514 EESEARCHES IN SOUND. 



In these figures and all the succeeding ones the direction of the wind 

 is indicated bj^ arrows. 



The hypothesis we have adopted in connection with the fact of the 

 lateral si^read of sound gives a simple explanation of various abnormal 

 phenomena of sound such as has been observed in the previous investi- 

 gations, and of which the following are examples : First, the audibility 

 of a sound at a distance, and its inaudibility nearer the source of sound ; 

 second, the inaudibility of a sound at a given distance in one direction, 

 while a lesser sound is heard at the same distance in an oi^posite direc- 

 tion ; third, the audibility of the sound of an instmment at one time 

 at the distance of several miles, while at another time the sound of the 

 same instrument cannot be heard at more than a- fifth of the same dis- 

 tance ; fourth, while the sound is heard generally farther with the wind 

 than against it, in some instances the reverse is the case ; fifth, the sud- 

 den loss of sound in i^assing from one locality to another in the same 

 vicinity, the distance from the source of the sound being the same. 



The first four of these i)henomena find a ready explanation in the 

 hypothesis adopted bj^ supposing an increase or diminution in the rela- 

 tive velocity of the currents of wind in the upper or lower strata of air. 

 The fifth is exiDlained by the interposition of an obstacle which casts, 

 as it were, a sound-shadow, disappearing at a given distance by the 

 divergence of the rays on each side of the obstacle into what would be 

 an optical shadow. 



Accounts of these investigations were presented from time to time to 

 the Light-House Board, and to the Philosophical Society of Washington 

 in 1872. Subsequently a series of investigations on the same subject 

 was instituted in England by the Elder Brethren of the Trinity House, 

 under the direction of their scientific adviser, the celebrated physicist. 

 Dr. Tyndall. While in the latter investigations various abnormal phe- 

 nomena, similar in most instances to those we have mentioned, were 

 observed, they were referred by Dr. Tyndall to an entirely different 

 cause, viz, to the existence of acoustic clouds, consisting of portions of 

 the atmosphere in a flocculent or mottled condition, due to the unequal 

 distribution of heat and moisture, which, absorbing and reflecting the 

 sound, produce an atmosphere of acoustic opacity. While we do not 

 deny the possible existence of such a condition of the atmosphere, we 

 think it insufficient to account for all the phenomena in question, and 

 believe that a more general and efllcient cause is that of the wind, in 

 accordance with the hypothesis of Professor Stokes. 



We regret to differ in opinion from Dr. Tyndall, and have published 

 our dissent from his views in no spirit of captious criticism or desire to 

 undervalue the results he has obtained, some of which are highly im- 

 portant. Our only object in our remarks and in our investigations is 

 the establishment of ti'uth. 



The determination of the question as to the cause of the abnormal 

 phenomena of sound we have mentioned, and the discovery of new phe 



