SKEW.BRIDGE, a bridge in which the passages over and under the arch intersect each other obliquely. In conducting a road or railway through a district in which there are many natural or artificial water courses, or in making a canal through a country in which roads are frequent, such intersections very often occur. As, however, the con struction of an oblique or skew arch is more difficult than that of one built at right angles, skew-bridges were seldom erected before the general introduction of railways ; it being more usual to build the bridge at right angles, and to divert the course of the road or of the stream to accommodate it, as represented in fig. 1, in which a b is a Aram crossed by the road, the general direction of which is indicated by the dotted line c d. In a railway, and sometimes in a common road or a canal, such a deviation from the straight line of direction is inadmissible, and it therefore becomes necessary to build the bridge obliquely, as represented in the plan, fig. 2. Where space and neatness do not require to be considered, an oblique arch may be avoided, either by building the bridge square with the upper passage, and making the stream a b. Here it is evident that c g is the actual span of the arch ; although c d, the breadth of the stream, would be the span of a straight arch, leaving the same width of passage underneath.
Very little is known respecting the origin of skew-bridges. It has been repeatedly asserted that those built by George Stephenson on the Liverpool and Manchester railway were the first erections of the kind ; but this is certainly incorrect, there being wine of earlier date even in Lancashire. A paper in the Transactions of the Institution of Civil Engineers; vol i., p. 185, alludes to an oblique arch erected about the year 1530 by Nicola, called " II Tribolo," over the river 1)I ugnone, near Porta Sangallo, at Florence. It appears however that the principle upon which such bridges should be constructed was too little understood to render an attempt at constructing them on a large scale advisable. The next informatiou the writer has met with on the subject is contained in the article Oblique Arches,' in Rees's Cyclo pedia ; ' an article which appears to have escaped the notice of modern writers on this branch of engineering science. It is written by an
engineer named Chapman, who mentions oblique bridges as being iu use prior to 1787, when he introduced a great improvement in their construction. Down to that time, as far as he was informed, such bridges had always been built in the same way as common square arches, the voussoirs being laid in courses parallel with the abutments. How very defective such an arch would be may be seen by reference to fig. 3, in which lines are drawn to indicate the direction of the courses. It is evideut that here the portion cdfe is the only part of the arch supported by the abutments ; the triangular portions c dg and fh being sustained merely by the mortar, aided by being bonded with the rest of the masonry. This plan could therefore only be ulopted for bridges of very slight obliquity, and even then with con. iiderable risk. About the time mentioned above, Mr. Chapman was nnployed as engineer to the Kildare canal, a branch from the Grand :Sinai of Ireland to the town of Naas. on which it was desired to avoid Everting certain roads which had to be crossed. He was therefore ed to look for some method of constructing oblique arches upon a wand principle, of which he considered that the leading feature must be that the joints of the vouseoirs, whether of brick or stone, should be rectangular with the abutment, instead of being parallel with the face of the arch. Thus the coureee, Instead of taking the direction shown in fig. 3, were laid in the manner indicated iu jig. 4. One of railway. The Jolly Sailor bridge, which crosses over this line near Norwood, consisted originally of four separate ribs of brickwork, each forming an elliptical arch of 50 feet span, with a versed sine of 12 feet 6 inches, supporting a flat viaduct of Yorkshire flagstones. Each of these ribs, which were three feet wide on the transverse face, was built square, so that the brickwork was of the simplest kind; but by making the respective abutments project beyond each other according to the oblique direction of the railway, the ribs, token collectively formed a skew-arch. In a bridge erected by Mr. Wood house on the line of the Midland Counties railway, the same principle is adopted, but the ribs are placed close together, so that no platform of flagstA.nes is required.