of the upper caisson . Any vessels that just ascended in the top caisson exit into the upper reach, and any new vessels making a transit of the lock then enter the bottom or top caisson from the lower or upper reach respectively. Once the vessels are secured, all gates are closed and the crossover valve in the connecting pipe between the ram shafts is opened. Since the upper caisson weighs more than the lower caisson (1,844 vs 1,700 tons), it pushes down on its ram, forcing out water from its shaft via the connecting pipe into the shaft of the bottom caisson. The force pushes up on the bottom caisson's ram, raising the caisson up to the top position. When the gate of the newly descended top caisson and lower reach gates open at the bottom, the extra foot of water flows out and equalizes with the water level in the lower reach of the canal, and any descended vessels exit, allowing the cycle to start over again. History The lift lock was designed by Richard Birdsall Rogers, a superintendent of the Trent Canal (part of the Trent-Severn Waterway). In 1896, he travelled to France, Belgium and England to see existing examples in operation. Part of the reason that the lift lock was built was political. At the time a federal election was taking place and in order to shore up local support the project was fast tracked. In 1896 construction was approved and contractors signed on prior to any real working drawings being ready. The government still fell, and Richard Rogers, who was concerned about his links to the former administration, only released portions of the working drawings bit by bit. It worked, allowing him to remain on the job as the main designer. The final project included many engineering firsts. It was the first lock to be built out of concrete, and at the time was the largest structure ever built in the world with unreinforced concrete. Construction was by Corry and Laverdure of Peterborough, which excavated the site and built the concrete towers and lock, and Dominion Bridge Company of Montreal, which completed the metal work including rams, presses and large caissons, and was finished in 1904. The lift lock officially opened to the public to a crowd of thousands on 9 July 1904, and remains in full use today. See also List of boat lifts