Modern weather forecasting relies on techniques such as systematic temperature and air pressure measurement, satellite photography and computer simulation. However in the past, a variety of folk means were used, with objects such as pine cones and seaweed being known for their predictive powers. But strangest of all natural forecasters is perhaps the humble, blood-sucking leech, and stranger too was the way a Victorian inventor sought to mechanize the leeches' forecast.

An astonishing example of the ingenuity of Victorian England, the Tempest Prognosticator was designed by Dr George Merryweather, and used the intuition of leeches to ring a bell to predict a coming storm. Described by its creator as the "Atmospheric Electromagnetic Telegraph, conducted by Animal Instinct", it was unveiled at the pinnacle of Queen Victoria's reign, the Great Exhibition of 1851.

The device was described thus by T.W. Hendrick:

This elaborate and highly ornate apparatus was evolved by a certain Dr. Merryweather (no epigram intended) who had observed that during the period before the onset of a severe storm, fresh water leaches [sic] tended to become particularly agitated. The learned Doctor decided to harness the physical energy of these surprisingly hysterical aquatic bloodsuckers to operate an early warning system. On the circular base of his apparatus he installed glass jars, in each of which a leech was imprisoned and attached to a fine chain that led up to a miniature belfry -- from whence the tinkling tocsin would be sounded on the approach of a tempest. (Quoted in Packer.)

The weather-predicting ability of the leech has been known for a long time. Leeches have a natural desire to rise to the top of a jar or tank before a thunderstorm or heavy rain to avoid drowning. Merryweather made use of this climbing propensity, by creating a machine that would detect the rising of the leeches, and convert it into an audible warning.

The device contained twelve glass pint bottles, each holding a leech. An inch and a half of rain water was poured into each bottle; one assumes this was to give the leech some incentive to believe the bottle was in danger of flooding next time it rained. When the leech sensed electrical disturbances in the atmosphere, it would rise. When it reached the top of the device, its movement would touch a whalebone button and trigger a mechanism like a mouse trap, which would cause a hammer to ring a bell. The mechanism would then have to be reset in time for the next storm. Dr Merryweather used twelve leeches to form what he called a "jury". The more leeches that reached the top, and the more bells that rung, the more likely a storm was.

Dr George Merryweather, M.D. was honorary curator of the Whitby Literary and Philosophical Society’s Museum in North Yorkshire, England. He was known for his inventing genius; earlier he had produced a "platina lamp", which burned a mixture of pure alcohol and whisky, and could run for a fortnight for the economical sum of one penny per eight hours. Merryweather developed the product for which he would be best known during 1850, based on long observations of the leech, and outlined his work in a paper delivered to the Whitby Philosophical Society, which he read on February 27, 1851.

The machine's gold-plated design was based on that of an Indian temple; a similar influence was found in the architecture of the enormous glass Crystal Palace built to house the Great Exhibition. In fact, Dr Merryweather came up with a number of different versions: the grand one he exhibited was suitable for the drawing room, but he also planned simpler ones for coastguard stations. It was built by local craftsmen; however it is not known what happened to it after the exhibition, or if any others were ever put into service. Its creator sought to persuade the British government to install a network around the coast of Britain, but they opted instead for Admiral Fitzroy's Storm Barometers.

A working replica has been constructed thanks to the work of Philip Collins. It is on display at the Barometer World museum in Merton, London. Collins commented: "Leeches have an uncanny ability to sometimes predict the weather; they are unlikely to replace barometers despite their lower cost and small size, due perhaps to their unattractive appearance and a rather disturbing method of maintenance, (although they do not hurt much). Barometers only cost money, not Blood!" (Quoted in Barometer World and Museum) A non-working model was made in 1951 for the Festival of Britain and can be found in Whitby Museum, North Yorkshire, where Merryweather was once curator.

So next time you fall asleep in front of the Weather Channel, or scoff at the predictions of some young and clueless forecaster, remember Dr Merryweather, and think how different it all could be. Either that, or imagine the arrogant TV meteorologists being consumed by hundreds of blood-sucking leeches.