INDIAN SARACENIC ARCHITECTURE. BOOK VIL should stand. If the section represented an arch or a vault, it is such as would not stand one hour; but the dome is itself so perfect as a constructive expedient, that it is almost as difficult to build a dome that will fall as it is to build a vault that will stand. As the dome is also, artistically, the most beautiful form of roof yet invented, it may be well, before passing from the most extraordinary and complex example yet attempted anywhere, to pause and examine a little more closely the theory of its construction. Let us suppose the diagram to represent the plan of a perfectly flat dome 100 ft. in diameter, and each rim conse- quently 10 ft. wide. Further assuming for convenience that the whole dome weighs 7,850 tons, the outer rim will weigh 2,826, or almost exactly as much as the three inner rims put together; the next will weigh 2,204, the next 1,568, the next 942, and the inner only 314; so that a considerable extra thickness might be heaped on it, or on the two inner ones, without their pre- ponderance at all affecting the stability of the dome; but this is the most unfavourable view to take of the case. To understand the problem more clearly, let us suppose the 418. Diagram illustrative of Domical Construction. semicircle A A A (Woodcut No. 418) to represent the section of a hemispherical dome. The first segment of this, though only 10 ft. in width, will be 30 ft. in height, and will weigh 9,420 tons; the next, 10 ft. high and 10 ft. wide, will weigh 3,140; the third, 10 ft. by 6 ft., will weigh only 1,884; the fourth will weigh 942; and the central portion, as before, 316. Now it is evident that the first portion, A B, being the most perpendicular, is the one least liable to disturbance or thrust, and, being also two-thirds of the whole weight of the dome, if steady and firmly constructed, it is a more than suffi- cient abutment for the remaining third, which is the whole of the rest of the dome.
