h as a pump, to a height greater than thirty-three
feet, but he was never able to offer a satisfactory explanation of the
principle. Torricelli was able to demonstrate that the height at which
the water stood depended upon nothing but its weight as compared with
the weight of air. If this be true, it is evident that any fluid will
be supported at a definite height, according to its relative weight
as compared with air. Thus mercury, which is about thirteen times more
dense than water, should only rise to one-thirteenth the 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 scale on the glass tube, indicating relative
atmospheric pressures, and the Torricellian barometer was complete.

Such a revolutiona