The Leech Machine

The parasitic creatures that could have been the key to Victorian-era meteorology, and the guy who had that weird idea.

By Nathan Lawrence

Today in Tedium: If you’re like me, one of the few remaining artifacts of the pre-Internet age that you’re able to regularly revel in is the mail order catalog. I particularly love the desk toys highlighted they show off—often, some of the most luxurious are vintage weather prediction devices. Today’s tedium is about the Victorian “Tempest Prognosticator,” a vintage weather forecast device you’re not likely to see as a desk toy any time soon—because maintaining one also means taking care of a dozen leeches. — Nathan @ Tedium

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The year Galileo and fellow polymath Evangelista Torricelli began work on the first known barometer. Ever since, measures of air pressure have been used in weather prediction. When pressure is decreasing, stormy conditions tend to be on the way.

Weather Sky

(Victor Rodriguez/Unsplash)

The physician who thought leeches could be the perfect tool for predicting the weather

George Merryweather, a member of Whitby, North Yorkshire’s then-thriving intellectual scene, masterminded the “Tempest Prognosticator” as a years-long hobby that culminated in its public display in 1851.

As a physician, Merryweather would already have been quite familiar with leeches, but in his essays, Merryweather said he was inspired by a poem, which spoke of how the common “medicinal leech” tends to move up in a jar of rainwater as a storm nears, then settle to the bottom in clear conditions.

To harness this instinct, Merryweather placed 12 leeches in their own jars of rainwater, arranged in a circle to keep each other company. Atop each jar, he rigged a piece of whale bone to a chain that, when yanked, would hit a bell he had placed in the center. As leeches rose to the top of their jars in advance of a storm, they would come into contact with the bone and sound the bell. The more bells that sounded, the more likely there was to be a storm, and the more intense it was likely to be:

As it would have been preposterous to have a bell for each leech, I made use of a simple contrivance, by placing a bell upon a pedestal, erected on the centre of a circular platform; which bell was surrounded by twelve hammers. From each of these hammers was suspended a gilt chain; each of which played upon a pulley, which was placed in a disk, that was a little elevated above the circle of bottles. By this method, every leech could have communication with the bell.

At the time, consensus among the leech-invested appears to have emerged that these behaviors were due to the creatures’ innate ability to sense electromagnetic energy gathering in advance of a storm. Merryweather himself was a major proponent of this belief, dedicating a significant portion of his essays to reiterating Michael Faraday’s contemporary work on electromagnetism.

Unfortunately for him, we now know this acknowledgement was likely both unnecessary and uncalled-for. Leeches’ faculties for weather prediction turn out to actually be pretty patchy, and their “instincts” for this are far simpler than it seemed to him at the time. Leeches “breathe” through their body walls by absorbing the dissolved oxygen in the water they inhabit. When atmospheric pressure drops, a fractional amount less oxygen remains dissolved, and they move toward the surface, where the water is more oxygen-rich.

In effect, the “Tempest Prognosticator” was one of the world’s most elaborate barometers.

“What is the reason of the extraordinary multiplication of the human species during the present century? What is the cause of the superabundant population of old countries rushing to the colonies and to previously unpopulated countries? What is it that has saved more lives, during the time of its operation, than have been destroyed by the sword, by famine, and by revolution...? [These questions] are all answered by one single word—‘VACCINATION.’”

George Merryweather, in his essays about the Tempest Prognisticator. Within his lifetime, the smallpox vaccine would transform medical science - and save millions of lives in the process.

Leech In Action

(David Bruce/Flickr)

What did Merryweather see in leeches, anyway?

Merryweather’s inspiration resonates a surprising amount with our present-day challenges. Thanks to his work in medicine, he knew firsthand just how damaging contagious diseases had been to previous generations. For hundreds of years, smallpox in its various forms had periodically ravaged communities or even countries with sudden resurgences.

Highly unpleasant, and perhaps well outside the scope of what you might consider tedious at all, smallpox was brutally efficient in its destructive effects, killing about 30 percent of those it infected, and leaving many who remained alive with permanent complications, from skin lesions across the body to blindness.

