produced across the face of the arch give the leading bed joint, and the point of junction of the respective arcs. The bed joints in each section are made to radiate from the centre from which that section is described.
The Horseshoe or Moorish Arch, Fig. i6, is generally associated with Saracenic or Arabic architecture ; elsewhere it is used as an ornamental rather than a constructional feature. Alternative treatments are shown, the principal characteristic being the earning of the curve around below the centre line, no other type of arch sharing this peculiarity.
The Stilted Arch. Fig. 12a, which springs, not from the impost where all other arches commence but at some distance above, was a constructional device in Early English architecture to bring the crowns of cross vaulting into line or level. Apart from this peculiarity, there is no distinctive feature from the ordinary arch, and "stilting" may be applied to arches of any contour.
The Basket-handle or Three-centred Arch. Fig. 13, is obtained by dividing the span into three equal parts and constructing an equilateral triangle upon the centre part, as shown. The three angles of the triangle contain the centres, and the sides produced across the face of the arch give the leading bed joint and junction of the curves.
The Ogee Arch. Fig. 14, is described in detail under Carpentry Arches.
1 he Sqliinch Arch. Fig. 15, is so named from its position, not its shape, which may vary with circumstances. Squinch is Old English, or Saxon, for corner, and in this connection means an arch turned across a corner. If is often used to support a diagonal wall beneath an octagonal spire, where the tower changes from square to octagonal.
The other sketches, Figs. 10, 11, 12, are comparative groupings of four chief types of arches: the " flat," the " segmental," the " round " and the " pointed."
Carpentry Arches, page 171.—The arched frames constructed by the carpenter, or, to be exact, the joiner, usually follow the contour of the openings provided by the
mason or bricklayer, consequently tlieir description or setting out, so far as the outlines are concerned, is similar to that described for stone arches. There are, however, differences in detail, and the necessity of dealing with these gives the opportunity of presenting alternative treatment of types that will be equally useful to the mason and the carpenter.
The Equilateral Arched Frame, Fig. i, is described upon an equilateral triangle, the radius of the two arcs being equal to the span; the centres, therefore, lie in the springing line at a and b. The back of the. frame is described from the same centres. The haunch joints are usually placed about one-quarter the height of the arch above 1 he springing, and radiate from the centre of the curve—i.e. they are made normal to the direction of pressure.
The Lancet Fig. 2, is described by constructing two semicircles on the springing line extended, with a radius equal to half the span, the centres being at a and h. The extremities of these semicircles, points 1 and 2, provide the centres for striking the curves. The haunch joint is one-fourth the perpendicular height above the springing.
The Drop Arch, Fig. 3, is made in varying degrees of depression to suit the designer. The span and rise being given, construct a triangle upon the springing line with the required altitude. Bisect the inclined sides of the triangle and produce the bisectors to intersect the springing line in points 1 and 2. These are the required centres.
The Tudor Arch, Fig. 4, is constructed by dividing the span on the springing line into four equal parts, describing semicircles equal in radius to the half span, upon points t and 3 as centres ; these describe the haunch arcs. For the centres of the crown arcs, describe quadrants from a and h, intersecting the jambs in points 2 and 4, which are the required centres. To lind the joint line, join points 1-2 and produce.
Two other Four-centred Arches are shown in Fig. 5. On the right halt the span is divided into six equal parts,
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