Over the site of the proposed pier, a large circular cylinder is sunk, which rests on the rock-bottom, and has its upper edge slightly above high water. A horizontal floor divides the cylinder into two chambers. The lower chamber, seven feet in height, is charged with compressed air by machinery situated on shore, and connected with it by flexible hose. The air under pressure excludes the water, enabling workmen to descend into the lower chamber—which is, in fact, a large diving-bell—and to excavate the rock on which the caisson rests. The excavated material is drawn up in buckets or ‘skips’ and thrown over, whilst the caisson gradually descends by its own weight until a level bed is formed. The upper and lower chambers of the caisson are then filled with concrete, and this circular monolithic foundation carries the granite pier on which rests the steel superstructure. A tube, connecting the air-chamber below with an air-lock on the upper platform of the caisson, gives access to the working beneath. In principle, the air-lock of a caisson in no way differs from the well-known lock on a canal. The air-lock is formed by a tube of larger diameter, which surrounds the upper end of the vertical tube leading to the air-chamber. Having entered this outer chamber, the door is closed behind us, and our connection with the outer world severed. A cock is turned, and with a steady hiss, the compressed air enters, a fact of which we soon become painfully conscious by the pressure that is brought to bear upon the drum of the ear. We follow the directions previously given us, and by copiously swallowing the compressed air and forcing it into the ears, with closed nostrils, we equalise the pressure on both sides of the drums, and succeed in accommodating ourselves to the novel atmospheric conditions. The inrush of compressed air at length ceases; and the pressure being now equal in the outer chamber—in which we are—and the internal tube, the door between them opens without difficulty. We enter, and descending a vertical ladder some ninety feet, we find ourselves in the air-chamber, and standing on the solid rock-bed of the Firth of Forth fifty feet below water-level. The scene is as striking as it is novel. A circular iron chamber, seventy feet in diameter and seven feet high, brilliantly illuminated by arc-lights suspended from the roof. Groups of foreign workmen—enlisted for this service, owing to continental experience in this class of work—are busily engaged in levelling the surface of the rock. The majority of these men wield pick and bar; whilst others fill the iron tubs or ‘skips’ with the fragments of rock, which are then drawn to the surface, passing through a lock similar in principle, though differing slightly in design from that we have ourselves traversed; and having discharged their contents over the edge of the caisson, return for another load.