The four principal piers are very carefully built of shaped stones, the joints, according to Procopius, being run with lead, but the Silentiary mentions a cement as being used here. The height from the floor to the springing of the great arches is seventy-three feet. The arches of seventy-two feet span have abutments of twenty-four and a half feet, which are increased above the vaults of the gynaeceum to twenty-nine feet. The great arches under the springing of the dome are about four bricks, or five feet, thick. The depth at the top, including the cornice of the dome, is about six feet and three quarters. The centre of the arches is two and a half feet above the springing, so that they are more than semicircular. In the internal angles formed by them are the four pendentives. The cornice has a projection of about two feet nine inches. The lead mentioned by the Silentiary may be found in the interstices of its stones. The dome springs from the cornice on forty piers, about three feet five inches broad on the inside, and about eight and a half feet deep in the direction of the radius. They are connected by arches which form windows four feet nine inches wide. On the outside the piers project beyond the arches, and may perhaps at one time have been connected with other arches, forming a drum for the dome: within they form part of the ribs of the dome. In the interior the ribs project at the springing six inches from the surface of the dome, which is there twenty-nine inches thick, but their projection gradually diminishes, till they are lost in the great circle of thirty-seven and a half feet diameter in the centre. In the interior from rib across to rib is 104 feet, so that all round on the cornice is the passage two feet nine inches wide, which, according to Paulus, was used by the lamp-lighter. The dome rises forty-six feet nine inches above this gangway, so that it is considerably less than a semicircle in section. The original dome, according to Agathias, must have been even flatter. Theophanes states the increase in height to have been twenty feet, and Zonaras twenty-five.[253] The dome has now many swellings and depressions which are not visible from the ground. At the same time we see how immovable domed vaulting is, if only its supports remain uninjured.