As will be seen from the plan, it is internally a square apartment, 135 ft. each way; its area consequently is 18,225 sq. ft., while that of the Pantheon at Rome is, within the walls, only 15,833 sq. ft.; and even taking into account all the recesses in the walls of both buildings, this is still the larger of the two.
At the height of 57 ft. from the floor-line the hall begins to contract, by a series of pendentives as ingenious as they are beautiful, to a circular opening 97 ft. in diameter. On the platform of these pendentives the dome is erected, 124 ft. in diameter, thus leaving a gallery more than 12 ft. wide all round the interior. Internally, the dome is 175 ft. high, externally 198 ft., its general thickness being about 10 ft.
The most ingenious and novel part of the construction of this dome is the mode in which its lateral or outward thrust is counteracted. This was accomplished by forming the pendentives so that they not only cut off the angles, but that, as shown in the plan, their arches intersect one another, and form a very considerable mass of masonry perfectly stable in itself; and, by its weight acting inwards, counteracting any thrust that can possibly be brought to bear upon it by the pressure of the dome. If the whole edifice thus balanced has any tendency to move, it is to fall inwards, which from its circular form is impossible; while the action of the weight of the pendentives being in the opposite direction to that of the dome, it acts like a tie, and keeps the whole in equilibrium, without interfering at all with the outline of the dome.
322. Pendentives of the Tomb of Mahmúd, looking upwards. (From a Drawing by Mr. Cumming.) Scale 50 ft. to 1 in.
In the Pantheon and most European domes a great mass of masonry is thrown on the haunches, which entirely hides the external form, and is a singularly clumsy expedient in every respect compared with the elegant mode of hanging the weight inside.
Notwithstanding that this expedient gives the dome a perfectly stable basis to stand upon, which no thrust can move, still, looking at the section ([Woodcut No. 323]), its form is such that it appears almost paradoxical that such a building 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 consequently 10 ft. wide.