Scientists Used Leaches to Predict the Weather

One late-1800s scientist used leeches in glass jars to predict the day's weather.
Trevor English

What would you do without your smartphone or the internet? You couldn't get 15 pounds of pizza delivered to your house at 2 AM, you couldn't request 20 bucks from your friend that stole your t-shirt, and you couldn't see what the weather was going to be like tomorrow. Back in the 1800s, people were struggling with all these issues – or maybe just the weather thing (Okay, definitely just the weather thing).

Nonetheless, predicting the weather was a useful aspect of life back in the day, and scientists were continuously working to devise better ways to do so.

Old ways to predict the weather

One option to predict the weather that scientists thought of at the time was animals. Frogs seemed to croak when storms were coming in, birds would fly back to their nest, and wild animals obviously had some innate way of sensing coming weather patterns.

Enter George Merryweather, a 19th-century English doctor who moonlit as an inventor. Being a doctor in the mid-1800s meant that he frequently worked with leeches for bloodletting. This also meant that he noticed their behavior changing when the weather changed. When the weather was normal, say a sunny day, the leeches would just sit at the bottom of their jars. However, hours before a storm would hit, the leeches would move out of the water and start getting agitated. They would even curl themselves into balls and stay like that while the storm was there. After the pressure front moved on, the leeches would return to relaxation.


It was this almost computer-like reflex of the leeches that gave Merryweather an idea. He built a device called an "Atmospheric Electromagnetic Telegraph, conducted by Animal Instinct.” But that was just the devices surname, off-hand he called it the tempest prognosticator. Those not adept at the meaning of those words – tempest, meaning windy storm, and prognosticator being someone who predicts the future.

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Scientists Used Leaches to Predict the Weather
Source: Wikimedia/Badobadop

Engineering the tempest prognosticator

This tempest prognosticator device was made up of 12 pint glass bottles, each with a live leech inside. The bottles were filled with an inch and a half of water, and the top of the bottle had a piece of whalebone in the neck. The whalebone was also connected to a small hammer that could strike a metal bell.

When storms would approach, the leeches would move out of the water and into the neck of the bottle. This action would be enough to dislodge the whalebone, and the bell would strike. When the bell would ring several times in a row, Merryweather would consider the storm prognosticated.

Merryweather being a somewhat quirky scientific mind, after all, made a weather-predicting bell with leeches and referred to his blood-sucking helpers as the "jury of philosophical counselors." He actually designed the machine as a circle with clear glass jars so the leeches wouldn't feel "the affliction of solitary confinement." A noble effort.


The English doctor spent a year perfecting the device and sent a letter to the Philosophical Society telling them about his weather-predicting leeches. He even lobbied the English government to get them to use his design on the coasts for sailing.

Unfortunately for Merryweather, the government went with the leech-less invention of a "storm glass," a liquid-filled jar that produced crystals when storms were near.

The Tempest Prognosticator never really caught on, even though it killed two birds with one stone – housing your blood-letting leeches for medical purposes when they weren't needed and predicting the weather. There's now a replica of the machine at the Whitby Town Museum in the United Kingdom.

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