Constructed in 1867, the caisson is one of the oldest surviving vessels built by Harland and Wolff.
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 hollow inside.
When filled with water it sank down into grooves at the entrance to the dock, so sealing it. When the dock was closed off, it was possible to cross the dock by walking along the wooden deck on top of the caisson. When it was pumped empty by a stream pump, it became buoyant and floated up to allow the ship to be drawn in and tugged out of the dock. Constructed in 1867, it is one of the oldest surviving vessels made by Harland and Wolff. It was launched by the firm in the same way as a ship and was designated 'Hull No. 50' on their shipping list.
Above, left The plan shows the caisson's interior lattice-like supporting framework. The handrail that ran along the top of the caisson is also detailed.
Left The plan shows how the gate fitted into the dock in cross-section
Far left This plaque is located on the opposite side of the caisson, and is visible from the other side of the dock
AboveThe caisson in place at Alexandra Dock was of a similar 'ship-like' design
[Balloon caption on this photo reads] The caisson
has a very similar structure to a ship's hull, and was assembled in the same way. It was even given a hull number in sequence with all the other vessels Harland and Wolff built.