Bridge 74A, Leeds and Liverpool Canal
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This is a most impressive skew arch bridge built to carry the Bolton and Preston Railway over the Leeds and Liverpool Canal at Cowling, south of Chorley.
Built by Alexander James Adie to an orthogonal or 'logarithmic' pattern proposed by Edward Sang. Some sources give a build date of 1838, but the report below suggests that it had not been built by January 1841.
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Geograph entry, with map, here.
See here for photographs and an expert insight into the design principles of this bridge.[1]
1841 Report by A. J. Adie: 'Contract No. 3. ..... The Bridge over Cowlin Brook (above Yarrow Bridge) is well advanced, and will soon ready for turning the Arches. The only other Bridges of consequence on this contract are yet to complete is a skewed Bridge over the Lancaster Canal, for which a temporary wooden Bridge has in the meantime been substituted for the passage of the earth to forward the banking, and a Bridge with skewed ends, similar to those built in Bolton, which is to carry the Chorley Turnpike Road over the Railway. Messrs. Nowell can easily finish these Bridges in time, and it is expected that they will make every exertion to drive forward the cutting at the beginning of their contract.'[2]
1842 Minutes of Proceedings of the Institution of Civil Engineers [3]
'On the Construction of the Bridges on the Bolton and Preston Railway’ by A. J. Adie.
'This Paper, which was written at the request of General Pasley, and by him communicated to the Institution, contains a description of the bridges over the Cowlin Brook, the Lancaster Canal, and the Chorley Road….
'The Lancaster Canal Bridge was originally intended to have been a direct span of 60 feet, constructed of iron, but the directors subsequently decided on building a skewed stone arch of 25 feet on the right angle. The arch is semi-elliptical on the square, with a transverse axis of 41 feet 2 inches, and a semi-conjugate axis of 8 feet 9 inches; the arch stones are 2 feet 3 inches on the square at the springing, and 1 foot 6 inches at the key stone; the bed joints intersect at right angles all the lines of sections of the intrados, made by vertical planes, parallel to the elevation; and it is that property that causes the chamfer lines of the beds of the stones to diverge from the springing to the crown. These lines of curved joints are easily laid down on the sheeting of the centres from a full-sized development, and by the lines drawn at the different heights parallel to the springing of the arch. The lines of the radiating bed joints are always perpendicular to the tangent of an ellipse of the same form as the elevation of the bridge, the moulds used to form this being applied in the plane of the elevation. The twist on the length of the beds of the courses was taken from full-sized skeleton moulds of the form of the oblique ellipse or elevation. The five courses running parallel to the abutment are all of the same form and have the same amount of twist on the beds of each stone, except the end stones of the courses, which are varied in length to suit the general breaking of the joints of the courses resting together. The centre part of the arch is plain square work.
'This mechanical method of finding the lines and the twist of the radiating beds for an elliptical skewed arch, is destitute of the scientific accuracy of the mode by which Mr. Buck calculates his spiral lines for oblique bridges, of which the section at right angles to the abutment is an arc of a circle; but the workmen had no difficulty in putting it in practice, and the author states that he would have had more trouble in constructing trussed centres for a flatter curve of a circular arc, and at the same time keeping the towing path of the canal open. He states that he has not met with any description of an arch executed in this manner, but he considers it the only true principle. Every very thin section parallel to the elevation is a proper elliptical arc, and there is a very great saving of stone from the smallness of the twist on the curved beds as compared to the common method of working them.'