Chap. XXIV.

MASONRY.

1461

one of the walls. The voussoirs are divided on the quarter circle, E N I, prolonged ftomNto E, representing the section on the diagonal of the vault.

The joints of the stones are only prolonged until they intersect the vertical PE, whichindicates the re-entering angles of the walls, prolonged in the part occupied by the vault;those of the pendentives form an arris, also at the intersection of the interior surface of thewalls to which they belong. To draw one of these stones, as shown in perspective, forthe beds, we use arcs marked on the plan bb and a a ; and for the elevation, those distin-guished by similar letters on the section : to form the surface of the stone, instead of a rulea pair of callipers is applied to the great circle, directed perpendicularly to the arcs a,b.

Fig. 2621.

Fig. 3524.

Fig .2622

Fig. 2625.

VAULTS ON A SQUARE PLAN, WITH SPHERICAL !LAID IN VERTICAL COURSES.

Fig. 2623.

VAULTS FORMED ON THB PORTION OPAN HEMISPHERE.

Vaults on a square Plan , with sphe-rical Heads laid in vertical Courses. •—

The rows of voussoirs are shown onthe plan by right lines, which aredrawn perpendicularly to the dia-gonal; these lines form on the ele-vation arcs of circles, shown in thesection as foreshortened, but deve-loped outside the plan, and havingfor radii other lines which arethe projection lines of the rows ofvoussoirs prolonged to the greatcircle passing through the diagonalG E, and expressing the projection ofthe springing of the entire vault.

To draw the voussoirs, we must forma horizontal and perpendicular planecorresponding to the line of projection on the plan; upon it is traced the curve

5 a S

Fig. 2626.

of ele-