PHILOSOPHICAL SOCIETY OF WASHINGTON. 34T 



investigatiocs at different light-stations, on our western coast of 

 California. 



The very important observation that a sound could best be heard 

 at an elevation vv^hen the wind is adverse (that is when it blows 

 from the observer towards the acoustic signal,) and that after it 

 had even been entirely lost to the ear in such case, it might be 

 regained in full force by simply ascending to a suitable elevation, 

 — admitted apparently but one explanation, namely that the line 

 of successive impulse constituting a sound beam was deflected or 

 bent upwards by the action of the opposing wind. If — as had 

 already been shown to be the ease sometimes, and as might there- 

 fore be expected generally, — the adverse wind were assumed to 

 be a little stronger at the elevation than at the surface such a re- 

 sult would at once follow. " 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 by friction 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 oppo- 

 site effect is produced, the upper portion of the sound waves is 

 more retarded than the lower, which advancing more rapidly in 

 consequence, inclines the waves upward and directs them above 

 the head of the observer."* 



From several observed and reported cases where the sound of a 

 fog-signal was exceptionally heard to a greater distance against 

 the wind than toward the direction of the' wind. Professor Henry 

 for a while hesitated to give the hypothesis of Professor Stokes 

 an unqualified acceptance ; but forced as he was constantly to recur 

 to it as the only plausible explanation of the ordinary influence of 

 wind on the transmission of sound, he finally was able to satisfy 

 himself that even the apparent exceptions to the rule were really 

 in accord with it. Having more than once observed that when 

 the upper current of air, as indicated by the course of the clouds, 

 is in an opposite or different direction from the lower or sensible 

 wind, the range of audibility is most affected and favored by the 

 upper current, it was a natural induction to extend such a condi- 

 tion in imagination to other cases of abnormal behavior of sound. 

 A large amount of subsequent labor and attention was devoted to 

 the determination of this important question. 



* Report of Light House Board for 1874, p. 106. 

 121 



