"You mean to say that the center of the earth is not a mass of molten matter at a white heat?"
"We have every reason to believe that this is not the case."
"Then how do you explain that, in mines, the temperature becomes warmer the deeper down you go? You will at least admit this fact, will you not?"
"Certainly," said Dr. Giles; "that fact is well established. But you must remember that our deepest mines barely extend a couple of miles into the earth."
"Still, in those two miles the increase in heat is considerable, the bottom of the mine being always hotter than the top."
"True; but this increase varies considerably in different parts of the earth, being much less in some mines than in others."
"Yes; but the average increase, as I understand it, has been found to be about 1° F. for every sixty feet, so that, if we accept this as the normal rate, the heat at the depth of a very few miles would be sufficient to melt the hardest rocks we know of."
"That would be true enough," said the doctor, "if the heat continued increasing at the same
