1877] WRITINGS OF JOSEPH HENRY. 505 



the weather as to clearness or fog, the direction and intensity 

 of the wind, and the surface of the ocean. 



From the observations made at these two points, for more 

 than two years at one station and over a year at the other, 

 the echo may be considered as produced constantly under 

 all conditions of weather, even during dense fogs, since at 

 Block Island it was heard 106 times out of 113, and at Point 

 Judith 50 times out of 57, and on the occasions when it 

 was not heard the wind was blowing a gale, making a noise 

 sufficiently loud to drown the sound of the echo. These 

 results would appear to quite effectually disprove the hypoth- 

 esis that the phenomenon is produced by an acoustic cloud 

 accidentally situated in the prolongation of the axis of the 

 trumpet. An occurrence of such regularity must be due to 

 something more permanent in its effects than a difference in 

 temperature or density of a portion of air — from that of the 

 general atmosphere ; since such a condition could not exist 

 in a dense fog embracing all the region in which the pheno- 

 menon occurs. Indeed, it is difficult to conceive how the 

 results can be produced, even in a single instance, from a 

 flocculent portion of atmosphere in the prolongation of the 

 axis of the trumpet; since a series of patches of clouds of 

 dififerent temperature and density (if sufficient) would tend 

 to absorb or stifle by repeated reflections — a sound projected 

 into their interior, rather than to transmit it to the ear of 

 the observer. 



The question therefore remains to be answered : What is 

 the cause of the aerial echo? As I have stated, it must in 

 some way be connected with the plane of the horizon. The 

 only explanation which suggests itself to me at present, is 

 that the spread of the sound — which fills the whole atmos- 

 phere from the zenith to the horizon with sound-waves, may 

 continue its curvilinear direction until it strikes the surface 

 of the water at such an angle and direction as to be reflected 

 back to the ear of the observer. In this case, the echo would 

 be heard from a perfectly flat surface of water; and as differ- 

 ent sound-rays would reach the water at different distances 

 and from different azimuths, they would produce the pro- 



