"I cannot," he said, "delicately explain in so many words what use I expect to make of this apparel. Nor do I yet know whether I shall have any use at all for it. That can only be a theoretical speculation until, within a few more hours, my theory is proven or disproven--and," he said, suddenly turning on me, "my theory concerning these invisible creatures is the most extraordinary and audacious theory ever entertained by man since Columbus presumed that there must lie somewhere a hidden continent which nobody had ever seen." He passed his hand over his protruding forehead, lost for a moment in deepest reflection. Then, "Have you ever heard of the Sphyx?" he asked. "It seems to me that Ponce de Leon wrote of something--" I began, hesitating. "Yes, the famous lines in the third volume which have set so many wise men guessing. You recall them: "'_And there, alas! within sound of the Fountain of Youth whose waters tint the skin till the whole body glows softly like the petal of a rose--there, alas! in the new world already blooming_, THE ETERNAL ENIGMA _I beheld, in the flesh living; yet it faded even as I looked, although I swear it lived and breathed. This is the Sphyx_.'" A silence; then I said, "Those lines are meaningless to me." "Not to me," said Miss Barrison, softly. The professor looked at her. "Ah, child! Ever subtler, ever surer--the Eternal Enigma is no enigma to you." "What is the Sphyx?" I asked. "Have you read De Soto? Or Goya?" "Yes, both. I remember now that De Soto records the Syachas legend of the Sphyx--something about a goddess--" "Not a goddess," said Miss Barrison, her lips touched with a smile. "Sometimes," said the professor, gently. "And Goya said: "'_It has come to my ears while in the lands of the Syachas that the Sphyx surely lives, as bolder and more curious men than I may, God willing, prove to the world hereafter_.'" "But what is the Sphyx?" I insisted. "For centuries wise men and savants have asked each other that question. I have answered it for myself; I am now to prove it, I trust." His face darkened, and again and again he stroked his heavy brow. "If anything occurs," he said, taking my hand in his left and Miss Barrison's hand in his right, "promise me to obey my wishes. Will you?" "Yes," we said, together. "If I lose my life, or--or disappear, promise me on your honor to get to the electric launch as soon as possible and make all speed northward, placing my private papers, the reports of Miss Barrison, and your own reports in the hands of the authorities in Bronx Park. Don't attempt to aid me; don't delay to search for me. Do you promise?" "Yes," we breathed together. He looked at us solemnly. "If you fail me, you betray me," he said. We swore obedience. "Then let us begin," he said, and he rose and went to the steel cage. Unlocking the door, he flung it wide and stepped inside, leaving the cage door open.