Tempest Prognosticator
How leeches were used to forecast 19th-century storms
One thing that the history of meteorology has in common with the history of medicine is a baffling enthusiasm for leeches.
In medicine, leeches have been used for millennia for bloodletting — based on the ancient “humoral” theory that argued the human health is reliant on good balance of four humors — blood, phlegm, black bile and yellow bile. Leeches were used to rebalance those fluids within the body, returning the patient to health.
But leeches have also played a role in the history of weather forecasting. They were a crucial component of the tempest prognosticator, a bizarre device that predicted storms by getting the leeches to ring a bell.
It was the creation of George Merryweather, a surgeon from Yorkshire. By the 19th century it was known that air pressure was important to weather forecasting, and barometers had been in use for almost two centuries to identify approaching storms. But Merryweather had other ideas.
He noticed that the medicinal leeches he used in his daily work were sensitive to electrical conditions in the atmosphere — they became agitated when a storm approached. He bottled twelve, referring to them as his “jury of philosophical councillors”, and positioned them in a circle so they wouldn’t feel “the affliction of solitary confinement”.
The whole device was inspired by two lines from Edward Jenner’s poem Signs of Rain: “The leech disturbed is newly risen; Quite to the summit of his prison.” The idea was that the leeches wouldn’t enter the tubes unless sufficiently agitated by a storm on its way.
Each bottle was topped with a tube containing a piece of whalebone, connected to a wire that was rigged up to a tiny hammer. The more that rang their bell, the more likely it was that a storm approached. Merryweather explained:
“After having arranged this mouse trap contrivance, into each bottle was poured rain water, to the height of an inch and a half; and a leech placed in every bottle, which was to be its future residence; and when influenced by the electromagnetic state of the atmosphere a number of leeches ascended into the tubes; in doing which they dislodged the whalebone and caused the bell to ring.”
Six designs for the device were created, from a simple, cheap version that Merryweather hoped would be used on ships, to an extravagant version that drew inspiration from the architecture of Indian temples. This latter one was actually built and shown off in the Great Exhibition of 1851, in the sky gallery of the “Dome of Discovery”.
He subsequently lobbied for the British government to use his design in ports around the coastline but it failed to react with much interest, opting instead for Robert FitRoy’s equally-ineffective storm glass.
Merryweather was undeterred, making 28 predictions with his “council” that are now in the library of Whitby Museum, including one where he sent a letter to the president of the museum and the Philosophical Society, Henry Belcher, to warn him of a storm approaching. The letter was sent first thing in the morning, and received around lunchtime, but it’s not recorded if the promised storm materialised.
The model shown at the Great Exhibition has since been lost to time but two replicas were created. The first was made in 1951 for the Festival of Britain, which is now in Whitby Museum.
A second was created at Barometer World, near Okehampton in southwest Britain from Merryweather’s original designs. If you’d like to see a small slice of bizarre meteorological history, then stop by next time you’re passing through Devon.
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