If you need a canal to cross another canal, you simply build a water bridge across, with one canal on top of another. If you need to build a land bridge across a waterway, you can make the bridge split in two or swing aside when tall ships need to pass. But what do you do when you need to have a canal make room for large ships on another canal? That was the situation in Manchester, UK, in 1885.
It was replaced by a unique swing aqueduct that was opened in 1893 and was an even more daring structure than the original aqueduct, consisting of a channel that could be sealed off at each end to form a 235 feet long and 18 feet wide tank, holding 800 tons of water, that swung round on its pivot, situated on an island in the middle of the Ship Canal.
In the links, you can find a video of the swinging aqueduct in action.
Link and Video via The Adventures of Roberta X | Photo: Happy Pontist
UPDATE 11/30/2010: Martin Clark writes with this clarification:
It was not the Manchester Ship Canal that the Bridgewater Canal crossed on an arched bridge - the arched aqueduct actually crossed the Mersey and Irwell Navigation, which was there before the ship canal was built.
The swing aqueduct was built at the same time as the ship canal. The stone arches had been high enough to allow the river barges to pass below, but the sea-going vessels that would use the ship canal were too big, so the swing aqueduct was designed - the first and only one of its kind!