 A connecting underwater chain passing through rollers, while not in itself doing any work, controlled the relative positions.Boats were admitted through double gates; the inside ones on the caisson "adjusted to fit close to those of the outer".Congreve's patent then envisaged that having achieved "absolute equipoise" (neutral buoyancy) a rack-and-pinion, removable weights or a small winch could overcome inertia and move the caissons.