; but beyond this was discovered a stratum of invariable
temperature, beneath which, if we descend, the heat increases at the
rate of 1 deg. Fahr. for every fifty or seventy feet. The uniformity of
this rate seemed to imply that, at depths quite insignificant, a very
high temperature must exist. This was illustrated by such facts that the
water which rushes up from a depth of 1794 feet in the Artesian well of
Grenelle has a temperature of 82 deg. Fahr. The mean temperature of
Paris being about 51 deg. Fahr., these numbers give a rate of 1 deg. for
every fifty-eight feet. If, then, the increase of heat is only 100 deg.
per mile, at a depth of less than ten miles every thing must be red hot,
and at thirty or forty in a melted state. It was by all admitted that
the rise of temperature with the depth is not at all local, but occurs
in whatever part of the earth the observation may be made. The general
conclusion thus furnished was re-enforced by the evidence of volcanoes,
which could no longer be regarded as merely local, depending on
restricted areas for the supply of melted material, since they are found
all over the land and under the sea, in the interior of continents and
near the shores, beneath the equator and in the polar regions. It had
been estimated that there are probably two thousand aerial or subaqueous
eruptions every century. Some volcanoes, as Aetna, have for thousands of
years poured forth their lavas, and still there is an unexhausted
supply. Everywhere a common source is indicated by the rudely uniform
materials ejected. The fact that the lines of volcanic activity shift
pointed to a deep source; the periodic increments and decrements of
force bore the same interpretation. They far transcend the range of
history. The volcanoes of central France date from the Eocene period;
their power increased in the Miocene, and continued through the
Pliocene; those of Catalonia belong to the Pliocene, probably. Coupled
with volcanoes, earthquakes, with their vertical, horizontal, and rotary
vibrations, having a linear velocity of from twenty to thirty miles per
minute, indicated a profound focus of action. The great earthquake of
Lisbon was felt from Norway to Morocco, from Algiers to the West Indies,
from Thuringia to the Canadian lakes. It absolutely lifted the whole bed
of the North Atlantic Ocean. Its origin was in no superficial point.

[Sidenote: Proof from the mean density.] A still more universal proof of
a high