"But the circumstance most remarkably corroborative of the mysterious truth to which these indications point, is the appearance of our midnight luminary during a total eclipse. By theory, she ought to disappear utterly from the heavens. She should vanish, and the sky seem as if no Moon were in being; but, on the contrary, and even when she passes the very centre of the Earth's shadow, she seems a huge _disc of_ bronze, in which the chief spots can easily be described by a telescope." And that, remember, when the moon is in the utter blackness of the earth's shadow. Of course, another explanation has been advanced; but it does not take the professor long to dispose of that. "It has been put forth in explanation," he says, "that a portion of the rays of the Sun must be reflected by our atmosphere and bent toward the eclipsed disc, from which again they are reflected to the Earth--thus giving the Moon that bronze color; but, the instant the hypothesis is tested by calculation, we discern its utter inefficiency. Nor is there any tenable conclusion save this:--That the matter both of Sun and planets is capable, in certain circumstances whose exact conditions are not known, of evolving the energy we term light." All this, and more, Rhodes explained to me, succinctly but clearly. "Oh, we'll find light, Bill," said he. All the same this subterranean world for which we were bound presented some unpleasant possibilities, in addition, that is, to those concomitant to its being a habitat of demons--and Heaven only knew what besides. "And then there is the air," I said. "As we descend, it will become denser and denser, until at last we will be able to use these ice-picks on it." Rhodes, who was removing his creepers, laughed. "We will have to make a vertical descent of three and one half miles below the level of the sea--a vertical descent of near five miles from this spot where we stand, Bill--before we reach a pressure of even two atmospheres." "The density then increases rapidly, doesn't it?" "Oh, yes. Three and a half miles more, and we are under a pressure of four atmospheres, or about sixty pounds to the square inch. Three and a half miles farther down, or ten and one half miles in all below the level of the sea, and we have a pressure upon us of eight atmospheres. Fourteen miles, and it will be sixteen atmospheres. At thirty-five miles the air will have the density of water, at forty-eight miles it will be as dense as mercury, and at fifty miles we shall have it as dense as gold." "That will do!" I told him. "You know that we can never get down that far." "I have no idea how far we can go down, Bill." "You know that we could never stand such pressures as those."