In a practical demonstration at Hampstead Road Congreve later elected to deploy an air compressor releasing into one caisson, estimating that one man could achieve raising and lowering in three minutes—the company's enabling act did not permit the installation of stationary steam engines. However it was found the fastest overall time was six minutes and that the effort required rendered the operator "incapable of further exertion". Furthermore, as the operation of the device depended upon air pressure inside the caissons (whether the means of moving them was by varying this air pressure or otherwise) when contractor Henry Maudslay delivered them and they were found to leak air, the scheme failed. In 1818, after many unsuccessful repair attempts, the company substituted conventional locks. A patent granted to Jonathan Brownill, a cutler from Sheffield, on 1 May 1828, while in principle operating as an open-air balance lock, was described as using three caissons. The main caisson was connected by ropes passing over grooved pulleys to two smaller, counterbalancing caissons. Brownill's innovation was to place wedges ("inclined planes") opposite the upper and lower fixed openings so that as the main caisson moved into place, powered by water being added or released from the counterbalancing caissons, rollers acting against the wedges forced it against a padded frame surrounding the opening. The "conductor" was to have control of a lever to release the rollers when the vertical gates were slid shut for the next ascent or descent. See also Balance lock Canal inclined plane Boat lift Diagonal lock Caisson (water transport) References General references Clew, Kenneth R (1977): Somersetshire Coal Canal and Railways. David and Charles, Newton Abbot, UK. . Uhlemann, Hans-Joachim (2002): "Canal Lifts and Inclines of the World" Internat, Horsham, UK. . Inline citations External links Somersetshire Coal Canal Society History of Somerset Locks (water navigation) Locks of the United Kingdom