Mercury Level?, English, Eighteenth Century?

Mahogany box with a hinged lid and 2 copper alloy clasps. It contains a thick white marble disc with hollowed out surface connected to a hole in the top edge, with a glass cover plate, and a boxwood bottle with copper alloy stopper, containing mercury. The mercury can be poured into the hollowed out portion via the hole to form a flat pool of mercury with a bubble, which can be used as a levelling instrument. There is also an iron alloy key.