Model sea gauge, maker unknown, before 1754. Once belonged to Stephen Demainbray.

This is a device for measuring the depth of the sea. It once belonged to Stephen Demainbray and was used by him in his lecfures on natural philosophy. The inner vessel 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 pressure increases with depth, so the depth could be calculated from the height of the treacle reached in the inner tube. Demainbray worked as superintendent at the King's observatory at Kew from 1768 and his collection of instruments and apparatus was absorbed into the King's own collection.