1442

THEORY AND PRACTICE OF ENGINEERING.

Book II.

whore the projections from joints on the plan cut with the horizontal line A B, andcontinued to the intersections of the joints of the new section. The three first voussoirs are

l-'ig. 2528.

Fig. 2530.

1 ig. 2529.

Fig. 2532. DESCENDING VAULTS INTERSECTING AT RIGHT ANGLES.

shown in isometrical perspective : the acute angles resulting from the double obliquity ofthe lesser vault, and the irregularity of the arris formed by the intersection requires theacute angle to be changed, as in a former example.

To construct a vault where both the extrados and intrados are conic surfaces having acommon vertical axis, the solid being equally thick between the conic surfaces, so that inthe joint lines those of the beds are horizontal, and those of the headings in vertical planespassing along the axis, the beds must be so formed that they will unite in horizontalplanes, and the headings in vertical planes. As the sides of the joints of the courses of allvaults terminate upon the intrados in a horizontal plane perpendicular to it, if the intradosbe cylindrical, the sides are straight lines parallel to the horizon, those of the coursingjoints will be in planes intersecting the intrados perpendicularly in straight lines, and thecourse will form one solid prism ; the sections will consequently all be equal and similarfigures, and in vertical planes.

Groined Arches. — When on a rectangular plan, and formed by two vaults of the sameheight, but differing in diameter, the smaller arch being set out, and perpendicularsdropped to the diagonal or line of intersection, by drawing others at right angles tothem, and setting off the heights of the several voussoirs of the smaller arch, we haveseveral points, which traced through by hand give the curvature of the greater arch. To