SOUND-SIGNALS. 



733 



10. As sounds may be inaudible at certain dis- 

 tances and elevations without being wholly extin- 

 guished, it follows that the comparative inaudibility 

 of sounds at different times can not always be cited as 

 an evidence of their relative intensities. The com- 

 parative inaudibility may be a function of variable 

 refraction, rather than of variable intensity. Hence 

 the law of inverse squares, though perfectly true in 

 its theoretical application to the measurement of the 

 intensity of all sounds, can not always be legitimately 

 used to calculate backward from the audibility of a 

 sound, as empirically ascertained at a given point and 

 elevation, to its relative intensity as previously heard 

 at the same point and elevation. 



11. The hypothesis of Stokes, as applied by Henry, 

 does not exclude the hypothesis of Humboldt, but 

 reduces the latter to a very subordinate and inappre- 

 ciable place in interpreting the abnormal phenomena 

 of sound. 



12. The hypothesis of Stokes, as applied by Henry, 

 does not exclude the reasoning or the experimental 

 proofs by which Prof. Eeynolds demonstrates that 

 differences in temperature exert a refracting power in 

 sound, but finds in that refraction an influence which 

 may sometimes accelerate and sometimes retard the 

 refraction produced by wind. 



The officer who made the reports as to the 

 fog-signals at Beaver Tail and Little Gull, after 

 the accidents to the steamers Rhode Island 

 and Galatea heretofore mentioned, was then 

 Assistant-Inspector of the Third Lighthouse 

 District, Lieut.-Commander F. E. Chadwick, 

 U. S. N. ; and it was he who had charge of the 

 lighthouse steamer while the foregoing observa- 

 tions were being made, after Capt. George 

 Brown, U. S. N., the inspector, was called else- 

 where. In answer to certain questions as to 

 the opinions which had forced themselves upon 

 him, he said : 



It seems to me that navigators should understand 

 that, when attempting to pick up a fog-signal, atten- 

 tion must be given to the direction of the wind, and 

 that if they are to windward (in a moderate breeze) 

 the chances are very largely against hearing it, unless 

 close to : that there is nearly always a sector of about 

 120 to windward of the signal, in which it either can 

 not be heard at all, or in which it is but faintly heard. 

 Thus, with the wind east-southeast, so long as they 

 are bearino- from the signal between northeast and 

 south, there is a large chance that the signal will not 

 be audible until it is very close. 



As they bring the signal to bear at right angles with 

 the wind", the sound will almost certainly, in the case 

 of light wind, increase, and it will soon assume its 

 normal volume being heard almost without fail in 

 the leeward semicircle. ' 



Fosr, to my mind, and so far as my experience goes, 

 is not a factor of any consequence whatever in the 

 question of sound. Signals may be heard at great dis- 

 tances through the densest fogs, which may be totally 

 inaudible in 'the same directions and at the Baine dis- 

 tances in the clearest atmosphere. It is not meant by 

 this last statement that the fog may assist the sound, 

 as at another time the signal may be absolutely inau- 

 dible in a fog of like density, where it had before been 

 clearly heard. That fog has no great effect, can easily 

 be understood when it is known, as it certainly is 

 known by observers, that even snow does not deaden 

 sound, there being no condition of the atmosphere so 

 favorable for the far-reaching of sound- signals as is 

 that of a heavy northeast snow-storm, due, supposably, 

 to the homogeneity produced by the falling snow. 



It seems to be well established by numerous observa- 

 tions that on our own northern Atlantic coasts the best 

 possible circumstances for hearing a fog-signal are m 

 a northeast snow-storm, and, so far as these observa- 

 tions have extended, they seem to point to the extra- 



ordinary conclusion that they are best heard with the 

 observer to windward of the signal ; and that in lijjht 

 winds the signal is best heard down tbe wind, or u 

 right angles with the wind. 



The worst conditions for hearing sound seem to be 

 found in the atmosphere of a clear, frosty mon. 

 which a warm sun nas been shining for two or three 

 hours. 



The curve of audibility in a light or moderate breeze, 

 in general, is similar to that plotted by Prof. Henry, 

 as in the accompanying diagram. 







Wind. 



FIG. 9. THE HENRY DIAGRAM. 



I think it is established that there are two great 

 causes for these phenomena : non-homogeneity of the 

 atmosphere, and the movement of the wind. How 

 this latter acts, no one can say. The theory of retarda- 

 tion of the lower strata of the atmosphere near the 

 earth's surface, as advanced by Prof. Stokes, of Eng- 

 land, seems good for moderate winds, but it hardly 

 holds in cases where the siren is heard from eighteen 

 to twenty miles to windward during northeastern gales. 



While the mariner may easily expect to hear 

 the sound of the average fog-signal normally as 

 to force and place, he should be prepared for 

 occasional aberrations in audition. It is im- 

 possible, at this point in the investigations, 

 which are still in progress, to say when, where, 

 or how the phenomena will occur. But cer- 

 tain suggestions present themselves even now 

 as worthy of consideration. 



It seems that the mariner, in order to pick 

 up the sound of the fog-signal most quickly, 

 should, when approaching it from the wind- 

 ward, go aloft; and that when approaching it 

 from the leeward, the nearer he can get' to the 

 surface of the water the sooner he will hear the 

 sound. 



It also appears that there are some things 

 the mariner should not do. He should place 

 no negative dependence on fog- signals; that 

 is, he should not assume that he is out of hear- 

 ing distance because he fails to- hear the sound. 

 He should not assume that, because he hears a 

 fog-signal faintly, he is at a great distance from 

 it. Neither should he assume that he is near 

 it because he hears the sound plainly. < He 

 should not assume that he has reached a given 

 point on his course because he hears the fog- 

 signal with the same intensity that he-did when 

 formerly at that point. Neither should he as- 

 sume that he has not reached this point because 

 he fails to hear the fog-signal as loudly as be- 

 fore, or because he does not hear it at all. 



He should not assume that -the^ fog-signal has 

 ceased sounding, because he fails'to hear it even 

 when within easy ear-shot. He should not as- 

 sume that the aberrations of audibility which 

 pertain to any one fog-signal pertain to any 

 other fog-signal. He should not expect to hear 



