In 1851 Dr. George Merryweather displayed his “Tempest Prognosticator” at London’s Great Exhibition. Remarkably, this “leech barometer” used the infamous blood-sucking worms to predict changes in air pressure and, thus, forecast inclement weather. Leeches require moisture to stay alive, and so they usually remain underwater unless it is rainy and wet outside their river and pond homes. It is believed that leeches possess a kind of natural barometer that tells them when a low pressure system is bringing wet weather and they can venture outside to seek victims to feed upon. Merryweather’s contraption worked by placing one leech in each of a series of twelve bottles filled with about an inch and a half of water. The bottles were arranged in a circle, and at the top of each bottle was a wire connected to a bell. When the air pressure was high, the leeches would remain in the water. When the air pressure began to fall, however, the leeches would become more active and climb up the bottles, eventually ringing the bells.