bridge was that thrown by Julius Cæsar across the Rhine, as described in his Commentaries. Germany was the school for wooden bridges, as Britain for those of iron. Perhaps the most celebrated of all wooden bridges was that which spanned the Rhine at Schaffhausen in Switzerland. This had a span of 364 feet, and was 18 feet broad. It was designed and executed by J. Ulric Grubenman, a village carpenter of Teuffen, in 1758, and was destroyed by the French in 1799.

Bridges of timber were formerly much used in the United States and Canada; and many magnificent and elaborately planned structures carried American railroads across rivers and ravines, often at enormous heights. But with the exception of some insignificant spans, steel, as before mentioned, has almost ousted wood.

Stone Bridges.-The theory of stone bridges involves mathematical demonstrations and calculations of the greatest complexity; the properties of the arch, the thickness of the piers, and the force of the water form separate elements in the computations. The arch is a series of wedge-formed bodies cemented together and rendered coherent by mutual pressure, the first and last being sustained by a support or abutment. The centre arch-stone or voussoir (a French term) is the highest part or crown of the arch, and is called the "keystone." The interior surface of the arch is called the "intrados," the part immediately under the keystone is called the "soffit," the remaining portions the "flanks." The term soffit, however, is sometimes used synonymously with intrados. The exterior surface is called the "extrados," or back of the arch." The points of junction between the flanks and the abutments are called the "springings"; the distance between them horizontally is the "span"; the perpendicular from the soffit to a straight line drawn between them is called the "rise," or height of the arch. The extreme supports of the arches at the ends are called "abutments"; the solid parts between the arches, piers, and the fences on the sides of the road or pathway, "parapets." It has been found that in the construction of an arch the slipping of the stones upon one another is prevented by their mutual pressure and the friction of their surfaces; the use of cement is thus subordinate to the principle of construction in contributing to the strength and maintenance of the fabric. The line of resistance of the materials forming any structure, whether straight or curved, being capable of geometrical calculation, it can be determined beforehand whether this line will fall within or intersect each successive joint of the structure; if it does, the mass will not be liable to turn over on the edge of any particular joint. By a more exact calculation of the line of resistance, even the slipping of one joint upon another can be obviated by the form of the structure. This is called determining the line of pressure. Upon these two lines the equilibrium of the structure depends. The lines of pressure and resistance in the arch of a bridge are calculated so as to pass into the abutments or piers, where they take a new direction; the strength of the abutments or piers, and the amount and direction of the strain upon them, being calculated by the architect so as to render them capable of permanently supporting the fabric. When the pressure upon an arch causes it to give way, it

breaks into four parts, the stones turning inward upon their superior edge at the crown, and outward on their inferior edge at two points in the haunches where the line of resistance reaches the interior edge, and which are called the points of rupture. The ancients, being unacquainted with the full theory of the arch, usually made their arches more numerous, of smaller span, and with stronger piers than are found necessary in modern structures. Another advantage which the modern architect of bridges has over the ancient is in the invention of the cofferdam (q.v.). This is a water-tight case made to protect the middle piers of a bridge, by driving piles down to the solid foundation, and filling them up with clay or other impermeable material, so as effectually to exclude the water. In constructing a bridge it is desirable to have the smallest possible number of points of support. Piers in the water-way are not only expensive to form, but obstruct the navigation of the river. When the water-way is obstructed the bridge itself is exposed to danger, the wearing action of the water sometimes undermining the piers on which it rests. The pier, at the point where it supports the arch, is usually oblong in form, and increases in breadth to the foundation. The rate of increase in modern bridges is frequently three inches to the foot. The ends of piers are provided with salient angles to act as cut-waters; the form of these differs according to circumstances. In building the arch of a bridge, a timber framework is used called a "centre," or "centreing." The centreing keeps the voussoirs in position till they are keyed in; that is, fixed by the completion of the central part; its construction is a matter demanding the utmost care of the architect. On removing this framework, which is called "striking the centre," what is called the settlement of the arch takes place, the central voussoirs move slightly downward, while a corresponding rise takes place in the flanks. Before striking the centre of the bridge of Nogent-sur-Seine, Perronet caused three lines to be cut, one horizontally over the crown, and two obliquely from the extremities of this line to the springings. On the striking of the centre these lines were found to have altered their forms and relative positions. From straight lines they had become curves. The horizontal line had sunk, and its greatest deflection was above the key, while the oblique lines from the springings were deflected upward. The "spandrils" of a bridge are the spaces between the haunches of the arch and the point of junction of the extrados with the roadway. These are frequently filled up with rubble-work or gravel. They are sometimes left open to give greater lightness to the structure. The former plan has been generally adopted in the large French bridges; the latter was recommended by Telford. Owing to the superior skill of modern engineering, the roadways of bridges are made much wider and more level than formerly.

