This being so, why should we withhold from the sun of primitive days the designation to which it seems to be so fully entitled? Why should we not speak of it as a nebula? The application of the laws of heat has shown that the great orb of day was once one of those numerous objects which astronomers know as nebulæ, and perhaps it may not be too fanciful to suppose that a trace of the primæval nebula still survives in what we call the Solar Corona (Fig. 20). ------------------------------------------------------------------------ CHAPTER VIII. THE EARTH’S BEGINNING. The Earth to be Studied—A great Experiment—The Diamond Drill—A Boring upwards of a Mile Deep—A Mechanical Feat—The Scientific Importance of the Work—Increase of Temperature with the Depth—A special Form of Thermometer—Taking the Temperature in the Boring—The Level of Constant Temperature—The Rate of Increase of Temperature with the Depth—One degree Fahrenheit for every Sixty-six Feet in Depth—Temperatures at Depths above a Mile—Conclusions as to the Heat at very great Depths—The Heat developed by Tidal Action—This will not account for the Earth’s Internal Heat—The Earth must be continually Cooling—Inferences from the incessant loss of Heat from the Earth—The Earth’s Surface once Red-Hot, or Molten—The Earth must have originated from a Nebula—The Earth’s Beginning. IN the last chapter we endeavoured to ascertain what can be learned from the radiation of the sun with regard to the history of the solar system. In this chapter we shall not consider any body in the heavens, but the condition of the earth itself. We have learned something of the history of the solar system from the celestial bodies; we shall now learn something about it in another way—from the condition of our globe at depths far beneath our feet. It will be convenient to commence by mentioning a remarkable experiment which was made a few years ago. Though that experiment is of great scientific interest, yet it was not designed with any scientific object in view. Not less than £10,000 was expended on the enterprise, and probably so large a sum has never been expended on a single experiment of which the sole object was to add to scientific knowledge. In the present case the immediate object in view was, of course, a commercial one. There was, it may be presumed, reasonable expectation that the great initial cost, and a handsome profit as well, would be returned as the fruits of the enterprise. Whether the great experiment was successful from the money-making point of view does not now concern us, but it does concern us to know that the experiment was very successful in the sense that it incidentally afforded scientific information of the very highest value.