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Rh addition to the air-shafts, a number of cast-iron pipes were fixed, reaching from the air-chamber ceiling to various parts of the outer shell, and these were used for the removal of silt and soft material, all of which, in combination with water, were ejected by means of a small quantity of compressed air being allowed to escape.

In the air-chamber itself, the main difference between the Queensferry and Inchgarvie caissons consisted in a different arrangement of the shoe.

In the former, the sloping plates were carried right down to the cutting edge, while in the latter they only went half-way down the slope, being then bent in a horizontal direction, and brought to the outside shell and then rivetted on. For the support of the lower remaining portion of the outer shell, strong brackets were fixed about 3 ft. 6 in. apart on the circumference, and these terminated at the lowest angle, which, with the 18-in. steel belt, formed the cutting edge of the caisson. This was done in order to allow the sinkers the fullest access to the rock immediately under the cutting edge, it being of the greatest importance that it should be undercut to the extent of at least 6 in. all round. The section of the Inchgarvie shoe is shown in Fig. 52.

It has already been stated that launching ways were laid down on the south shore to the east of the jetty. These timbers (Fig. 37) were laid on the natural slope of the ground, holes being dug at intervals which were filled up with concrete, and thus provided strong piers to carry the weight of the caissons. At the higher end of the timbers several trestles were set transversely, of such height as to keep the work out of the reach of the water. The tops of the trestles formed the platform upon which the construction of the caisson was commenced and carried on until about 25 ft. or 30 ft. of it, counting from the lower edge upwards, had been built. The weight was then about 300 tons. By means of hydraulic jacks the caisson was then slightly raised and packed, and the trestles withdrawn. It was then lowered down to within half a foot of the top of the launching cradle. This was made up of transverse girders laid on longitudinal timbers, which in their turn were laid on the timbers forming the launching ways. The top of the cradle was, of course, horizontal, and it was essential to support on it as large a portion of the ceiling of the air-chamber as could be reached, and also as much as possible of the lower or cutting edge of the caisson, which latter was also supported for the time being by a number of timber blocks placed on the ground.

In the building of the caissons, the general practice was to commence by laying down the bottom booms of the large lattice girders upon the timber trestles at the head of the launching ways, to bolt the 3-ft. cross-girders between them, and fix to both the floor-pistes. Outside the trestles, any projecting parts of the caissons were supported from the ground by timbers. The plates forming the inner shell were next put on, and also the sloping plates forming the shoe, the two forming one joint with the floor-plates. All joints were rivetted up as far as possible by ordinary hydraulic rivetters, and meanwhile the bracings of the big girders were put on and the top booms laid on these, and more plates were added to the inner and outer shells. Upon the girders the first platform was laid down, and a hand crane set up as well as forges, concrete mixer, and other gear. By the time the caisson was about 25 ft. high—much of the lower work being then done—it was got ready for lowering, as already described. It was even then still accessible in the lowest portions soon after half-tide for further work and for making preparations for launching. Previous to this, all joints in the plates forming the ceiling—the sloping face of the shoe and the outside shell of the caisson—were carefully caulked to make a good water-tight job.

In all cases, previous to launching a caisson, the shoe and the whole space over the ceiling of the air-chamber were filled up with concrete, the latter generally to a depth of from 4 ft. to 6 ft. This not only acted as ballast, but it gave much stiffness to all the lower portion of the caisson, and produced a draught of from 9 ft. to $10 1/2$ ft. after it floated.

When finally the caisson had been built to the required height and weight, all the points of contact between cradle and caisson were carefully adjusted, and all the weight allowed to rest on the cradle, it being then ready to be launched. The cradle was loaded with pieces of iron to hold it down to the launching ways when the water lifted the caisson off its bearings and floated it away. The slope of the launching ways being about 1 in 11, it required a push from a 12-in. hydraulic jack, placed horizontally, to set the mass in motion, and this it did most effectually on all occasions.

The launching of these caissons took place generally at or near to the time of high water of spring tides, owing mainly to the shallowness of the shore in front of the launching ways—the caissons drawing from 9 ft. 6 in. to 10 ft. 6 in. of water at the time. As soon as afloat a tug-boat was attached and the floating monster at once towed to its final resting-place, or else to the end of the jetty, where convenience existed for charging concrete into it and placing all the necessary machinery on board, as also the temporary caisson on top. Should any tide not rise as much as was expected, it was the practice to hermetically seal the air-shafts and other outlets from the working chamber and force air into the latter, in order to increase the buoyancy.

The first caisson—that for the south-west pier, Queensferry—was launched on May 26, 1884, the ceremony being performed by the Countess of Aberdeen, the Earl being at the time Lord High Commissioner to the General Assembly in Edinburgh. The last caisson was launched on May 29, 1885—the south-west Inchgarvie.

