Seventeen feet within this outer wall the basin is crossed by an arch, supporting a light wall of brick and timber, which was the rear wall of the tower. This arch is a very remarkable piece of construction. It springs from two half-octagonal piers, and is segmental, of 61 feet span and 15½ feet rise. The voussoirs form two ribs, and the inner one is composed of seventy-five stones, united by a simple joggle or rebate. The rectangular tower, which thus rises from the walls of the outer half of the basin, is 86 feet east and west by 18 feet north and south, and capped at its two Thames-ward angles by light cylindrical turrets, three-quarters engaged, and rising above the parapet of the tower. The side walls, prolonged backwards an additional 30 feet, terminate in two square turrets, which occupy the northern angles, and also rise above the battlements. Two doors on the north faces of these turrets, on the ground level, open into a mural gallery in each wall, looped on one face towards the ditch and on the other towards the basin. These galleries communicate with the southern turrets, and are continued within the south wall, having each two loops towards the Thames and two towards the basin. They do not meet, being stopped by the jambs of the sluice-gate. They were also used to receive the slack of the chain when drawn up. The floor of the tower, which is above the level of these galleries, contained the machinery for lifting the sluice, which must have been heavy. The walls of this floor are also pierced with loops, and contain two garderobes. In the river-front are two early Decorated windows, of two lights each. The two upper chambers in the south turrets open into this floor, which is reached by a winding stair in the north-east turret, and by an exterior stone stair in the north-west turret. [Illustration: ST. THOMAS’S TOWER.—PISCINA.] The four chambers in the two cylindrical turrets are of excellent design and delicate workmanship, as may be seen from the plate in the “Vetusta Monumenta.” They are in plan octagons, having slender columns, with high bases and bell caps, all cylindrical, and slightly engaged, in each angle, from each of which springs a light chamfered rib, meeting in a plain joint at the centre. There are thus eight cells, each with a lancet gable, supported by two half-ribs. Three of the faces have windows or loops, and another is occupied by the door. In the lower rooms the loops have chamfered recesses beneath drop arches. In the upper, the loops are larger, but still square-headed, and their recesses have an arch-rib with a hollow chamfer. The south-east upper chamber was an oratory. The window-sills on each side the east windows are Purbeck slabs, hollowed into bowls,—on the south for a piscina, on the north for holy water. Both slabs projected, and have been broken off when the walls were wainscoted.