"It often does," he answered. "You trifle with me," I said, acrimoniously. "No," he replied; "I am telling you the truth. Have you never heard of what men call artesian wells?" "Yes, and" (here I attempted in turn to become sarcastic) "have you never learned that they are caused by water flowing into crevices in uplands where layers of stone or of clay strata separated by sand or gravel slant upward. The water conducted thence by these channels afterwards springs up in the valleys to which it has been carried by means of the crevices in these strata, but it never rises above its source." To my surprise he answered: "This is another of man's scientific speculations, based on some facts, it is true, and now and then correct, but not invariably. The water of an artesian well on an elevated plane may flow into the earth from a creek, pond, or river, that is lower than the mouth of the well it feeds, and still it may spout into the air from either a near or distant elevation that is higher than its source." "I can not admit the truth of this," I said; "I am willing to listen to reason, but such statements as these seem altogether absurd." "As you please," he replied; "we will continue our journey." INTERLUDE.--THE STORY INTERRUPTED. CHAPTER XXVIII. A CHALLENGE.--MY UNBIDDEN GUEST ACCEPTS IT. The white-haired reader, in whom I had now become deeply interested, no longer an unwelcome stranger, suspended his reading, laid down his manuscript, and looking me in the face, asked: "Are you a believer?" "No," I promptly answered. "What part of the narrative do you question?" "All of it." "Have you not already investigated some of the statements I previously made?" he queried. "Yes," I said; "but you had not then given utterance to such preposterous expressions." "Is not the truth, the truth?" he answered. "You ask me to believe impossibilities," I replied. "Name one." "You yourself admit," I said warmly, "that you were incredulous, and shook your head when your guide asserted that the bottom of the ocean might be as porous as a sieve, and still hold water. A fountain can not rise above its source." "It often does, however," he replied. "I do not believe you," I said boldly. "And, furthermore, I assert that you might as reasonably ask me to believe that I can see my own brain, as to accept your fiction regarding the production of light, miles below the surface of the earth." "I can make your brain visible to you, and if you dare to accompany me, I will carry you beneath the surface of the earth and prove my other statement," he said. "Come!" He arose and grasped my arm. I hesitated. "You confess that you fear the journey." I made no reply.