At what period the lee-board was transferred from the lee side to the center, being lowered in a fore-and-aft well amidships, is not known to the writer.
It was introduced into England by Admiral Schank, probably from Holland, and was first applied to a government exploring-vessel destined for New Holland.
The wide and strong board which formed the slidingkeel was connected to three vertical planks, which slipped up and down in wells, and were raised and lowered by racks and pinions.
Willoughby made a massive iron keel suspended by iron bars, which slipped in vertical wells amidships.
The bars were worked by racks and pinions, as before.
The heavy keel acted as ballast.
Shuldham's metallic sliding-keel (English) is of a somewhat triangular shape, and is pivoted at its apex forward.
When lowered, the after end is deepest in the water, and resembles the ventral fin of a fish.
It moves in a longitudinal well amidships, passing through the true keel.