SKIN, DISEASES OF. Most of the diseases of the skin are described in this work under their particular names. In this article we shall supply an arrangement of them, and a description of those which are not described under special heads. The following arrangement is that adopted by Rayer in his work on diseases of the skin :— the first bridges built on this plan, the Finlay bridge, near Naas, crossed the canal at an angle of only the oblique span being 25 feet, and the height of the arch 5 feet 6 inches. Mr. Chapman observes that the lines on -which the beds of the voussuire lie are obviously spiral lines, and to this circumstance may be attributed much of the singular appearance of oblique arches. The Finlay bridge stood well, but the ingenious designer did not think it prudent in any other case to attempt se great a degree of obliquity, although he built several other bridges on the same principle, over the Grand Canal in Ireland, and over some wide drains in the East Riding of Yorkshire. He recommends carrying up the masonry as equally as possible from each abutment, in order to avoid unequal strains on the centering.
On the Liverpool and Manchester railway, out of rather more than sixty bridges, about one-fourth were built on tLe skew ; one, built of stone, conducting the turnpike-road across the line at Rainhill, being at an angle of only 34', by which the width of apau is increased from 30 feet, the width of the railway from wall to wall, to 54 feet, the width on the oblique face of the arch. Skew-bridges have since become very common, and some have been erected of even greater obliquity. That at Box-moor, on the London and North-Western railway, was for long unrivalled for obliquity by any other brick arch. Its angle is 32 , the square span 21 feet, and the oblique span 39 feet. There are also brick arches of great obliquity on the Greenwich and Blackwell rail ways, but with their precise angles we are unacquainted ; on the Paris and Rouen railway there is a skew-bridge of brickwork teith stone bond courses of 25' of obliquity.
From Mr. Buck's treatise on oblique bridges, it appears that the difficulty of building skew-bridges increases with the obliquity of the angle from 90° to 45°, which is supposed to be the most hazardous angle for a semicircular arch ; but that beyond that point, instead of increasing, it rather diminishes to about 25', which appears to be about the natural limit for a semi-cylindrical arch. Mr. Buck, whose ex
perience renders his opinion highly valuable, considers that oblique arches of the elliptical form should not be attempted, as they are deficient in stabdity, more difficult to execute, and more expensive than semicircular or segmental arches.
The construction of skew-bridges of iron or timber is comparatively simple, the ribs or girders of which such bridges are composed being of the usual cenetructiou, laid parallel with each other, but the end of each being in advance of that next preceding it. Fig. 5 represents the ground-plan of such a bridge, the dotted lines indicating the situation of the ribs upon which the platform is supported The extraordinary iron bridge by which the Manchester and Birmingham railway is con ducted over Fairfield-street, Manchester, at an angle of only 241°, is a fine example of this kind of skew-bridge. It consists of six ribs, of rather more than 128 feet span. although the width of the street is only 48 feet. resting upon very massive abutments of masonry. The total weight of iron in this bridge, which is considered to be one of the finest iron arches ever built, is 540 tons. It was erected from the design of Mr. Buck, who has constructed several other oblique bridges of great size and very acute angles. Timber bridges, formed of trussed ribs or girders, are built on the same principle. One of very great obliquity, on the Sechill railway, is represented in the second series of Brees's ' Railway Practice.' A somewhat similar mode of constructing skew-bridges in brickwork was introduced by Mr. Gibbs on the Croydon Distention of the Skin, Cicatrices, Vegetations, nevus hmmatodes, Subcutaneous vascular tumours, warts, pearly granulations ; Corns, Ichthyosis, Horny appendages.