Caisson Lock - Method of Operation

Method of Operation

The system depended on the submerged, sealed box (the "caisson", from the French for "large chest") being heavily ballasted to achieve neutral buoyancy, so it was never possible in ordinary operation to lift it to water level to allow a descending boat to float in. Instead, a masonry chamber ("cistern") was built with walls higher than the water level in the top pound and itself filled completely with water, so that even at its upper position the box remained below the surface. A vertically sliding door sealed the caisson from the top pound and kept the water in.

The mechanism was operated from the top level. For a descent, the box was first wound into its upper position using a double rack-and-pinion mechanism, then drawn tightly against the frame of the opening using a ratchet mounted on the top of the wall. The outer door was then drawn up with another rack-and-pinion. At this point the water levels in the top pound and inside the box would have been roughly equal, but as the inner door – the box door – swung outwards horizontally (like a normal single lock gate) it would not open if the outer level was to any extent higher. A small equalising cock was therefore provided. The door was opened, the boat was floated in, the doors closed and the ratchet released. Because the entering boat would displace its weight of water back into the pound, the total weight of the box was always the same and no great endeavour was needed to wind it up and down. However, the operators could release a little water into the box to assist the descent. Water pressure against the outward-opening doors kept them firmly closed and watertight.

At the lower position the process was reversed. Here the water pressure was strong enough to press the box tightly into position against the exit opening. Another rack and pinion (again operated from above) lifted the outer gate, the levels were equalised again, the inner door on the box was swung open and the boat floated out. Apart from the inevitable leakage, no water had been used in the process.