An arch is an arrangement of bricks, placed to any shape or curve over an opening, so that each brick is supported by mutual pressure from its neighbours, the whole being wedged in, as it were, over the jambs on each side of the opening.
Before proceeding to explain the application of bricks to the formation of arches, it will be as well to make the student acquainted with the different parts of an arch, their names, and the various kinds of arches in use.
Fig. 127 is an elevation of a semicircular arch, the capital letters referring to the text with explanations, as follows : A represents the abutments which support the arch, and from which it springs; they are really the jambs of small openings, and a pier will act as abutment to two arches, as shown at P on the illustrations : C is the crown or top of an arch; E indicates the extrados, top, back, or outside; F is the face; If are the haunches, sides, or shoulders, comprising the sides of an arch up to a point halfway between the springing and the crown; I represents the intrados, soffit, bottom, or under-side of the arch; J shows the jambs or sides of the abutments and piers; K is the keystone, or central voussoir or arckstone; S indicates the spandrils, the spaces over the extrados and haunches up to a line horizontal with the crown; Sp is the springing, or place from which the arch starts - called skew-backs in other than semicircular arches - they radiate to the centre, and form a key to keep the arch in and wedge or tighten it up; Sn represents the span, or width of the opening over which the arch is built; Spg is the springing or horizontal line, from which the arch commences; R is the rise of the arch, from the springing to the soffit of the crown; V are the voussotrs, blocks, or bricks, which form the arch, radiating from the centre-point, and supported by mutual pressure from their neighbours.
Fig. 128 is a segmental arch, shown for the purpose of indicating the position of the skewbacks, which do not occur in semicircular arches. The same reference-letters and nomenclature apply as in the last case.
The semicircular arch (shown in Fig. 127) is made from one centre situated on the springing line, halfway between the jambs of the abutment. The segmental or "scheme" arch (shown in Fig. 128) is formed with a curve which is part of a circle, also struck from one centre, halfway between jambs of abutments, but at a distance below the springing line, which may vary according to the rise of the arch.
Fig. 129 is a straight ox flat arch, the bricks (or voussoirs) varying in hickness from a maximum of 3 inches at the extrados to a minimum thickness at the intrados, so as to radiate to a centre below it.
Fig. 130 is an elliptic arch, has half an ellipse for its curve, and is used for wide spans, where only a small rise is obtainable.
Fig. 131 is a cambered arch, which is similar to a straight arch, except ing that the intrados or soffit rises slightly in a flat curve towards the centre.
As a rule, in fact, all flat or straight arches should be thus made with a small camber, to allow for settling.
Fig. 132 is an equilateral arch - i.e., one whose curves have each an angle of an equilateral triangle as its centre, the two meeting at the third angle. This is also called a Gothic arch.
Fig. 133 represents an arch struck from three centres, as shown, and is, as will be seen, akin to the elliptic arch (fig. 130).
Fig. 134 shows an arch struck from four centres, and called a Tudor arch; being also a Gothic arch, but of a later period than fig. 132.
In setting out three- and four-centred aches, the line joining the centres should be at an angle of 30 degrees with the vertical, which can always be contrived by altering the radii.
Having thus become acquainted with the different kinds of arches, and the names of the various parts, the student will the more easily be enabled to master the constructional points and details.