As far as about 350 years before Merryweather’s birth, records of medical practice in China suggest that physicians were practicing rudimentary inoculation techniques, but Europe’s saving grace came in Merryweather’s own lifetime, when Edward Jenner proposed and proliferated the first iteration of what closely resembles the modern smallpox vaccine.

By his death in the 1820s, Jenner had become one of the most important figures of his age. His work became so central to the conception of public health that two short years after Merryweather presented the Tempest Prognosticator in public, the UK would institute a smallpox vaccine mandate—leading to the era’s very own anti-vaccine movement.

Jenner was a personal hero to many physicians of the era, and Merryweather was no exception—the leech poem he was inspired by came from Jenner’s personal notes, and in his essays, Merryweather makes clear that he sees the Tempest Prognosticator as a tribute to Jenner’s own work on vaccines, in addition to his own on the meteorological powers of the leech.

Tempest Prognosticator

A replica of the tempest prognosticator in the Whitby Museum. (Badobadop/Wikimedia Commons)

Tempest Prognosticators: The wrong solution to the right problem

Though colorful accounts of Merryweather’s ambitions are occasionally spread as trivia in academic or “fun fact” circles, very few amount to more than dubious synthesis from modern sources.

For example, while Merryweather may have documented entreaties to the British government suggesting his work be officially employed at sea, very little historic evidence seems to exist that would suggest he was ever taken seriously.

Even if he had managed to attract the attention of decision-makers, his work would have very quickly faced stiff competition. The 1840s and 1850s marked a period of explosive growth for all sorts of barometers, as industrial manufacture made the devices possible to create at a previously impossible scale—and none of these rivals forced the owner to care for new annelid pets.

In addition, Admiral Robert FitzRoy, who had risen to public prominence for his role as captain of the HMS Beagle during the voyage that inspired Charles Darwin’s work on evolution, had his own pet devices—among them the “storm glass,” a glass phial filled with chemicals that arrange themselves into crystal configurations, seemingly at random. Like Merryweather, FitzRoy thought his storm glass was drawing on electromagnetic signals to manifest weather predictions, but within years of his endorsement, naysayers emerged to observe that it mostly just seemed to respond to changes in temperature.

Merryweather had not anticipated these developments. In the hopes of hitting it big, he designed several different variations on his apparatus, ranging from small devices that would handle only a few leeches to his own personal model, the one that would eventually be publicly showcased at Prince Albert’s Great Exhibition in 1851—it was a design so grandiose and ornate he claimed it was modeled after the architectural feats of 17th-century India.

Merryweather seemed to envision some level of commercial production for his devices, too—he told press during the exhibition that he intended to confine this work to his hometown of Whitby, intending to build a new cottage industry of “Prognosticator” artisans.

Perhaps the most ambitious element of Merryweather’s plans was the ability to distribute signals from a centralized machine. In his essays, he points out that a bell could easily be swapped for electrical contacts, and the signal broadcast over wire—“I could cause a little leech, governed by its instinct, to ring Saint Paul’s great bell in London as a signal for an approaching storm,” he asserted.

If this were ever implemented, the Tempest Prognosticator may have become one of the earliest examples of a networked sensor array. Alas, like many of Merryweather’s hopes for his apparatus, no variation on this appears to have ever come to pass.

There are no known additional machines that Merryweather himself produced or oversaw production of, and all that remain today are replicas of his original machine from the exhibition: There’s one at the Whitby Museum, which Merryweather himself once served as curator of, and one at Barometer World—which is exactly what it sounds like—in Devon.


Thanks again to Nathan for uncovering this bizarre quirk in the way we once tried to track the weather. Find this one an interesting read? Share it with a pal!

Nathan Lawrence

Your time was just wasted by Nathan Lawrence

Nathan Lawrence is a former public radio journalist and journalism teacher who somehow managed to turn his love of computers into a steady income. In his spare time, he enjoys woodworking, reading old newspapers, and wishing we still used Geocities.

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