A SUBAQUEOUS EXCURSION.
Our good-fortune in obtaining permission to descend a caisson of the gigantic Forth Bridge—which when completed will be one of the most stupendous railway viaducts in the world—obtained additional zest from the fact that comparatively few structures are founded on what is termed the pneumatic principle in this country—the employment of compressed air being more in vogue on the continent—and still fewer are open to the passing visitor, uninfluenced alike by professional or scientific ardour.
Arrived at North Queensferry, on the Fife side of the Firth of Forth, we embark for the island in mid-channel, and rounding the easternmost promontory of the rock, see before us a huge iron cylinder, which, but for the incongruity of its position, we should take for a gasometer, and not a caisson. We land, and are forthwith marshalled to the dressing-room. Leathern caps and garments of a sombre blue hue are donned, and we are ready to descend. Before, however, proceeding, a brief outline of the working of a caisson, the end in view, and the means adopted in the attainment of that end, may be given, which will enable the reader to follow our movements.
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.
We would fain linger amid a scene so weird and wonderful; but time fails, and we must return to ‘bank.’ We take a last look at the air-chamber with its busy occupants, and ascending the ladder, not without exertion, for a vertical ladder at all times calls muscle into play, and the pressure we are under by no means lightens our labours, we find ourselves again in the air-lock. The reverse process now takes place. The inner door is closed, the compressed air is allowed to escape from the outer chamber in which we now are, and causes a thick mist, cold and chilly. Before long, the pressure ceases; the outer door opens, and we again tread terra firma. The pressure-gauge records thirty pounds per square inch.
We now discard our exploring garments, and having enjoyed a not unneedful wash, we quit the works, and returning homewards, congratulate each other on having trodden the very foundations of the wonderful Forth Bridge, and ponder how little the future traveller, as he lightly skims the estuary at sixty miles an hour, will think of the practical ingenuity and patient labour that wrought, deep down beneath the waters of the Forth, the foundations on which repose the huge structures through which the flying express is whirling him.