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The caisson is a hollow gate which closed off the entrance to the dock. Shaped like a ship’s hull, it is made of wrought iron and is hollow inside. When filled with water, it sank down into the grooves at the entrance to the dock, so sealing it. When the deck was closed off, it was possible to walk cross the dock by walking along the wooden deck on top of the caisson. When it was pumped empty (by a steam pump) it became buoyant and floated up to allow the ship to be drawn in or tugged out of the dock.

Constructed in 1867, this caisson is one of the oldest surviving vessels made by Harland and Wollf. It was assembled and launched by the firm in the same way that a ship was. It was even given a hull number (50) in sequence with all the other vessels built by Harland and Wollf. It is on display at Hamilton Dock, alongside J3575 : SS Nomadic, Belfast.