"He intends," he explained, "to send forth at intervals sharp notes of inaudible sound which, acting like audible sound, will come back from an iceberg, a ship, or a coast as an echo, and the time elapsed, recorded by a suitable receiver, and divided by two, will give the distance." "Inaudible sound," I answered. "That seems anomalous." "Hypothetical rather--not yet proven. But who knows? There are sounds of too low and long a vibration--I do not mean the roll of a drum, which is merely a succession of beats--that cannot be cognized by the human ear. There are sounds too high to be heard, such sounds as the chirp of a cricket, or tweet of a bird. Some people never hear these sounds. Why not use these sounds, and receive their echoes by delicate instruments?" "Give it up," I said hopelessly. "They call silence the negation of sound," he continued, "as they call darkness the negation of light. There is polarity throughout the universe. We are aware of plus and minus quantities in mathematics, each calculable by the same methods. "We know of the north and south poles of the magnet; we know of positive and negative electricity; we know, by the expansion of gases, that there is a force the opposite of gravitation, and which repels instead of attracts. Some theorists have called this force apergy. I believe that all these negative forces exist, but only in the presence, or because of the existence, of their opposites. For I know that cold, which, more than all other conditions, may seem to be a mere negation, will answer to the laws of other radiant energy, and, like heat, decrease in force as the square of the distance." I looked stupidly at him, then at Old Bill, who, no more stupid than myself under this avalanche of erudition, was puttering about, cleaning up the studio. Not noticing my bewilderment, the old artist went on. "But an iceberg," he said, "is too large to be cognized by the law of inverse squares. It would need a diaphragm bigger than itself--bigger than the largest ship, to collect its radiations of cold. Could it be reduced to a point, however, and its cold concentrated, we could calculate its distance and direction, at least, if not its size. You know the nature of a searchlight, do you not?" He looked at me now, and, trying to bring an intelligent expression to my face, I nodded. "An arc light in the focus of a parabolic reflector placed behind it," he continued. "Stand in the path of its beam and you will sensibly feel the heat. Place a large lens--as large as the aperture--in the way, and focus the beam on yourself, and you will feel a heat equal to that of the arc light--about seven thousand degrees Fahrenheit--less what has radiated into the surrounding air.