The "Tempest Prognosticator" consisted of twelve pint bottles of white glass, round the base of a circular stand, at the top of which was a bell surrounded by twelve hammers. Each bottle was connected with one of the hammers through a metal tube in its neck, containing a piece of whalebone and a wire, to which was attached a small gilt chain. Here is the inventor’s description of how the Prognosticator works: "After having arranged this mouse trap contrivance, into each bottle was poured rain water, to the height of an inch and a half; and a leech placed in every bottle, which was to be its future residence; and when influenced by the electromagnetic state of the atmosphere a number of leeches ascended into the tubes; in doing which they dislodged the whalebone and caused the bell to ring."
Paul Collins also reports that some guy has built a working replica of the Prognsticator, and has it on display at the Barometer World Museum in Devon, England. No word on whether it actually worked.