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The tempest prognosticator comprises twelve pint bottles in a circle around and beneath a large bell. Atop the glasses are small metal tubes which contain a piece of whalebone and a wire connecting them to small hammers positioned to strike the bell. In his essay Merryweather described the workings of the device:
“ 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. ”
— George Merryweather
The leech would have difficulty entering the metal tubes but would endeavour to do so if sufficiently motivated by the likelihood of bad weather. By ringing the bell it would signify that that individual leech is indicating that a storm is approaching. Merryweather referred to the leeches as his “jury of philosophical councilors” and that the more of them that rang the bell the more likely that a storm would occur.
In his essay Merryweather also noted other features of the design, including the fact that the leeches were placed in glass bottles placed in a circle to prevent them from feeling “the affliction of solitary confinement”