Science tells us that the Earth was once a shining star, a globe of liquid fire. As it cooled down, a crust formed over its surface, composed chiefly of rocks and metals. This crust was rent by the force of the gases shut up within, and thus the mountains, valleys, gorges, and volcanoes were formed. The Earth, indeed, is still upheaving and subsiding, but so slowly that we rarely feel it. Through these agencies the distribution of land and water on the surface of the earth has undergone great changes. The shape of the Earth is that of a sphere somewhat flattened at the poles, and it has a diameter of about 8,000 miles. The solid crust is called the lithosphere—which is surrounded by an envelope of air—the atmosphere—and in part by an envelope of water—the hydrosphere.
HOW THE EARTH WOULD APPEAR IF CUT THROUGH THE CENTER
Beneath the rocky crust of the earth, thirty-five miles in thickness, there is a broad belt of heavier material to a depth of nine hundred miles. Within this shell lies the great metallic core.
OUR EARTH: ITS STRUCTURE AND SURFACE
Our first glimpse of the earth as a planet shows it as a nebulous star, still intensely hot, and with no solid nucleus, rotating on its own axis, and at the same time revolving around the sun in a nearly circular orbit.
WHAT THE HEAT OF THE
EARTH SHOWS
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.