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ot| D(Je » for though they are frequently built of brick and(jjt| Materials, the vast majority of such constructions areformed of stone, or stone and brick combined.

“tique, or Skew bridges, have been introduced contem-

ane °usly with railways; for few bridges of the kind were

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Instructed anterior to that time. We hear, indeed, of oneeat l' n g been thrown over the river Mugone, at Florence, soas the year 1550 ; but the true principles on whichbridges depended were not previously known orif le< h as their application to general purposes was rarely,tj 0 er > absolutely called for, until tbe plan of rapid locomo-it s ’ w hich the system of railways gave rise to, made theirj constant, and in many cases indispensable.lj e /* the construction of oblique bridges, the main object top 0g r'®Pt in view is to bring the thrust of the arch into such ado * tlon as will enable the abutments, or piers, properly tothe eract '*■ 5 an( J the most effective way of so doing, and at<li r Sa ! Be time the most simple, is found to be by making the^i(3?. tl0n Courses at right angles to the face of the

tk nere have been various methods suggested for buildingf 0f c bridges ; but the one most commonly adopted, is to5 a , ^he arch by regular spiral courses, all parallel to onen rUf , aer > and so arranged, that any section taken at righttlig 68 to l ' le ax ' s °f the cylindrical segment which constitutes^i r ar ch. shall exhibit the coursing-joints tending in a radialAll 011 to the centre of the cylinder at the point of section.u n( j , le voussoirs are rectangular in plan, both on the extradostl, e j'^trados, with the exception of those on the faces of

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fulu abutments, would form a curve of contrary flexure,

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r *dge, because a spiral line, touching the outer edge of

tli e ' n S Partly within and partly without the line bounding(> ar . ar °h. In this case, the face-line will evidently not bew t 1 . to the heading-joints of the voussoirs, which, there-f ilC(; ’ ;i W| ll be of a constantly varying shape and size on thetli at . °f the bridge. Again ; it is important to remark,lh a ’ n this particular system of construction, the joints oftl‘e ^ 0Uss °hs are not, properly speaking, straight lines ona!) t evat ion of the arch, but segments, whose chords shouldta 0fe na to a common centre, which, in every case, must be0r less below the axis of the cylinder, according to theq 6 °f the skew bridge.

W r ;' le °f the best works on the subject of skew bridges, wastin, ,® n by Euck, wherein formulae are given, from which allat ..tensions of skew arches of every kind may be arriveddor,J t l mathematical accuracy, and the principles of theirr>IJ bl ,r , Uct i° n are clearly and correctly explained. Nicholsonjif ^ e d two admirable works on this important departmentr l(! 'l^ge-building—one in 1828 , entitled, “A Practical^39 * Se on Masonry and Stone-cutting; and the other in\ a ’ Ca )bed, “ A Treatise on the Oblique Arch.” To him*ipl e re indebted for the first sound exposition of the prin-ce on . which such constructions are founded; and from3te a ( act *cal bearing of this, as of all his works, it was of theJotj at use in presenting the subject to men’s minds in a'h V,. ar >d intelligible point of view, and setting it forth inf[| 0 Per light.

, method of forming oblique arches, as above explained"6 a ea ted of by Buck and Nicholson, supposes the arch toSe gment of a circle on the square section. It is not,

j, a , r ' necessary that such should be the case ; the form ofmay be elliptical on the square section, in which case

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Nation is a much flatter ellipse, according to the amount

as it is clear that the minor semi-axis is constant,

*ki 8 major semi-axis will increase with the skew ; andhiu,,,, . Ca %ay be again modified by making the joints of thers perpendicular to the radius of curvature on the

square section, instead of making them tend to the axis of thecylindrical segment.

Many changes of this kind have been recommended andtried, with varying success, by Messrs. Hart, Adie, Roebuck,and several others.

Stone Columns, such columus as are constructed of stone;in which, the fewer the joints, the better the appearance.If columns are necessarily composed of more than one piece,from the difficulty of procuring stones of sufficient magnitude,they ought to consist of three, five, or some odd number ;more especially if the joints be few in number, than whenthey are many.

In columns constructed of several pieces, a mould mustbe made for every joint; and in setting the pieces, a sheetof thin milled lead, well bedded in white-lead, ought to beinserted between the joints, so as to come within about three-quarters of an inch of the convex surface of the column.

In the act of building, the diminishing rule ought to beapplied to every course as it is set; and when the wholecolumn is completed to its height, it ought to be tried again,and all the inequalities in the height reduced to the regularcurve ; for it is hardly possible, even though constructed bythe best workmen, that some irregularities should not occur:and, at the same time, the circularity, or roundness, of thecolumn ought to be attended to.

The most accurate method of fluting columns is, to cut theflutes after the columns are built, and brought to the curve ;but in performing this, great care ought to be taken to pre-vent the chipping of the stone at the joints ; which is bestavoided by working each adjoining stone, or piece, from thejoint, at least so far as to be out of danger of splitting the partof the stone next to it.

Stone Stairs, those constructed of stone. "When stone stairsare supported by a wall at both ends, nothing difficult can occurin the construction ; in these the inner ends of the steps mayeither terminate in a solid newel, or be tailed into a wall sur-rounding an open newel. Where elegance is not required,and where the newel does not exceed two feet six inches, theends of the steps may be conveniently supported by a solidpillar ; but when the newel is thicker, a thin wall surround-ing it would be cheaper.

In the stairs of a basement story, where there are geo-metrical stairs above, the step next to tbe newel are generallysupported upon a dwarf wall.

Stone geometrical stairs have the outer end fixed in the wall,and one of the edges of every step supported by the edge ofthe step below, and constructed with joggled joints, so thatthey cannot descend in the inclined direction of the plane,nor yet in a vertical direction; the sally of every joint formsan exterior obtuse angle on the lower part of the upper step,called a back rebate, and that on the upper part of the lowerstep, of course an interior one, and the joint formed of thesesallies, is called a joggle, which may be level from the faceof the risers, to about one incli within the joint. Thus is theplane of the tread of each step continued one inch withinthe surface of each riser, and the lower part of the joint is anarrow surface, perpendicular to the inclined direction, orsoffit, of the stair at the end next to the newel.

In stairs constructed of most kinds of stone, the thicknessof every step at the thinnest place of the end next to thenewel, need not exceed two inches for steps of four feet inlength, measuring from the interior angle of every step per-pendicular to the rake. The thickness of steps at the interior ■!angle should be proportioned to the length of the step : butallowing the thickness of the steps at each interior angle tobe sufficient at two inches, the thickness of steps at theinterior angles will be in inches half the length of the steps