CIRCULAR CENTRES.

Plate 15 exhibits several designs, more or less convex, for the construction of centres, which are skeletons used by stone and brick masons to build an arch upon, but which are to be taken away when the arch is sprung and the mortar set.

First, draw the line AB upon a floor, two inches less than the width of the skeleton required, and take OA as a radius, and describe the semi-circumference. Inch boards are then to be fitted to this curved line, and their ends beveled to fit each other, as represented in the Plate. The bevel is determined by simply drawing a straight line from any point of the curve to the central point O. Other boards are then to be nailed over the joints, on the inside of these; and the long brace AB nailed at each end at the bottom.

Having prepared the other end of the skeleton in the same manner, strips of boards, an inch thick, two inches wide, and of a length equal to the thickness of the wall, are nailed upon their convex edges, as represented.

Should the arch have more than 12 feet span, it would be proper to use thicker boards; but for any thing less than 12 feet, inch boards are amply sufficient.

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ELLIPTICAL CENTRES.

This Plate illustrates the manner of constructing elliptical centres. The elliptical curve is described most accurately by means of a trammel, the construction and use of which are explained in Plate 2, page 20.

In order to describe the curve for these centres, take AB equal to the span of the arch, less 2 inches, and set the trammel so that the intersection of the arms will fall upon O, the middle point of AB. Then set the pin B, so that PB will equal the height of the arch less 1 inch; and set the pin C so that PC will equal AO.

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