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.
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.
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.
During the last century, a German professor, named Huth, made a model of the horn, and found it answered every purpose of a speaking-trumpet with most powerful results, but we beg leave to doubt whether the instrument really carried the voice to any very great distance.