This is a device for measuring the depth of the sea. The inner vesel contained mercury with honey or treacle above it. A weight was attached to make the gauge sink and it was dropped overboard. When it hit the sea bed the catch released the weight so that the gauge floated back up again again. Since presure increases with depth, so the depth could be calculated from the height of the treacle reached in the inner tube. This apparatus is asociated with Dr Stephen Demainbray (1710-82), a science lecturer of the 18th century.
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