ft 4 in by 2 ft 9 in high (21.3 by 4.1 by 0.84 metres). The caissons were profiled to conform to the hull of the boats, so as to contain as little excess water as possible, to a depth of , and were only to convey empty boats. The caissons ran on a railway track of gauge with flat bottom rails on longitudinal timbers, on a gradient of 1 in 10. There were two tracks. They ran on ten pairs of wheels, of diameter except the uppermost pair were and the second pair were , to accommodate the wedge shape of the carriage. A continuous rack was fixed to the longitudinal timbers, and a pawl—Leslie calls it a "pall"—attached to the carriage was arranged to engage the rack if the tension was let off the haulage cable. The caissons were counterbalanced, the ascending caisson and the descending operating together. At the top of the incline, the caisson was brought close to the end of the upper canal reach; it was then forced close to it by hand-operated screw jacks to obtain a water seal, and two guillotine gates, at the end of the caisson and at the end of the canal respectively, were opened. At the bottom, the caisson simply descended into the lower canal reach and became immersed. There were two high pressure steam engines. For most of the ascent, the caissons balanced, but as the lower caisson entered the canal water, it gained buoyancy, and the final closing-up required the engines to operate at full power to bring the upper caisson to the canal. The engines operated two shafts carrying the winding drums, geared to work in opposite directions, so as to achieve the counterbalancing of the caissons. The speed of ascent and descent was about , with a passage time of 5 to 6 minutes. The screw jacks for closing the ascending caisson to the canal gate proved unsatisfactory, requiring the engine operator to make too fine a stop, within , and a hydraulical accumulator system was later adopted, which could ram the caisson closed over a longer range: up to . When a descent was about to start, the gates were closed and about of water from the space between was lost; it was pumped back to the upper reach. The practice was to bring up empty boats partly grounded in the caisson, and to