1874] WRITINGS OF JOSEPH HENRY. 415 



velocity of the sound, yet this bending of the wave being 

 continuous throughout its entire course, a marked effect 

 must be produced, 



A precisely similar effect will be the result, but perhaps 

 in a considerably greater degree, in case an upper current 

 is moving in an opposite direction to the lower, when the 

 latter is adverse to the sound; and in this we have a logical 

 explanation of the phenomenon observed by General Duane, 

 in which a fog-signal is well heard during the occurrence of 

 a north-east snow-storm. Certainly this phenomenon can- 

 not be explained by any peculiarity of the atmosphere as to 

 variability of density, or of the amount of vapor which it 

 may contain. 



The first phenomenon of the class mentioned by General 

 Duane which I had the good fortune to witness was in com- 

 pany with Sir Frederick Arrow, and Captain Webb, of the 

 Trinity House, London, in their visit to this country in 1872. 

 At the distance of two or three miles from an island in the 

 harbor of Portland, Maine, on which a fog-signal was placed, 

 the sound which had been distinctly heard on approaching 

 the island was lost for nearly a mile, and slightly regained 

 at a less distance. On examining the position of the fog- 

 signal, which was situated on the farther side of the island 

 from the steamer, we found it placed immediately in front of 

 a large house with rising ground in the rear, which caused 

 a sound-shadow, into which the sound was projected at a dis- 

 tance, (on account of the lateral divergence of the rays,) but 

 not in the immediate vicinity of the island. In the same 

 year, I made an excursion in one of the light-house steamers, 

 with Captain Selfridge, to an island on the coast of Maine, 

 at which abnormal phenomena were said to have been ob- 

 served ; but on this occasion no variation of the sound was 

 noted, except that which was directly attributable to the 

 wind, the signal being heard much farther in one direction 

 than in the opposite. 



