Chap. XXIV.

MASONRY.

1431

A flat Vault on a circular plan has*ts voussoirs arranged concentrically,*nd locked by a conical key.

Flat Vaults supported upon fourmiliars require a different arrangement;the rows of voussoirs are parallel to the

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Fig- 2147. FLAT VAULT ON A CIRCULAR PLAN. Fig. 2448. PLAT VAULTS SUPPORTED ON FOUR PiKKS.

inner faces, and intersect at right angles on the diagonals, on which are placed voussoirscommon to the two sides: the last row on each side is cut to receive the diagonal voussoirs.

To trace the Caissoons in spherical and spheroidal Vaults is the most difficult portion ofa mason’s work : after the circumference of the dome is divided into the number of intendedcaissoons, from the centre of each perpendiculars are raised, united in the key of the dome;these lines may be considered as the circumferences of several vertical semicircles whichintersect at the axis of the vault: whence it results, that if a cord be stretched, to representthe diameter of one of these circles, the points of the circumference will be found byelevating with a plumb-line several corresponding points on the surface of the vault, a linepassed through which will be the circumference of the circle. To draw them, a templateor mould cut to the curvature of the vault is employed, which should not be more than3 feet in length. The points raised upon the diameter represented by the cord should benot less than 18 inches apart, so as to have always three points to fix the mould: instead ofraising these points perpendicular to each diameter, we may effect this in a far moreexpeditious and simple manner, by means of a plumb-line suspended from the centreof the key; if at night, a light may be placed at each division opposite to that suc-cessively, through which a vertical circle is to he drawn, which will be indicated in itswhole extent by the shadow of the thread of the plumb-line, serving to place the woodenmould as before described. After having thus drawn the centre of the caissoons,their height may be thus determined : circles are inscribed, one above each other in thedivisions or parts of the development, comprised between the vertical arcs -which passthrough the middle of the sides, as is shown in the figure B. In order that the first rowof caissoons may experience less foreshortening, the first may be raised above the

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