PHILOSOPHICAL SOCIETY OF WASHINGTON. Ill 



heard against the wind is greater than the number of times it has 

 been heard with the wind ; a result which, though unexpected, 

 is not in discordance with previous assumptions. 



It will be recollected by the Society that I have, in previous 

 years, mentioned a remarkable phenomenon, which I have deno- 

 minated the '■^ ocean echo.'''' This has also been observed by the 

 scientific adviser of the Trinity Board, and is considered by him 

 as the key of all the abnormal phenomena of sound observed, and 

 as a special illustration of the truth of his hypothesis that 

 such abnormal phenomena are produced by invisible clouds of 

 flocculent atmosphere. The phenomenon in question consists in a 

 reverberation in the form of an echo from a point in the verge of 

 the horizon to which the axis of a fog-trumpet is directed. 



In regard to this I first adopted the provisional hypothesis 

 that this was produced l)y a reverberation from the crests of the 

 waves of the ocean, but it having been stated that the same phe- 

 nomenon is exhibited while the sea is smooth, this assumption must 

 be abandoned, or in some way modified to suit the observed facts. 

 To test the hypothesis of the reverberation being due to a re- 

 flection from an invisible cloud on the verge of the horizon, the 

 trumpet of the large syren on Block Island was gradually ele- 

 vated from a horizontal to a vertical position, and while in this 

 position it was sounded at intervals for several days; but in no 

 case was an echo heard from the zenith, but in every instance an 

 echo was returned from the horizon around i^s whole circumference. 



In another experiment with a vertical trumpet at Little Gull 

 Island, a small cloud, from which a few drops of rain fell on the 

 area of the base of the Lighthouse, passed directly across the 

 zenith, and during this passage no echo was observed from the 

 cloud, although the trumpet directed toward it was sounded 

 several times in succession. 



Again, in order to obtain additional facts in regard to the 

 nature of this echo, observation was made from a vessel, bj 

 steaming out directly as if into the region of the echo ; i.e, in 

 the direction of that point in the horizon from which the echo 

 appeared to emanate. 



In this case the loudness of the echo appeared, as we advanced, 

 to gradually diminish, and to spread itself through a much longer 

 arc of the horizon, while the duration of the echo increased ia 

 time. 



