The conditions of working of a modern dry dock frequently lead not unnaturally to the result that the caisson is the last floating structure to be cared for and docked. Repairs are postponed to a convenient period, which is sometimes long in coming, and the caisson has, as regards scraping and painting, to take its chance, for some years.

Such considerations are obviously opposed to any unnecessary refinements in the thinning down of scantlings, although, on the other hand, undue excess of weights must be avoided, as demanding correspondingly large buoyancies and increase in size of air chamber.

In the examples of caissons now before us the thicknesses of plates and bars range from ⅞ inch or ¾ inch to ⅜ inch, no thickness less than the latter being allowed; and, broadly, it may be stated that from ⅛ inch to 1/16 inch more metal is allowed than merely theoretical requirements would determine.

There may be, and probably are, examples of this class of work in private establishments, where such a view of the maintenance of the life of a caisson or gate is not taken, and where considerations of first cost have influenced the selection of scantlings on a less liberal scale.

With these preliminary remarks on the general subject, we may now consider in detail certain examples of both classes of caissons, floating and sliding, the former being taken first in order, being somewhat simpler in construction than its rival.

The type of caisson shown in midship section in Fig. 398 consists, as regards its internal arrangements, of five principal subdivisions, the lowermost of which, termed the bilge, is open to the water, by means of a series of flood openings on one side only through the skin, placed below the lower deck of the air chamber.

The bilge contains in its lowermost position, just above the keel, the ballast necessary for the stability of the caisson when floating, and is so arranged as to be capable of being drained dry into the dock when the latter has been pumped out, suitable outlets with plugs being provided, the key for opening which can only be withdrawn when the outlet is closed.

Above the bilge the next subdivision to be observed is the air chamber. This chamber is of capacity sufficient, when combined with the buoyancies of all immersed materials, to float the caisson, with all weights and ballast on board, at the level of the upper deck of the air chamber.

The level of this upper deck is consequently determined by the lowest level of tide at which it is required that the caisson shall be capable of being removed from the groove, and by the amount of vertical lift necessary to enable the stems of the caisson to clear the corners of the groove. The amount of lift required will depend upon the degree of batter in the side walls of the dock entrance, together with the clearance left between caisson stems and back of groove, referred to and shown in Fig. 395.

If, for example, a lift of, say, 5 feet is necessary before the caisson can be safely manipulated out of its grooves, then the level of the upper deck of air chamber, being the flotation plane, must be 5 feet below the tidal level at which it is determined that the caisson shall be worked, and this tidal level will usually have some relation to the depth at which the sill of the dock has been laid.

Fig. 398. Scale 1 inch =10 feet.

The position of the air chamber having thus been determined in the relation of its upper deck to the total height of the structure from sill to coping, the volume of the chamber is then determined in accordance with the buoyancy required, and the level of its lower deck will follow.

But the conditions of practical working demand a certain stability of the caisson when floating, and practical experience has shown that a pendulum of from 1 foot 6 inches to 2 feet 6 inches, in accordance with the size and total height of the structure, is desirable; that is to say, that the centre of combined buoyancies (C.B.) shall be from 1 foot 6 inches to 2 feet 6 inches above the centre of combined weights, or the centre of gravity (C.G.) of the caisson when ballasted. This pendulum will be reduced if the caisson is worked at any time with the top tanks full. Further reference to these tanks will be made.

It is thus seen that the capacity and position of the air chamber must be such as to meet all the conditions above described, as the amount of ballast required for the stipulated stability is frequently considerable, especially where, in consequence of the relative position of tidal levels and the conditions as to floating out, the air chamber is comparatively low down in the structure.

The air chamber will be frequently found to contain scuttliug tanks, which, being flooded, reduce the buoyancy of the chamber, and are usually brought into action when rapid sinking is desired, or to counteract excessive buoyancies under certain conditions of working. Such tanks must, however, be emptied before the caisson can be restored to its normal working condition, and this implies pumps, which may be worked by compressed air, electricity, or other power, commonly supplemented by hand gear, to be applied in case of a breakdown. The weights of all such internal machinery must, of course, be taken into the account when estimating the combined centre of gravity (C.G.) of the structure.

In the example before us, the scuttling tanks in the air chamber consist of two steel cylindrical tanks or boilers, seated on stools attached to the lower deck of the air chamber, stayed to prevent any motion consequent upon heeling over of the caisson, and so arranged as to be blown out and emptied of their contents by compressed air, admitted by suitably arranged pipes, and connected at the upper deck level by means of flexible high pressure hoses to the compressed air mains adjacent on the dock side.