Communication between the tanks and the sea is maintained at all times by pipes and valves worked from the upper deck.

The blowing-out apparatus is associated with hand pumps, the latter being used in case of breakdown or failure of the compressed air supply.

The space above the upper deck of the air chamber is subdivided into three compartments, separated by watertight bulkheads ; the central compartment communicates with the bilge by two watertight trunks passing up through the air chamber, so that the water in the central division will rise or fall with the level of the water outside, which is in communication with the bilge by means of the flood-holes before mentioned.

The two end compartments are not in communication with the bilge, but are connected by trunks and valves controlled from the upper deck, with the water on the side opposite to the flood-holes. These compartments can therefore be flooded to any extent desired, and control is thereby obtained over the longitudinal trimming of the caisson, should such be necessary. Water admitted into the end chambers at a high tide can also be retained and locked in, an essential element under certain conditions, or at such times as the caisson is turned end for end in the grooves.

The uppermost deck of the caisson is designed to accommodate a mixed traffic of passengers and horses or carts, together with a single line of rails calculated for the heaviest rolling load which can occur in practice.

This load has sometimes been taken as high as 72 tons on four wheels, having a wheel base of 6 feet 6 inches, and the whole of the rail-bearers and cross-girders have been calculated on this basis. The remainder of the deck is laid with oak planking, with steel wheel tracks and guards for ordinary wheeled traffic, horse treads being applied between the rails, while the various trunks and manholes, openings for valve spindles, etc., are covered with movable or hinged flaps.

The groove in the masonry of the dock sides and sill for a floating caisson is usually about 4 inches wider than the outside width of the keel or stems, and the caisson "fleets" over from side to side of the groove by this amount, as the water pressure is on one side or the other. See Figs. 394, 395. To provide for this movement an automatic switch is usually provided, whereby the rails on caisson deck are kept in correct adjustment with those on the dock side.

In the case of flaps over valve spindles, an automatic arrangement is frequently adopted whereby any condition of the valves below which implies danger to the stability of the caisson by reason of a temporary excess of buoyancy (the danger to be the most carefully guarded against) is indicated by the flap standing up, and its inability to be closed until the valves have been adjusted to a "safe" position, while such a position of the flap acts as a danger signal to the caisson master.

Handrailing is' provided on both sides of the upper deck, and is usually arranged to be turned down on the roadway when required, so as to be out of the way during warping or docking.

Immediately below the uppermost deck, but placed as high as possible, so as not to be overlapped by the highest recorded tides, is constructed the top tank before referred to. The function of this tank is to contain a sufficient amount of water, either salt or fresh, as may be convenient, to overcome and destroy the buoyancy of all immersed material above the level of the upper deck of the air chamber, and it is by the filling or emptying of this tank that the caisson is caused to sink into, or rise out of, the grooves.

The tank is usually filled by hose from water mains on the dock side, and is emptied by valves controlled from the upper deck.

When a floating caisson has been removed from its groove, it is sometimes found convenient, as a saving of time, to fill the upper tank prior to warping back again. This may be done, providing the stability of the caisson permits, by closing the valves to end chambers, thereby providing the additional buoyancy required, but adding to the draught of water.

These valves being opened, when the caisson is in its groove, the buoyancy of the end chamber is destroyed and the caisson sinks.

In the type of caisson shown in Fig. 398 the same result can be obtained by the admission of water into the cylindrical tanks in the air chamber, the valves to end chambers being closed. In this case, before the caisson can rise again the contents of the cylindrical tanks must be either blown out or pumped up.

It has been previously hinted that one of the most serious dangers to which a caisson can be exposed is a temporary excess of buoyancy, whereby an upward lifting tendency is developed.

This has led to serious accidents in certain cases, resulting in the forcing-in of the caisson and the immediate flooding of the dry dock, and consequent risk of damage to vessels on the blocks.

Caissons unsymmetrical about their centre line, with a flat side to the dry dock, and the whole of their displacement on the tidal side, are perhaps most exposed to this risk; but in all classes of caissons the point is deserving of the most careful attention, and calculations for buoyancy should always include the worst possible conditions as regards the proportion of weight to buoyancy, when, for example, the bilge has been drained into a dry dock, and the readmission of water into the dock produces a set of conditions temporarily out of the normal.

To meet such contingencies, and to provide for the accidental emptying of tanks through leaky valves or the like, holding-down straps, bolted to a sufficient mass of masonry at the upper part of the side grooves, are frequently adopted, so arranged as to hold down the caisson, and provide an anchorage sufficient to supply the place of any temporary loss of weight resulting in an excess of buoyancy.

It will generally be found that before a serious excess of buoyancy can arise, a difference of head of water, as between the two sides of the caisson, affords an additional safeguard against uplifting by reason of friction between the stems and keels of the caisson, and the faces of the groove.