112. Diagram of Roofing.
The simplest mode of roofing a small square space supported by four pillars is merely to run an architrave or stone beam from each pillar, and cover the intermediate opening by a plain stone slab. Unless, however, slabs of great dimensions are available, this mode of construction has a limit very soon arrived at. The next step therefore is to reduce the extent of the central space to be covered by cutting off its corners; this is done by triangular stones placed in each angle of the square, as in Woodcut No. 112, thus employing five stones instead of one. By this means, the size of the central stone remaining the same, the side of the square space so roofed is increased in the ratio of ten to seven, the actual area being doubled. The next step in the process ([Woodcut No. 113]) is by employing three tiers and nine stones instead of two tiers and five stones, which quadruples the area roofed. Thus, if the central stone is 4 ft., by the second process the space roofed will be about 5 ft. 8 in.; by the third 8 ft. square; by a fourth process ([Woodcut No. 114])—with four tiers and thirteen stones—the extent roofed may be 9 ft. or 10 ft., always assuming the central stone to remain 4 ft. square. All these forms are still currently used in India, but with four pillars the process is seldom carried further than this; with another tier, however, and eight pillars (as shown in Woodcut No. 115), it may be carried a step further—exactly the extent to which it is carried in the tomb at Mylassa above referred to. In this, however, as in all instances of octagonal domes in this style, instead of the octagonal form being left as such, there are always four external pillars at the angles, so that the square shape is retained, with twelve pillars, of which the eight internal pillars may be taken as mere insertions to support the long architrave between the four angular pillars.
| 113. | Diagrams of Roofing. | 114. |
115. Diagram of Roofing.
It is evident that here again we come to a limit beyond which we cannot progress without using large and long stones. This was sometimes met by cutting off the angles of the octagon, and making the lower course of sixteen sides. When this has been done an awkwardness arises in getting back to the square form. This was escaped from, in all the instances I am acquainted with, by adopting circular courses for all above that with sixteen sides. In many instances the lower course with sixteen sides is altogether omitted, and the circles placed immediately on the octagon, as in the temple at Vimala Sah ([Woodcut No. 130], p. 236). It is difficult to say how far this system might be carried constructively without danger of weakness. The Indian domes seldom exceed 30 ft. in diameter, but this may have arisen more from the difficulty of getting architraves above 12 ft. or 13 ft. in length to support the sides, than from any inability to construct domes of larger diameter in themselves. This last difficulty was to some extent got over by a system of bracketing, by which more than half the bearing of the architrave was thrown on the capital of the column, as shown in Woodcut No. 116. Of course this method might have been carried to any extent, so that a very short architrave would suffice for a large dome; but whether this could be done with elegance is another matter. The Indians seem to have thought not; at least, so far as I know, they never carried it to any extent. Instead of bracketing, however, they sometimes used struts, as shown in Woodcut No. 116, but it is questionable whether that could ever be made a really serviceable constructive expedient in stone architecture.