The most ancient stone bridge in England is said to be the Gothic three-way bridge at Croyland, or Crowland, in Lincolnshire, said to have been built in 860, though the present work is much later. It originally spanned three watercourses, and is so steep that none but footpassengers_can go over it. The longest old bridge in England was that over the Trent at Burton, in Staffordshire, built in the 12th century, of squared freestone. It consisted

of 36 arches, and was 1,545 feet long. It has recently been pulled down. Old London Bridge was commenced in 1176 and finished in 1209. It was built on each side with houses like a regular street till 1756-8. In 1831 it was altogether removed, the present bridge, which had been begun in 1824, having then been finished. It consists of five elliptical arches, of which the centre arch has 152 feet span with a rise of 292 feet above high-water mark. The two arches next the centre are 140 feet span with 272 feet rise. The total length of the bridge is 1,005 feet, and the roadway is 53 feet wide between the parapets. It was erected, after designs of the elder John Rennie, by his son, Sir John Rennie, and cost about $2,500,000. Waterloo Bridge, London, is generally considered one of the finest and handsomest stone bridges yet built. The architect was the elder John Rennie, and the bridge was completed in 1817. The material is Aberdeen granite, and it consists of nine elliptical arches of 120 feet span with a rise of 32 feet, the total length, including the approaches, being 2,456 feet. The width between the parapets is 412 feet. The roadway is almost perfectly horizontal, and the bridge was one of the first in which this construction was adopted. Other English bridges of importance are the Grosvenor Bridge at Chester (1826), consisting of one arch 200 feet in span, one of the largest stone archways yet constructed; the Royal Border Bridge (railway), Berwick, 2,160 feet long, with 28 arches, designed by Robert Stephenson; the Congleton Viaduct on the railway from Manchester to Birmingham, 2,870 feet long, with 41 arches; the Tyne Viaduct, 902 feet, with four arches, etc.

There are very few bridges of notable antiquity in Scotland. One of the oldest is the bridge over the Don, near Aberdeen, called the Brig of Balgownie, said to have been built by Bishop Cheyne in 1281. It consists of one large Gothic arch of 67 feet span. The bridge at Perth is a handsome structure, designed by Smeaton, finished in 1772, and replacing an old bridge swept off by an overflow of the river more than a century before. The bridge of Tongueland, near Kirkcudbright, over the Dee, consists of one arch of 112 feet span. Union Bridge, Aberdeen, built in 1800-3 from Telford's designs, has an arch of 132 feet span with 29 feet of rise. Dunkeld Bridge, a noble structure built in 1805-9, from designs by Telford, has five large arches and two smaller, the middle arch being 90 feet span. The Dean Bridge, Edinburgh, is a beautiful structure of 1,832 feet (by Telford), remarkable for its situation and height. It crosses the deep ravine formed by the stream called the Water of Leith, a little above St. Bernard's Well. It is 447 feet long, 39 feet wide, and 106 feet from the roadway to the bed of the river. At Glasgow there are two stone bridges of modern construction and remarkable solidity and beauty, spanning the Clyde and connecting the southern with the northern division of that city. One of these was finished in 1854; the other, designed by Telford, called the Broomielaw Bridge, was erected in 1833-6, but has recently been taken down and rebuilt, being now widened from 60 feet to 80 feet. The arches (as before) are segments of circles, seven in number, the span of the centre arch being 57 feet 9 inches, and the length of the bridge about 560 feet. The Ballochmyle Via

duct, by means of which the Glasgow and Southwestern R.R. crosses the river Ayr, has a central semicircular arch of 180 feet span, the largest span of any railway stone bridge.

Comparatively few large stone bridges are found in America; but a bridge constructed in 1861, to carry the Washington aqueduct over Cabin John Creek, has a stone arch of 220 feet span. The High Bridge, New York, was built to carry the Croton aqueduct across the Harlem River. It consists of 13 granite arches, the highest one being 116 feet above the river. The bridge, crossing the river and valley, is 1,460 feet long. The Washington Bridge, a composite structure, is situated a short distance north of the High Bridge and consists of nine arches, three of granite on the east side, four of granite on the west, and two central steel spans connecting them and crossing the water-way. The entire length of the bridge is 2,300 feet, and its width 80 feet; the central spans being each 510 feet long and 135 feet above high water. The largest single stone span is in Luxemburg, being 277 feet long.

Iron Bridges. The use of iron in the formation of bridges is a comparatively modern invention, by which greater strength is secured in proportion to the weight of the material employed. The earliest iron bridges were erected in the form of arches, and the material was cast-iron, but the arch has now been generally superseded by the beam or girder, with its numerous modifications; and wrought-iron is likewise found to be much better adapted for resisting a great tensile strain than cast metal. The greater cohesion and adaptability of the material give more liberty to the architect, and both in suspension and in arched bridges of iron much greater width of span is possible than in arched bridges of stone. As already stated, however, steel is now almost exclusively employed in bridge construction (q.v.). The first bridge of cast-iron ever erected was that over the Severn, England, about two miles below Coalbrookdale, Shropshire. It is an arch composed of five ribs, forming the segment of a circle. Its chord is 100 feet long and its height 45 feet. It was erected in 1779. Near it there is a more recent iron bridge of greater span (200 feet), designed by Sir John Fowler for the railway here. The bold and elegant cast-iron bridge over the Wear at Sunderland was opened in 1796, and was widened by Robert Stephenson in 1858-9. It is formed of a single arch of 236 feet span and 34 feet rise, composed of six ribs. The height above low water is about 100 feet. Close to it there is a more recent single-arched iron bridge. Westminster Bridge, over the Thames (opened in 1862), consists of seven low arches of wrought and cast iron, supported on piers of solid granite, and is no less than 85 feet wide. Blackfriars Bridge, London (opened in 1869), consists of five arches of wroughtiron, of which the centre arch is 185 feet span, with its summit 25 feet above high water. The width between the parapets is 75 feet, the roadway in the centre being 45. In front of the piers there are short massive columns of polished red granite, with carved capitals of Portland stone. The Southwark Bridge (opened in 1819), over the Thames at London, designed and constructed by the elder Rennie, consists of three arches, the span of the middle arch being 240 feet and its height 24 feet. One of