scale on the glass tube, indicating relative atmospheric
pressures, and the Torricellian barometer was complete.

Such a revolutionary 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