The general model of a Christian church was that of a cross; the stem represented by the nave, the cross by the transepts, and the upper part by the choir: but from the inequality of the parts, the western churches laboured under a disproportion from which the eastern were exempt. The arms of the Greek cross are all of equal length, and Santa Sophia is built on its model; it has therefore a symmetry which the Latin churches have not, though founded on the same symbol. The ground-plan is that of a cross enclosed in a square whose sides measure 243 feet, but, including the portico, its length is 269. Over the centre of the cross rises the dome. This dome is called “aërial,” because it is so constructed that its height is only one-sixth of its diameter, and its curve so flat that its convexity seems to correspond with that of the sky, and be a portion only of the great firmament, let down, and suspended, as the Greeks say, by a chain. To effect this, it is built of materials of the least possible gravity, pumice-stone, specifically lighter than the water on which it floats, and bricks from Rhodes five times less weighty than those of ordinary burnt clay. The vast dome, thus reduced in weight, is further secured by the pillars on which it rests. These are ponderous piles of freestone, made of blocks hewn into cubes and triangles, united by huge cramps of iron. It is partly by this judicious distribution of its materials, that the vast edifice has stood so long unshaken by those shocks of earthquakes, which have prostrated so many other edifices in the same period. The mosque is entered by a portico twelve yards in breadth; this communicates with another by nine gates with marble arches, closed by valves of rich bronze cast in high relief: this opens into another parallel to it. These vestibules formed what is called the narthex, or pronaos, of the Greek Christian church. Here stood the font where catechumens were baptized, and penitents were placed before they presumed, or were deemed worthy to enter the naos, or body, of the sacred edifice. From hence they passed into the interior by five doors of plain bronze.