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Tempest Prognosticator: Predicting Storms With Leeches

Some animals have the instinctive ability to predict changes in the weather. Frogs croak when a storm is approaching, birds return to their nest, cows, sheep and ants become restless.

George Merryweather, a 19th century English doctor and inventor, observed that medicinal leeches he used to work with behaved differently when the weather got worse. Housed in small glass jars of water, the leech would lie relaxed at the bottom when the weather was fine, but several hours before the skies became cloudy and the wind started blowing, the sucker would show signs of agitation. If rain was coming, it would move out of the water, and if a storm was imminent, a leech would curl itself into a ball and remain thus for its duration. Once the weather had settled down, the leech would return to the bottom of the bottle.

Tempest Prognosticator

The Tempest Prognosticator.

Dr. Merryweather decided to harness this physical energy of the leeches by constructing a storm forecasting apparatus he described as “An Atmospheric Electromagnetic Telegraph, conducted by Animal Instinct”, but preferred to call it by its shorter name—Tempest Prognosticator.

The “Tempest Prognosticator" consisted of twelve pint glass bottles each containing a live leech in about an inch and a half of water. The top of the glass bottle had a piece of whalebone loosely set in the neck of the bottle and connected by wires to a small hammer positioned to strike a large metal bell. The twelve bottles were arranged in a circular fashion around the metal bell.

When a storm was approaching, the changes in atmospheric pressure drove the leeches out of the water and into the neck of the bottles where they dislodged the whalebone and rang the bell at the top of the device. When several bells rang in succession a storm was “prognosticated”.

In an essay describing the apparatus, Merryweather noted that the leeches, which he referred to as the “jury of philosophical councilors”, were placed in glass bottles in a circle to prevent them from feeling “the affliction of solitary confinement.”

Merryweather spent over a year in 1850 testing the device, and sent a letter to the president of the Philosophical Society and the Whitby Institute, each time his jury of leeches predicted a storm. Merryweather later lobbied for the government to make use of his design around the British coastline, but the government decided to use Robert FitzRoy's storm glass instead.

Merryweather’s Tempest Prognosticator failed to catch on as he had hoped. Even the original instrument exhibited at the Great Exhibition of 1851 was lost. A replica of the weather forecasting machine can now be seen at the Whitby Town Museum.

Tempest Prognosticator

A replica Tempest Prognosticator at the Whitby Museum, Whitby, UK. Photo credit: Badobadop/Wikimedia

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