488 EESEARCHES IN SOUND. 



lu the comments we have made on the report of General Duaue, the 

 intention was not in the least to disparage the value of his results, which 

 can scarcely be too highly appreciated ; but inasmuch as the true explana- 

 tion of the i)henomena he has observed has an important bearing on the 

 location of fog-signals and on their general ai^plicatioii as aids to naviga- 

 tion, and are as well of great interest to the physicist, who values every 

 addition to theoretical as well as practical knowledge, we have not only 

 thought the remarks we have offered necessary, but also that special 

 investigations should be made to ascertain more definitely tlie condi- 

 tions under which the abnormal phenomena the general has described 

 occur, and to assign, if possible, a more definite and efficient cause than 

 those to which he has attributed them. 



We have, therefore, given much thought to the subject, and since the 

 date of General Duane's report, have embraced every opportunity which 

 occurred for making observations in regard to them. The first s^ep we 

 made toward obtaining a clew to the explanation of the phenomena in 

 question resulted from observations at New Haven, namely : 1st, the 

 tendency of sound to spread laterally into its shadow; 2d, the fact that 

 a sound is frequently borne in an opposite direction to the wind at the 

 surface by an upper current ; and 3d, that a sound moving against a wind 

 is heard better at a higher elevation. The first point to consider is in 

 what manner the wind affects sound. That it is in some way connected 

 with the distance to which sound can be heard is incontestably settled by 

 general observation. At first sight, the explanation of this might seem 

 to be very simple, namely, that the sound is borne on in the one direc- 

 tion and retarded in the other, by the motion of the wind. But this ex- 

 planation, satisfactory as it might appear, cannot be true. Sound moves 

 at the rate of about 780 miles an hour, and therefore, on the above sup- 

 position, a wind of 7.8 miles per hour could neither retard nor accele- 

 rate its velocity more than one per cent., an amount inappreciable to 

 ordinary observation ; whereas we know that a wind of the velocity we 

 have mentioned is frequently accompanied with a reduction of the pene- 

 trating power of sound of more than 50 per cent. 



The explanation of this phenomenon, as suggested by the hypothesis of 

 Professor Stokes, is founded on the fact that in the case of a deep current 

 of air the lower stratum, or that next the earth, is more retarded bj' fric- 

 tion than the one immediately above, and this again than the one above it, 

 and so on. The effect of this diminution of velocity as we descend toward 

 the earth is, in the case of sound moving with the current, to >carry the 

 upper part of the sound-waves more rapidly forward than the lower parts, 

 thus causing them to incline toward the earth, or, in other words, to be 

 thrown down upon the ear of the observer. When the sound is in a 

 contrary direction to the current, an opposite effect is produced, — the 

 upper portion of the sound-waves is more retarded than the lower, wliich 

 advancing more rapidly, in consequence inclines the waves upwar<l and 

 directs them above the head of the observer. To render this more clear, 



