At first it seems hardly possible that the earth could have been a star.
But, if we go down beneath the surface of the earth, we find that at a
depth of forty or fifty feet there is very slight variation in
temperature. When we go yet deeper, as in mines, we find that the earth
grows hotter as we descend. The temperature increases on an average
about one degree Fahrenheit for every sixty-four feet descent. But this
amount is variable according to the locality, geological formation, and
dip of strata. In the Calumet and Hecla Mine, observations show an
increase of one degree in about every one hundred and twenty-five feet.
At Paris, the water from a depth of 1794 feet has a temperature of
eighty-two degrees; at Salzwerth, in Germany, from a depth of 2144 feet,
a temperature of ninety-one degrees. Natural hot springs, rising from
unknown depths, are sometimes scalding hot. One in Arkansas has a
temperature of one hundred and eighty degrees.
At a depth of twenty miles, with this continual increase of temperature,
the ground must be fully red-hot; and not very much farther down the
heat must be sufficient to melt every known substance. The solid earth,
then, is merely a thin crust, covering a sea of liquid fire below. The
streams of lava poured forth from volcanoes are a proof of the existence
of this molten mass beneath our feet.