If we have two equal strings, each performing an equal number of vibrations in a second, they will produce equal sound waves, and the sound produced by both together will be uninterrupted, and twice as loud as one of them. But if one string vibrate, say one hundred times, and the other one hundred and one times in a second, they will _not_ be in unison, and one will gain upon the other string, till after it has got to fifty vibrations it will be half a vibration ahead. At that moment they will neutralize each other, and silence will ensue for an appreciable time. [Illustration: Fig. 185.—The Megaphone.] In the case of light suppose a _red_ ray strikes the eye, and another red ray to come upon it from somewhere else. If the difference between its distance and the other point from the spot in the retina on which the first ray fell, is the 258/1000 part of an inch, or exactly twice, thrice, four times as much, etc., that distance, the light will be seen twice as strong. But if the difference in the distances between the points whence the light comes be only _one-half the 258/1000 part of an inch_, or 1½, 2½, 3½, or 4½ times that distance, one light will extinguish the other, and darkness will be the result. Now this is precisely what happens in the case of the Topophone. To return to our simile of water waves. If two stones be cast into a pond, and two equal and similar waves produced, and if they reach a certain place at the same moment, they will make one large wave. But if one followed the other a little, so that the hollow of one coincided with the crest of the other, and _vice versâ_, the waves would obliterate each other, and a dead level would result. One tube of the Topophone is half a wave length longer than the other, and when the resonators are in a line and receive the wave at the same time, one ear hears the elevation of the sound wave, and the other the depression,—the sound is neutralized, and comparative, if not actual, silence results. The sailor knows in what direction the land lies, and can calculate his distance, or anchor if he please. If amongst our readers there be any who wish to make for themselves an acoustic signalling apparatus there is physically nothing to prevent them from constructing such an instrument as that shown in the annexed woodcut (fig. 185). It is founded upon the speaking-trumpet principle, which is supposed to have been originated by Samuel Markland, in 1670. Kircher, in his “_Ars magna et umbra_,” and in his “_Phonurgia_,” mentions a kind of speaking-trumpet, or _porte voix_, of gigantic dimensions, and called the “Horn of Alexander.” According to Kircher, the instrument was used by Alexander the Great to summon his soldiers from a distance of ten miles. The diameter of the circumference was about eight feet, and Kircher conjectured that the instrument was mounted upon three supports.