Now, suppose a person up to his shoulders in a pond of water, and someone throws a stone into it. If that person extend his arms and hands at right angles facing the sound, each hand would touch the edge of a ripple as it came towards him across the pond. He would then be facing the source of the ripples or waves, and look along a radius of the circle formed by the waves. But if he please, he can move his body so that both hands shall touch the same wave at the same time, or he might turn away from the source, and only one hand would touch the wave. But when both hands are actually washed by the same circular ripple he must be facing the source of it. Any position in which his fingers did not touch the ripple almost at the same instant, would not be facing the source of the wave ripples. So by turning and extending his hands, he could with his eyes shut find out whether he was or was not facing the original source of the waves. This applied to sound waves in the air is the whole theory of the Topophone, which, however, depends for its usefulness upon the same note being sounded by all horns and whistles. One note must be better than all the others, and that note, probably C (treble), caused by about two hundred and sixty vibrations per second, has been found most applicable. If all whistles and horns can by law be compelled to adjust themselves to this note, then the Topophone will be a real and lasting benefit. Let us now look at the apparatus itself. It being conceded that the resonators are in the same key as the Foghorn,—and this is necessary,—they are placed upon the deck of the vessel. An ear-tube of indiarubber is carried from each of these “resonators” into the cabin. These tubes unite and again separate, ending in small pieces ready to be fitted to the ears. The apparatus is fixed on deck, and the arrangement which supports it passes into the cabin, and can be turned about in any direction. Of course in this case a dial point is necessary to indicate the direction in which the instrument is turned. If the machine be worn on the shoulders of the officer of the watch he can move as he pleases, and wants no indicator. The Topophone when used is so constructed, that when a horn is heard, and when the listener is facing the sound, he can _hear nothing_! When not facing the origin of the sound he can hear the horn very well, but the moment the resonators receive the sound together as they face the source, a very low murmur is heard, or perhaps no sound at all.—Why? A certain pitch of tone is composed of vibrations or waves of equal length. In all waves there is a hollow and a crest. One neutralizes the other. The hollow of a sound wave meeting the crest of another wave “interferes” to produce silence, stillness, a dead level. So in “light”; two rays will produce darkness. We will endeavour to explain this.