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Water Thief
Mon Sep 15, 2003
Listen in RealAudio 
It took thousands of years of recorded human history before anyone figured out that air
actually has weight. The man who did was probably also responsible for the first recorded
scientific experiment. I'm Bryan Yeaton for the Weather Notebook.
The year is 450. BCE. And the man in question is Empedocles of Agrigentum [or Acragas,
depending on your source] (500-435).
Empedocles used a water clock, or clepsydras - a Greek term that means "water thief."
Clepsydras have been around since the time of the Pharaohs - one was found buried with the
other treasures in the tomb of Amenhotep I, interred around 1500 BCE. Empedocles' version of
the water thief consisted of a brass sphere, with drain holes in the bottom. An open tube
protruded from the top of the sphere.
Normally, the clock was filled by immersing the sphere in water with the tube left unplugged.
If you put your thumb over the end, locking out air, the sphere could be drawn up filled with
water. If you released your thumb, the water streamed out in a relatively predictable period -
kind of like an hourglass.
However, Empedocles found that if your thumb were over the tube when you sunk the globe, water
couldn't get into the sphere - but there was nothing there to block the water. Except,
reasoned Empedocles, the air. Air had stolen water from the "water thief."
Empedocles theorized about deeper matters including the universe itself which he postulated
was made up of earth, air, fire, and water. It was a new idea for the time and although it's
been long been supplanted, Empedocles experiment with the water thief laid the groundwork for
a later invention - the barometer.
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