ry theory and such an important discovery were, of
course, not to be accepted without controversy, but the feeble arguments
of the opponents showed how untenable the old theory had become. In
1648 Pascal suggested that if the theory of the pressure of air upon the
mercury was correct, it could be demonstrated by ascending a mountain
with the mercury tube. As the air was known to get progressively lighter
from base to summit, the height of the column should be progressively
lessened as the ascent was made, and increase again on the descent
into the denser air. The experiment was made on the mountain called
the Puy-de-Dome, in Auvergne, and the column of mercury fell and rose
progressively through a space of about three inches as the ascent and
descent were made.

This experiment practically sealed the verdict on the new theory, but
it also suggested something more. If the mercury descended to a certain
mark on the scale on a mountain-top whose height was known, why was not
this a means of measuring the heights of all other elevations? And so
the beginning was made which, with certain modifications and corrections
in details, is now the basis of barometrical measurements of heights.

In hydraulics, also, Torricelli seems to have taken one of the first
steps. He did this by showing that the water which issues from a hole
in the side or bottom of a vessel does so at the same velocity as that
which a body would acquire by falling from the level of the surface of
the water to that of the orifice. This discovery was of the greatest
importance to a correct understanding of the science of the motions of
fluids. He also discovered the valuable mechanical principle that if any
number of bodies be connected so that by their motion there is neither
ascent nor descent of their centre of gravity, these bodies are in
equilibrium.

Besides making these discoveries, he greatly improved the microscope
and the telescope, and invented a simple microscope made of a globule of
glass. In 1644 he published a tract on the properties of the cycloid in
which he suggested a solution of the problem of its quadrature. As soon
as this pamphlet appeared its author was accused by Gilles Roberval
(1602-1675) of having appropriated a solution already offered by him.
This led to a long debate, during which Torricelli was seized with a
fever, from the effects of which he died, in Florence, October 25, 1647.
There is reason to believe, however, that while