The Temporary Caissons.—The temporary caissons, placed on the top of the permanent ones for the purpose of keeping the water out while the circular granite pier was being built, were of the same construction as already described in connection with the Inchgarvie north piers. (See Figs. 32 and 51.) The lower tier of caisson segments, in addition to the large number of small bolts which made the water-tight joint, had two long 2-in. bolts to each section of caisson, or twenty-eight bolts on the circumference. These bolts terminated at top in a screw thread with nuts, and at bottom in a long shackle, which grasped a lug rivetted to the outside of the permanent caisson. The temporary caisson was made large enough to allow space round the completed pier for the masons pointing the joints in the granite blocks. Two tiers of caisson total height, 20 ft. were generally held sufficient to keep out the water; but in places where rough seas and much spray were likely to occur, a third tier, or at any rate a portion of it, was put on. The south-east caisson on Inchgarvie had a special temporary caisson constructed for it, consisting of plates 22 ft. in length. (See Figs. 36 and 53.)

The four Queensferry caissons, after being launched, were at once towed out and placed in their proper positions, being secured to the jetty in such a manner as to allow their rising and falling freely with the tide. The Inchgarvie caissons, on the contrary, were merely placed in a temporary berth near the end of the jetty, and they were completed there, and had the temporary caissons put on and all necessary machinery placed inside, before they were removed to their permanent places. This was done, not only because it was essential that these caissons should touch ground as soon as possible after their arrival in their berths, but also because a large proportion of the machinery inside and of the temporary caissons, was taken from the Queensferry piers, then already built up to above high water, and put on board, and the transport and double handling thus saved. It may not be amiss to state, as an instance, that the last caisson—No. 6, or south-west Inchgarvie—was launched on May 29, 1885, weighing a little over 500 tons, with a draught of 10 it. 3 in., was towed to Inchgarvie on July 16, drawing 31 ft. of water, and weighing 2877 tons, which was made up as follows:

Before entering upon the description of the mode of sinking the caissons, some account is needed to be given of the apparatus and machinery in connection with that part of the work.

There was nothing in these different from any ordinary type of compressors. They consisted of a pair of horizontal engines coupled, and directly acting upon a pair of double-acting air-compressing cylinders. There were two pairs of $16 1/2$-in. cylinders with 2-ft. stroke, and three pairs of 12-in. cylinders with 2-ft. stroke, all working at 60 lb. steam pressure. They were run up to any speed required to produce the necessary maximum pressure, which in the case of Inchgarvie on one occasion amounted to 37 lb. per square inch in the air-chamber of the south-west caisson. The air-compressing cylinders were water-jacketted, with a continuous current of cold water passing through. For the service of the rock drills inside the working chamber, a separate engine was used, which forced air down by a separate pipe lead at 70 lb. per square inch—the rock drills receiving, of course, the benefit of the difference between that pressure and the prevailing one in the air-chamber only, as they had to discharge into the general pressure of the working chamber.

On the top of the men's air-lock a whistle was set which worked by air pressure and which could be regulated from below; by means of this a constant communication was kept up, and more or less air pressure put on in accordance with requirements.

The pipes supplying the air were laid from the compressors straight to the caisson side, but the joint to the pipes inside the caissons had to be made by a long piece of flexible tube, the end of which was attached to a check valve placed upon the air-lock for the admission of the men. The air was forced down this shaft into the working chamber, thus preventing the ascent of any foul air in these parts.

A supply of water for flushing purposes was also laid on, being taken from an overhead tank set about 40 ft. above the level of the staging. Of great importance both for the safety and convenience of the men, and for the progress of the work, were the air-locks. Those by which access was got to, and exit from, the air-chamber, consisted of an inner portion, 3 ft. 6 in. in diameter, in continuation of the ascending shaft, and a circular space all round it about 21 in. wide and 6 ft. high. (See Figs. 38 and 39.) The outer chamber was divided by two partitions into two equal spaces. Each of these spaces was provided with a square door giving communication on the one side to the air-shaft, and on the other side to the outer atmosphere. All doors opened to the inside. To gain admission to the air-chamber it was necessary to enter one of the outer spaces, to close the door (which was pressed against an india-rubber joint), and by turning a small cock to gradually admit compressed air. As soon as the pressure on both sides of the door leading into the air-shaft was equal, this latter fell open and the air-chamber could be reached by an iron ladder fixed in the shaft. To return from the air-chamber to the outside was the reverse of this, the pressure from the outer chamber being exhausted by opening a small cock communicating with the atmosphere.