while attempting to raise water to a very great elevation,
found that the column ceased at a height of thirty-two feet. Beyond
this all the skill of the pump-maker could not get it to rise. The
fact was brought to the notice of Galileo, and he, soured by a world
which had not treated his science over kindly, is said to have twitted
the philosophy of the time by remarking that nature evidently abhorred
a vacuum only to a height of thirty-two feet. Galileo, however, did
not solve the problem. It was taken up by his pupil Torricelli, to
whom, after due pondering, the thought occurred, that the water might
be forced into the tube by a pressure applied to the surface of the
liquid outside. But where, under the actual circumstances, was such a
pressure to be found? After much reflection, it flashed upon
Torricelli that the atmosphere might possibly exert this pressure;
that the impalpable air might possess weight, and that a column of
water thirty-two feet high might be of the exact weight necessary to
hold the pressure of the atmosphere in equilibrium.

There is much in this process of pondering and its results which it is
impossible to analyse. It is by a kind of inspiration that we rise
from the wise and sedulous contemplation of facts to the principles on
which they depend. The mind is, as it were, a photographic plate,
which is gradually cleansed by the effort to think rightly, and which,
when so cleansed, and not before, receives impressions from the light
of truth. This passage from 'facts to principles is called induction;
and induction, in its highest form, is, as I have just stated, a kind
of inspiration. But, to make it sure, the inward sight must be shown
to be in accordance with outward fact. To prove or disprove the
induction, we must resort to deduction and experiment.

Torricelli reasoned thus: If a column of water thirty-two feet high
holds the pressure of the atmosphere in equilibrium, a shorter column
of a heavier liquid ought to do the same. Now, mercury is thirteen
times heavier than water; hence, if my induction be correct, the
atmosphere ought to be able to sustain only thirty inches of mercury.
Here, then, is a deduction which can be immediately submitted to
experiment. Torricelli took a glass tube a yard or so in length,
closed at one end and open at the other, and filling it with mercury,
he stopped the open end with his thumb, and inverted it into a basin
filled with the liquid metal.