 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 outer door sealed the lock chamber from the top pound and kept the water in.