A WEIRD SCENE.

Timaru Herald, Volume XLII, Issue 3438, 5 October 1885, Page 3

A WEIRD SCENE.

(Scotsman.) The process of sinking the caissons for the Forth bridge 13 of so interesting a nature that a short account of a descent which the writer of thiß article made to see the workers below water may not bo unacceptable. Each caißßon, or cylinder, used m founding these great piera is 70 feet m diameter. Its bottom is 7 feet from its lower or cutting edge, so that when it restß on the bed of the estuary there is a chamber 70 feet m diameter and 7 feet high, out of which the water is kept by compressed air. In this chamber workmen labor, digging downwards through the mud and clay until a sure, level foundation is secured. In sinking a oaisson m the first instance below the water, it is loaded, as already indicated, with the concrete which will afterwards, of course, take its place as part of the foundations of the pier. But m filling m this concrete, as the caisson slowly descends, two strong iron pipes, and m some instances three, are run up from the pneumatic chamber right through the solid mass to the top, to a staging erected over the caisson. Each tube is fitted at the top with an air-lock, worked exactly on the principle of the canal-lock. One of the tubes is for the descent and ascent of the workmen, the other one or two jb for the bucket which takes to the surface the material excavated. There are several engines constantly employed compressing the air, which is pumped into each shaft. This part of the work is let on sub-contract, and is all being performed by a Bquad of Italian workmen, who, it seems, acquired great experience of the kind required here m the construction of the beautiful quay at Antwerp, made by the Belgian Government at a cost of over two millions sterling. They are quiet, hard-working men, toiling for twelvo hours out of the twenty-four m shifts of six hours — six mi and six off, and appear to do their work m a purpose-like way. The air-pressure neceosary to keep the pneumatic chamber clear of water is reckoned at so many lb to the square inch, according to the depth of water m which the caisson lies. At Inchgarvie about 331 b to the inch will be needed to found the last and deepest pier ; but at the time of our visit the gauge showed that about 71b less than that was then required. The caisson had been sunk about GO feet below high water, and it was still descending. It should be mentioned, perhaps, here that above the caisson which is to be left permanently m the bod of the eßtuary another jb fixed to keep out the water at high tide. The upper one remains m this position until the granite masonry is built from low-water level to its required height ; then, being of no more use, it is removed. Attired m overalls obtained at an adjoining office, the writer, accompanied by two of the chief officials of the bridge, entered the air-lock, an airtight iron chamber, just high enough to stand upright m. In the centre rose the inner tube communicating with the chamber below. Only a faint glimmer of light from the outside penetrated it through two thick bulls' eyes placed on the roof. The outer door is shut with a bang, and, all being ready, a valve is opened between the inner shaft and the air-lock, and immediately the high-pressure air begins to rush into the chamber with a deafening noise, like what is made when steam hisses through the valves of a locomotive eDgine. As the pressure betwen lock and shaft is gradually equalised the sensation which a casual visitor experiences are by no means of a pleasant kind. The pressure seems to be most felt m the ears, and stinging pains shoot through the head. Thesn can be slightly modified by swallowing the saliva of the mouth ; but that commodity soon becomes exhausted, and, though three or four minutes suffice to equalise the pressure, it seems an age. The pains gradually subside, and by the time the inner door is opened they will, m all likelihood, be nearly gone. At the top of the Bhaft an electric lamp burns, but it does little more than make the darkness visible. Entering the shaft, we descend by means of a verticle ladder — with something like 120 iron rungs fixed on the side of the tube — and, proceeding with all caution, duly arrive m the pnuematic chamber, where one is at first dazed by the glare of the electric lamps and the extraordinary resounding noises which assail the ear on every side. The scene is certainly weird m the extreme — the feeling being intensified when the thought is recalled that one is standing on the bed of the Firth of Forth, CO feet below water level, with a solid mass of concrete weighing 6000 tons overhead. The caisson m question has got down to the solid rock — a hard blue trap— and more than three-fourths of its circumference rests upon it. The other fourth is projected over the sloping face of rock, and the caisson, it is explained, will require to be sunk eight feet further before a level rock bottom is secured. There are about thirty workmen spread over the chamber, which iB well lighted by three arc lamps which shed around a light of a distinctly violet hue. Each blow of the pick and hammer or stroke of the spado on the rocky floor has a sharp resonant ring ; and one has to shout pretty loudly to make oneself heard. In one part there is a diamond drill driven by compressed air at work on tho rock ; and this adds considerably to the prevailing noise. At short intervals the bucket descending through the second tube is filled with stones and wcund by machinery to the top to its own airlock, from which it io drawn out upon the jetty and emptied of its contents. In one corner of the pnuematic chamber — that where the caisson overhangs the rock — is a deep pool of water, the heaving Burface of which tells of tho conflict going on between it and the high-pressure air. Were the air pressure withdrawn for a moment the water of course would assert its supremacy ; but it is comforting to be assured that there is not the slightest chance, humanly speaking, of such a thing occurring. The whole mechanism is under the most perfect control ; and the workmen are as safe from accident as they would be m, say, an ordinary outside quarry. The whole of the arrangements connected with this part of the work strike one as a splendid triumph of science over the forces of nature m the cause of human progress and oivilisation. The great advantage of sinking the caissons m this way is the absolute certainty it givoß that the work shall be done m a thorough manner. When divers are employed to do such work, the engineer can nevor bu sure that his instructions are carried out m all their detail. A «ood deal must be loft to the conscientiousness of the men. I Here tho work of founding the piers is as systematically and carefully supervised by the chief officials an any part of the bridge above the water level. Tho chief outdoor inspector descends the tube every other day, and the chief resident engineer once a week. When the caisson is solidly founded, the workmen fill up the chamber with concrete, beginning at the outer edge, and gradually retreating before their work until the mouth of the tube is reached. Portions of the tube are then withdrawn, and tho hole filled up by pouring concrete into it from the top. In working m the throe atmosphere* of (he pneuraatio chamber

the workmen do not Beem to feel any inconvenience. The air is much warmer below than above, and there is risk unless care be taken of catching cold ; but the breathing of the compressed air for six hours on end does not Beem to trouble them ; certainly the casual visitor remaining below for a quarter of an hour or bo experiences no inconvenience of this kind. After climbing to the top of the shaft we step once more into the airlock, and this time the inner door is firmly shut behind us. The pressure m the lock has now to be equalised with that of the outside air before the outer door can be opened. A valve communicating with tho outside is accordingly opened and the air rushes out of tho chamber with a similar noise to that which it made when coming into it. About eight minutes elapae before it is safe to open the outer door, and moantime as the pressure is lessened the chamber is enveloped m steam from the condensing of the compressed air. Another perceptible result is the withdrawal of the pressure from the earß and the leaving m them of a curious open feeling. Once into the outer world again, that feeling, however, soon posses away, and no bad after reßult3 are experienced.