. Kirchen, in his “China Illustrated,” mentions one which is still to be seen in the province of Yun-nan, and according to tradition was built by the emperor Ming in A. D. 65; it is formed of chains, supporting a roadway of plank resting directly upon them, and is 330 ft. in length. The ancient Peruvians also constructed numerous bridges over the Andes, the principal material being ropes formed of the bark of trees; sometimes a roadway was constructed, and at others the transit was effected by means of a basket supported by the rope, and drawn over alternately from one side to the other. The same plan is used at the present day. Rope suspension bridges have also been used in Europe; they were employed in France, at the siege of Poitiers, to cross the river Clain, and Douglass mentions their use in Italy in 1742. Iron suspension bridges of large span, however, are of modern date. The first of this kind erected in England was in 1819, and was built across the Tweed at Berwick by Capt. Sir Samuel Brown; it was constructed with chain cables, 12 of which were used in all, six being placed on either side of the roadway; its span was 449 ft. and versed sine 30 ft. The same engineer constructed the Brighton chain pier, and the bridge at Montrose. The former was built in 1823, and destroyed by a gale of wind in November, 1836; its entire length was 1,136 ft., in four openings, each of 255 ft. span and 18 ft. deflection. The latter was erected in 1829, and in October, 1838, the roadway was totally destroyed by a hurricane; Mr. Rendell reconstructed it, and materially stiffened the structure by the system of trussing he adopted. The bridge over the Menai strait, by Telford, was built in 1819-'25; its span was 580 ft., and the clear height of the roadway above the water 102 ft.; it was seriously injured by a violent gale, which produced so great an oscillation of the main chains as to dash them against each other and break off the rivet heads of the bolts; a recurrence of the accident was provided against by suitable bracing, and the iron roadway beams strengthened by an additional number constructed of timber, as it was found that the former were frequently bent and even broken by the undulations of the bridge in a gale. The Conway bridge was also built by Telford, being completed in 1826; it crosses an estuary that divides the towns of Bangor and Chester; its span is 327 ft., with a deflection of 22⅓ ft. The Hammersmith bridge over the Thames was built by Tierney Clark in 1824-'7, and has a span of 422 ft. One of the most remarkable suspension bridges in Europe is that of Fribourg in Switzerland; the cables are of wire, and the span is 870 ft.; it was erected in 1831-'4 by M. Chaley; the roadway is 174 ft. above the surface of the river, and although the whole is remarkably light and fragile in appearance, it has withstood several severe tests uninjured, and is still considered a safe bridge. The Pesth suspension bridge over the Danube was commencedin 1840 by Tierney Clark, and opened in 1849, when it was crossed by a part of the Hungarian army retreating before the Austrians, and followed immediately by the latter, both armies with their heavy trains of artillery, ammunition, and baggage wagons; it is estimated that of the Austrian troops alone 30,000 crossed the bridge in two days; the clear waterway is 1,250 ft., and the centre span 670 ft., while the towers are 200 ft. in height from the foundations. In the United States, the first suspension bridges were built by Mr. Finley between 1796 and 1810, and were all of small dimensions, and constructed with chain cables. During the last 25 years, however, a large number of structures have been erected, and some of great size; the plan of wire cables has been universally adopted in their construction. The Wheeling bridge over the Ohio was built in 1848, by C. Ellet, and blown down in May, 1854; its span was 1,010 ft. The Belview bridge at Niagara, a slight structure, was built by the same engineer in 1848, and had a span of 759 ft.; it was removed in 1854, and its cables incorporated in the bridge constructed by Mr. Roebling. One of the finest structures of this kind in the country, and perhaps in the world, is Roebling's railway bridge at Niagara; its span is 821 ft., and deflection 59 ft.; 14,560 wires are employed in the cables, and their ultimate strength is estimated at 12,000 tons; the elevation of the railway track above the water is 245 ft., and so great is the stiffness of the roadway that the passage of ordinary trains causes a depression of only three to four inches; the bridge was completed in 1855, and, though continually subjected to the passage of heavy trains, has thus far proved a complete success.
Fig. 8.—Suspension Bridge over the Ohio at Cincinnati.
This able engineer has added to his fame by the construction of a suspension bridge over the Ohio river at Cincinnati. It has a total length of 2,220 ft. and a clear span of 1,057 ft., and is 103 ft. above low water in the river. The two cables supporting the roadway are 12¼ inches in diameter.
Fig. 9.—New York and Brooklyn Suspension Bridge.
A still more remarkable work of this kind was designed by Roebling to connect New York city and Brooklyn, and is now (1873) in process of construction, under the charge of his son, Washington A. Roebling. It will be 3,475 ft. long between the anchorages, with a clear span over the East river of 1,595 ft., the bottom chord of which will be 135 ft. above the water. The superstructure will consist of an iron framing 85 ft. wide, suspended from four main cables, each 16 inches in diameter, composed of galvanized cast-steel wire, having a strength of 160,000 lbs. per square inch of section. The aggregate strength of the main span will be 5,000 tons.—Cast-iron bridges are of recent origin. The first that was erected in England was at Colebrook Dale in 1779, and consists of five curved ribs, nearly semicircular in shape, and each formed of three concentric arcs, connected by radial pieces; its span is 100 ft., and rise 40 ft. The Wearmouth bridge was built in 1790, and has a striking appearance from its great span as well as height above the water; it is 100 ft. above high-water level, and has a span of 240 ft., with a rise of 30 ft. The pont d'Austerlitz, at Paris, has five arches, each with a span of 107 ft., and a rise of 1⁄10 the span; it was erected in 1801-'7 by Lamande. The pont du Carrousel, in the same city, was built by Polonceau in 1838, and consists of three arches, with a span of 150 ft., and a rise of 16 ft. The largest iron arch bridge is the Southwark bridge over the Thames, built by Rennie in 1815-'19; it consists of three arches, 240 ft. in span, and with a rise of 24 ft.
