RESEARCHES IN SOUND. 489 



let ns recall the nature of a beam of sound, in still air, projected in 

 a horizontal direction. It consists of a series of concentric waves per- 

 pendicular to the direction of the beam, like the palings of a fence. 

 Now, if the upper part of the waves has a slightly greater velocity 

 than the lower, the beam wiU be bent downward in a manner some- 

 what analogous to that of a ray of light in proceeding from a rarer to a 

 denser medium. The effect of this deformation of the wave will be 

 cumulative from the sound-center onward, and hence, although the ve- 

 locity of the wind may have no perceptible effect on the velocity of 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 perhai^s in a consid- 

 erably greater degree, in case an upx^er 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 exjilanation of the phenomenon observed by 

 General Duane, in which a fog-signal is only heard during the occurrence 

 of a northeast snow-storm. Certainly this phenomenon cannot be ex- 

 plained 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 company with Sir Frederick 

 Arrow and Cai^tain Webb, of the Trinity House, London, in their visit 

 to this country in 1872. At the distance of two or three miles fr^om an 

 island in the harbor of Portland, Maine, on which a fog-signal was 

 placed, the sound, which had been distinctly heard, was lost on ap- 

 proaching the island for nearly a mile, and slightly regained at a less 

 distance. On examining the position of the fog-signal, which was sit- 

 uated on the farther side of the island from the steamer, we found it 

 placed immediately in front of a large house with rismg ground in the 

 rfear, which caused a sound-shadow, into which, on account of the late- 

 ral divergence of the rays, the sound was projected at a distance, 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 observed, 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. 



