Predicting Weather By The Moon - Xavier University Libraries

The Barometer
In 1638 Galileo noticed that a suction pump used to
pull water from a well could not raise water more than 34
feet. His pupil, Torricelli, realised that this was due to the
weight of the atmosphere, and confirmed his theory with
an experiment. Knowing that mercury is 14 times heavier
than water, he realised that if 34 feet of water was held up
by the atmosphere in a column(such as a well, or a tube)
then an amount of mercury one fourteenth that of water
could also be held up. So he took a tube three feet long
and filled it with mercury, sealed the end momentarily with
his finger, inverted the tube and immersed the end in a bowl
of mercury. The health hazards of this were not of course
known. The level of mercury in the tube was about 30
inches. The empty space at the top was a vacuum. Watching
it and recording over several days, Torricelli discovered
that there were variations in the height. Without realising
the full implications, he had demonstrated the existence
of air pressure and invented the barometer and altimeter.
The French scientist Blaise Pascal repeated
Torricelli’s experiment, and realised that any air closer to
earth would be compressed by the weight of air immedi-
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