542 RESEARCHES IN SOUND. 



uniformity of temperature more than compensate for the dimiuislied 

 elasticity. Tlie same may be said with regard to the audibility of sound 

 during a fog, which usually exists during extreme stillness of the air. 



Whatever be the cause of the variation in the limit of audition as 

 exhibited in the diagrams, it is less efiBcient than the ordinary action of 

 the wind in producing the same phenomena. This is evident from the 

 fact that while the ratio of the extreme variation in the limits of audi- 

 tion in the first case is not more than 1:3, in the second it is that of 

 1:5. 



Moreover, when the effect of the wind on the audition of sound iu 

 relation to elevation is considered, we think we are fully warranted in 

 asserting, as we did in our last report, that the wind is a more efficient 

 cause of the variability of the penetration of sound than the invisible 

 acoustic clouds adopted by Professor Tyndall for the explanation of the 

 phenomena. 



The object of these investigations, as stated at the beginning of this 

 report, was to obtain facts which might serve to establish the true theory 

 of the abiiormal phenomena of sound, an object, independent of its 

 scientitic interest, of much practical importance in its application to fog- 

 signals. Although the observations were not as perfect as we could 

 wish in many respects, from want of certain appliances, they are yet 

 sufficient wje think to establish principles of much practical value. For 

 example, if the mariner in api)roaching a fog-signal while the wind is 

 blowing against the sound fails to perceive it on deck, he will probably 

 hear it by ascending to the mast-head ; or, in case a sound from a given 

 station is constantly obscured in a certain direction, while it is audible 

 in adjacent directions, we may attribute it to a sound-shadow produced 

 by some interposing object. If again, the obscuration of sound in a 

 given direction is only observed during a wind moving against the sound, 

 the cause will probably be found in a lateral refraction, due to the re- 

 tardation of the current of wind against a peri^endicular wall or cliif, as 

 in the 3ase observed at Block Island, August 19. The subject however 

 is one of great complexity, and requires further investigation, but the 

 results thus far obtained may be considered as furnishing the preliminary 

 data on which to found more precise observations. These should be 

 made with the aid of a number of steamers simultaneously employed, 

 each furnished with anemometers and balloons for determining with more 

 accuracy the direction and velocity of the wind. 



We hope to renew the investigations during next summer, and in view 

 of this have directed that in the mean time the light-keepers at Block 

 Island and at Point Judith shall continue to sound their sirens a certain 

 length of time every JVIonday, noting the direction and velocity of the 

 wind, the temperature and pressure of\he air, and the audibility of the 

 sound as it comes reciprocally from each instrument. 



It is shown, from the results thus far obtained from these reciprocal 

 observations, that sound is occasionally heard more distinctly against 



