have seen convince you that you are not in the world of your birth? And the horizon—could it present the strange aspects which we both noted unless we were indeed standing upon the inside surface of a sphere?"
"But the sun, Perry!" I urged. "How in the world can the sun shine through five hundred miles of solid crust?"
"It is not the sun of the outer world that we see here. It is another sun—an entirely different sun—that casts its eternal noonday effulgence upon the face of the inner world. Look at it now, David—if you can see it from the doorway of this hut—and you will see that it is still in the exact center of the heavens. We have been here for many hours—yet it is still noon.
"And withal it is very simple, David. The earth was once a nebulous mass. It cooled, and as it cooled it shrank. At length a thin crust of solid matter formed upon its outer surface—a sort of shell; but within it was partially molten matter and highly expanded gases. As it continued to cool, what happened? Centrifugal force hurled the par-
