of heat can not have been so much as the 1/300 of a Fahrenheit
degree, on the hypothesis that the mean dilatation of all terrestrial
substances is equal to that of glass, 1/180000 for one degree. If a
decline had taken place in the intrinsic heat of the earth, there must
have been a diminution in her size, and, as a necessary consequence, the
length of the day must have become less. The earth has therefore reached
a condition of equilibrium as respects temperature.

[Sidenote: Its ancient decline.] A vast body of evidence has, however,
come into prominence, establishing with equal certainty that there was
in ancient times a far higher temperature in the planet; not a
temperature concerned with a fraction of a degree, but ranging beyond
the limits of our thermometric scale. The mathematical figure of the
earth offers a resistless argument for its ancient liquefied
condition--that is, for its originally high temperature. But how is this
to be co-ordinated with the conclusion just mentioned? Simply by the
admission that there have elapsed prodigious, it might almost be said
limitless, periods. [Sidenote: Necessity for a long time.] As thus the
true state of affairs began to take on shape, it was perceived that the
age of the earth is not a question of authority, not a question of
tradition, but a mathematical problem sharply defined: to determine the
time of cooling of a globe of known diameter and of given conductibility
by radiation in a vacuum.

In such a state of things, what could be more unwise than to attempt to
force opinion by the exercise of authority? How unspeakably mischievous
had proved to be a like course as respects the globular form of the
earth, which did not long remain a mere mathematical abstraction, but
was abruptly brought to a practical issue by the voyage of Magellan's
ship. And on this question of the age of the earth it would have been
equally unwise to become entangled with or committed to the errors of
patristicism--errors arising from well-meant moral considerations, but
which can never exert any influence on the solution of a scientific
problem.

[Sidenote: Indications of the interior heat of the earth.] One fact
after another bearing upon the question gradually emerged into view. It
was shown that the diurnal variations of temperature--that is, those
connected with night and day--extend but a few inches beneath the
surface, the seasonal ones, connected with winter and summer, to many
feet