Fig. 10.—Washington Aqueduct Bridge.
A remarkable cast-iron bridge, combining the purposes of viaduct and aqueduct, was designed and constructed by Capt. M. C. Meigs, for the purpose of carrying the Washington aqueduct over Rock creek, between Georgetown and Washington: the peculiarity of this bridge is that its arches are constructed of cast-iron pipes, which carry the roadway and the water supply at the same time; its span is 200 ft., its rise is 20 ft., and the diameter of the two pipes of which it is formed is 4 ft. in the clear.—Of wrought-iron bridges, the Britannia and Conway tubular bridges, both erected by Stephenson, are widely celebrated.
Fig. 11.—Britannia Bridge.
The Britannia bridge crosses the Menai strait at 103 ft. above high water, and consists of four spans, two of 230 ft. each, and two of 459 ft., forming a huge tube of wrought iron, through which passes the Chester and Holyhead railway. The Conway bridge has a single span of 400 ft., and is only 18 ft. above the level of high water; it was finished in 1848, and the Britannia bridge in 1850. The tubes were constructed, in each instance, at a distance from their respective destinations, and afterward floated to their places by pontoons, and raised by hydraulic presses, forming the most gigantic application ever made of these powerful machines. The Victoria railway bridge, over the St. Lawrence at Montreal, constructed after the plan of the Britannia bridge, is two miles long, cost over $5,000,000, and contains 10,500 tons of iron and 3,000,000 cubic feet of masonry. Since 1860 many iron viaducts have been built in the United States. Among the most notable of these are the railroad bridge across the Ohio river at Louisville, constructed under the supervision of Mr. Albert Fink, and in accordance with the system known by his name, the essential feature of which is that the bridge carries its load by the aid of suspension rods, and at the same time possesses the stiffness of a truss.
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Fig. 12.—Lattice Girder.
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Fig. 13.—Lattice Girder.
Fig. 14.—Modified Fink trussed Girder.
Fig. 15.—Viaduct Bridge at Louisville.
The length of this bridge is 5,218⅔ ft., divided into 25 spans, supported by 24 stone piers. Its entire cost was $2,016,819. Its height is about 96½ ft. above low water. Its width is about 27 ft. The drawing (fig. 15) will show at a glance the peculiarities of this structure.
Fig. 16.—Truss Bridge at Rock Island.
The railroad bridges over the Mississippi river at Quincy, Keokuk, and Burlington, and over the Missouri river at St. Charles, are good examples of truss bridges constructed exclusively of wrought iron.—Many modifications have been made in the form of the truss since the substitution of iron for wood.
Fig. 17.—St. Louis Bridge.
A very remarkable bridge, the superstructure of which is mainly composed of steel, is now (1873) in the process of construction over the Mississippi river at St. Louis. It was designed by James B. Eads, and was begun in 1869. The piers, four in number, are composed of granite and limestone, and rest on the bed rock of the river, to which they were sunk through the sand, in one case as much as 120 ft. by the use of wrought-iron caissons and atmospheric pressure. There will be one span of 520 and two spans of 515 ft. each over the waterway. The rise of the arches is 60 ft., sufficiently high to permit the passage of steamboats at all stages of water. The two roadways, one for railways and one for carriages, will be carried by four arched trusses connected with each other by diagonal braces, the top and bottom chords of which are composed of steel tubes 16 inches in diameter. These tubes are made in sections of about 12 ft., and consist of steel staves, banded and held together by jackets or hoops of the same material; the segments being connected with each other by steel thimbles or couplings, accurately fitted and firmly secured over and about the joints. The top and bottom chords or tubes are 12 ft. apart, connected with each other by a triangular system of bracing so arranged as to constitute, with the tubes, an arched truss of great lightness and strength. The width of this bridge is 54 ft., and its cost, together with the tunnel under the city of St. Louis by which access is gained to its western end, will be about $9,000,000. (See fig. 17.)—Movable bridges are of several kinds, and receive different names from the manner in which they are constructed and operated. The term is usually applied to a platform properly supported between two points of a fixed bridge, and so constructed as to be readily removed and replaced. Drawbridges are those which are raised or lowered by means of a horizontal axis and counterpoise connected with the platform. Turning or swinging bridges are those which turn horizontally about a vertical axis, while rolling bridges are those which rest upon rollers, and can be propelled horizontally on them, so as to close or open the passage. We sometimes meet with a still different class of movable bridges, where the platform is supported by boats, or any other buoyant body, and which can be introduced in the waterway, or withdrawn from it, at convenience.