Tempest Prognosticator

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In 1851 Dr George Merryweather designed a Tempest Prognosticator or ‘Leech Barometer’. This worked on a simple principle. Leeches are very sensitive to changes in pressure. As pressure falls they will look to gain height, moving away from potential rising water due to rainfall. Merryweather used this principle to design his barometer. When the pressure dropped the leeches would make their way up to the top of the bottles. Here they would come into contact with a fine metal chain attached to a bone lever and trigger the release of a hammer onto the bell at the top of the device. If enough leeches triggered their bells the owner could be fairly confident of poor weather. If they stayed at the bottom of their bottles the owner could assume good weather.

The only Prognosticator known to have been made was of the size and scale of our instrument and was displayed at the Great Exhibition of 1851, but Merryweather also proposed smaller versions for use at ports and harbours to act as forecasting devices for mariners. Perhaps unsurprisingly no further examples were made and the original is long lost to history. This exact replica based on Merryweather’s detailed descriptions, was built by Phillip Collins and was formerly on display at Barometer World in North Devon (now closed).

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Record copies