It is quite otherwise, however, with the solution of the great
problem of the internal heat of the whole Earth. As we may judge of
uniformity of temperature from the unaltered time of vibration of a
pendulum, so we may also learn, from the unaltered rotatory velocity of the
Earth, the amount of stability in the mean temperature of our globe. This
insight into the relations between the 'length of the day' and the 'heat of
the Earth' is the result of one of the most brilliant applications of the
knowledge we had long possessed of the planet. The rotatory velocity of the
Earth depends on its volume; and since, by the gradual cooling of the mass
by radiation, the axis of rotation would become shorter, the rotatory
velocity would necessarily increase, and the length of the day diminish,
with a decrease of the temperature. From the comparison of the secular
inequalities in the motions of the Moon with the eclipses observed in
ancient times, it follows that, since the time of Hipparchus, that is, for
full 2000 years, the length of the day has certainly not diminished by the
hundredth part of a second. The decrease of the mean heat of the globe
during a period of 2000 years has not, therefore, taking the extremest
limits, diminished as much as 1/306th of a degree of Fahrenheit.
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