SOUND-SIGNALS AT SEA. 



89 



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 followicg 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, 18Y2, 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-sur- 

 face, and this wave-surface be a spherical one, that is, be not deformed, tlien a 

 perpendicular, S, erected to the center of a chord drawn between these two 

 points, wiil, 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. Sup- 

 pose both resonators at the same / \ ^ 

 instant on the wave-surface, then / Y? ^7" 

 they both receive, at the same in- 

 stant, 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 M'ill 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 de- 

 scribed by Prof. Mayer before the National Academy in April, 187(3, 



While this contrivance may not yet be entirely practicable, 

 its use, as detailed, makes the fact evident that some apparatus 



