...the thing which Mr. Merryweather became truly famous for was his "Atmospheric, Electromagnetic Telegraph, conducted by Animal Instinct," or, more shortly, his Tempest Prognosticator," which he built for the Great Exhibition of 1851. It is a beautiful structure, with a bell at the top designed to look like the dome at St. Pauls. Around the bottom are placed a dozen glass bottles; threading from tiny hammers around the edge of the bell are threads, which connect to a piece of whalebone just inside the neck of each bottle. Inside each bottle is poured an inch of rainwater and then -- oh happy home! -- each bottle is occupied by a leech. A common, ordinary surgical leech.
Being a doctor, Merryweather had observed that medical leeches responded to barometric pressure or electrical charge in the air, or whatever it is that allows smaller animals to know when bad weather is afoot. The leeches' response was to climb -- probably a good response for water-dwelling creatures just before a rain, so that they don't get washed away. So when Merryweather's leeches climbed to the top of the bottle, they nudged the piece of whalebone, which caused the string to move and ring the bell. It's not clear, but it appears that the more the bell rang before a storm, the worse the weather to come.
The Tempest Prognosticator proved to be surprisingly accurate, but did not catch on because it was not considered scientific enough. Read more about the device at Cabinet of Wonders, on a visit to the Whitby Museum, where Merryweather's device is housed. Link