height of a column of water--that is, about thirty inches.
Reasoning in this way, Torricelli proceeded to prove that his
theory was correct. Filling a long tube, closed at one end, with
mercury, he inverted the tube with its open orifice in a vessel
of mercury. The column of mercury fell at once, but at a height
of about thirty inches it stopped and remained stationary, the
pressure of the air on the mercury in the vessel maintaining it
at that height. This discovery was a shattering blow to the old
theory that had dominated that field of physics for so many
centuries. It was completely revolutionary to prove that, instead
of a mysterious something within the tube being responsible for
the suspension of liquids at certain heights, it was simply the
ordinary atmospheric pressure mysterious enough, it is
true--pushing upon them from without. The pressure exerted by the
atmosphere was but little understood at that time, but
Torricelli's discovery aided materially in solving the mystery.
The whole class of similar phenomena of air pressure, which had
been held in the trammel of long-established but false doctrines,
was now reduced to one simple law, and the door to a solution of
a host of unsolved problems thrown open.

It had long been suspected and believed that the density of the
atmosphere varies at certain times. That the air is sometimes
"heavy" and at other times "light" is apparent to the senses
without scientific apparatus for demonstration. It is evident,
then, that Torricelli's column of mercury should rise and fall
just in proportion to the lightness or heaviness of the air. A
short series of observations proved that it did so, and with
those observations went naturally the observations as to changes
in the weather. It was only necessary, therefore, to scratch a