Page:Popular Science Monthly Volume 33.djvu/99

Rh at sea, seven miles away, and that, too, after he had purposely deprived himself of a knowledge of even the direction of the shore by having the steamer turned in her course from time to time. President Morton describes it thus:

This apparatus consisted of the following parts: A vertical rod passing through the roof of the deck-cabin, on the upper end of which was attached a horizontal bar carrying two adjustable resonators. Below these was a pointer set at right angles with the above bar. Rubber tubes passed through the roof of the cabin and were connected with a pair of ear-tubes. A handle attached to the vertical rod served to turn it in any direction.

The principle upon which the operation of this apparatus depends was first announced by Prof. Mayer in 1872 (see "American Journal of Science and Art," November, 1872, p. 387), and its general operation may be explained as follows:

Let S of the diagram be the source of a sound, and let the circle represent a wave-surface produced by that sound. On this surface all the molecules of air have, at the same instant, the same direction and the same velocity of vibratory motion. If we can accurately determine two points, R and R', on this wave-surface, and this wave-surface be a spherical one, that is, be not deformed, then a perpendicular, S, erected to the center of a chord drawn between these two points, will, when produced, pass through the source, S. The method consists in determining these two points on a sonorous wave-surface, as follows: Let R and R' be two resonators accurately tuned to the note given by the vibratory body at S. Suppose both resonators at the same instant on the wave-surface, then they both receive, at the same instant, the same phase of vibration, on the planes of their mouths. If two tubes of equal length lead from the resonators and join into one tube just before they reach the ear, E, then the sound-pulses will act together, being of the same phase, and the ear will receive double the action which it would if only one resonator were connected with the ear. But suppose that one of these tubes, T', differs in length from the other tube, T, by one half of a wave-length of the tone given out by S, then the same pulses will no longer work together at E, but will be opposed to each other in their action, neutralizing each other's dynamic effect, and producing silence at the ear, E. This last condition is the one used in the apparatus above described.

We connect the two resonators, R and R', by a rigid rod, and it is evident, if a pointer be placed at the center of this rod at right angles to its length, that when the resonators, R and R', are on the wave-surface, the rod, S, will point toward the source of sound at S. The rigid rod connecting the resonators, R and R', turns on a vertical rod passing through 0. This arrangement was described by Prof. Mayer before the National Academy in April, 1876.

While this contrivance may not yet be entirely practicable, its use, as detailed, makes the fact evident that some apparatus