The tempest prognosticator, also known as the leech barometer, is a 19th-century invention by George Merryweather, in which leeches are used in a barometer. The twelve leeches are kept in small bottles inside the device; when they become agitated by an approaching storm they attempt to climb out of the bottles and trigger a small hammer which strikes a bell. The likelihood of a storm is indicated by the number of times the bell is struck.

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When you prefer the agony of leeches writhing in glass bottles to a wind sock.

The invention was inspired by a couple lines of poetry of Edward Jenner found in Signs of Rain: “The leech disturbed is newly risen; Quite to the summit of his prison.” It was shown off at the Longon Great Exhibition of 1851, but never gained popularity, despite its supposed accuracy.

Dr. George Merryweather tried to show it off at the Great Exhibition of 1851