The Patent Tempest Prognosticator [demonstrated by Dr George Merryweather in 1851] was based on the well known response of leeches to sudden changes in barometric pressure. Their soft, gelatinous bodies were squeezed and made drowsy and inactive by normal air pressure, but low pressure refreshed and awoke them...From Falling Upwards by Richard Holmes
The Prognosticator...was an ingenious form of multiple leech barometer. It consisted of a circular display of twelve glass flasks, each containing a leech partially immersed in rainwater. The flasks were enclosed at the top with a system of whalebone springs, and these in turn were linked to a set of counterweights that connected to metal hammers arranged to strike against an impressive brass bell at the centre of the apparatus.