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The Caisson was a device for taking boats from the upper to the lower canal and vice versa.
It comprised an immense stone-lined hole in the ground with the upper canal on one side and the lower canal on the other. It was filled with water to a level higher than the upper canal
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A watertight door prevented the chamber from emptying into the lower canal through the exit tunnel. A similar door closed-off the top canal level.

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Floating beneath the surface in the chamber was a large watertight box, a kind of wooden 'submarine'. It was connected to rods which could be used to control its movement, allowing it to be wound up and down.

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The control rods did not need to provide much force because the box was part filled with air and part filled with water and then ballasted with heavy material so that it floated 'neutrally buoyant'. It was easy to make it rise or sink.