Chap. XXIV.

MASONRY.

1173

Staircase with Quarter Spaces supported by rampant Vaults and Trompes on Hie Angles, or

by the Vault called the Arc de Cloitre -A B C D is a square plan in which the staircase is

placed : commence by drawing the parallels E II, I M, N P, at a distance correspondingwith the length to be given to the steps; these lines by intersecting form two squares,A E F N, I KP13, and a rectangle E F K I : it is evident that in the entire plan a squareshould be formed at each angle, indicating the quarter landings, and four rectangles similarto EFKI for the steps: the well-hole will be expressed by a square or rectangle,FIIMK, and NFHC and K M D P will indicate the halves of the ascending and de-scending ramps.

SEOTtOV.

\ p q T

Fig. 2674. plan.

To make the projection, having drawn the diagonal I P, and described a semicircle orsemi-ellipsis upon it for the primitive centre of the trompe, divide the voussoirs upon it,which set off on the diameter 1 P, and draw m n parallel to I P, to indicate the trompil-lion ; then draw from the point B to all the points of the diameter a,b,c,d,e,f,h lines pro-longed until they meet the sides KI and KP, which lines will be the projection on theplan of the joints of the trompe.

The cone with a circular or elliptic base which forms this trompe, being cut by twovertical planes K I, K P parallel to its sides, their section is a parabola; and this curveforms the profile of the vault to the right of the lines K I, K P, and not an arc of a circle,as some have supposed.

The figure in perspective shows the voussoir at the junction of the two vaults; afterhaving drawn on the plan and sections to indicate its greatest height and projection, cutprisms, on the faces of which the moulds taken from the sections are applied, showing

5 B