348 MEMORIAL OF JOSEPH HENRY. 



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, 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 more affected and favored by the 

 upper current, it was a natural induction to extend such a condition 

 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. 



In 1872 it was observed from on board a steamer approaching 

 Portland Head station in the harbor of Portland ( Maine ) that the 

 fog-signal which had been distinctly heard through many miles, 

 was lost to the ear when within two or three miles of the point, 

 that it continued inaudible throughout the nearer distance of a mile 

 or so, and that it was again heard as the station was neared. At 

 Whitehead light station on a small rocky island about a mile and a 

 half from the coast, (being some 65 miles northeast of Portland 

 Head,) it was observed on board a steamer approaching the station 

 during a thick fog, that the signal (a 10-inch steam whistle) though 



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



