"Whether we are fated, also, to reveal it to the world, to _our_ world--well, as for that, _quién sabe_?" he said. "Then," I remarked, my fingers busy removing my ice-creepers, "what we read about the state of things in the interior of the earth--the temperature, the pressure, the density--then all that is pure theory?" "Of course. How could it be anything else? All theory, save, that is, the mean density of the earth. And that mean density gives us something to think about, for it is just a little more than twice that of the _surface materials_. With all this enormous pressure that we hear so much about and the resultant increase of density with depth, the weight of the earth certainly ought to be more than only five and one half times that of a globe of equal size composed of nothing but water."[6] [Footnote 6: "It has been calculated that at the depth of 35 miles, air, subjected to the pressure of a column of matter of the mean density of that at the surface of the earth, would acquire the density of water; that at the depth of 173 miles, water itself, which is eminently incompressible, would acquire the density of marble; and at the centre, marble would have a density 119 times greater than at the surface. _But the comparatively small mean density of the mass [of the earth] proves that none of these effects take place._"--Brande.] "Kind of queer, all right," was my comment. "It is queer, all right--as the old lady said when she kissed the cow. However, as old Dante has it, 'Son! our time asks thrifter using.'" As the last words left his lips, I straightened up, the toothed shoes in my hand; and, as I did so, I started and cried: "Hear that?" Rhodes made no answer. For some moments we stood there in breathless expectation; but that low mysterious sound did not come again. I said: "What _was_ that?" "I wish that I knew, Bill. It was faint, it was--well, rather strange." "It was more than that," I told him. "It seemed to me to be hollow--like the sound of some great door suddenly closing." My companion looked at me rather quickly. "Think so, Bill?" he said. "I thought 'twas like the sound of something _falling_." There was a pause, one of many moments, during which pause we stood listening and waiting; but the gallery remained as silent as though it had never known the tread of any living thing. "Well, Bill," said Milton Rhodes suddenly, "we shall never learn what Drome means if we stay in this spot. As for the creepers, I am going to leave mine here." The place where he put them, a jutting piece of rock, was a conspicuous one; no one passing along the tunnel could possibly fail to notice the objects resting there. Mine I placed beside them, wondering as I did so if I should ever see this spot again.