524 WRITINGS OF JOSEPH HENRY. [1877 



In order to collect data for further use in the explication 

 of the phenomena, the light-keepers at Block Island and 

 Montauk Point (the eastern portion of Long Island) have 

 been directed to blow the fog-signals for an hour on every 

 Monday morning, each noting whether he can hear the sound 

 from the other station; observing at the same time the direc- 

 tion of the wind and the apparent motion of the clouds. 



From the result of these observations during the year it 

 appears that the clouds give frequent indications of adverse 

 wind currents, and that the number of times the sound has 

 been 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 pre- 

 vious years mentioned a remarkable phenomenon, which I 

 have denominated the " ocean echo." This has also been ob- 

 served by the distinguished scientific adviser of the Trinity 

 Board, and is considered by him as the key of all the ab- 

 normal 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 hypothe- 

 sis that this was produced by a reverberation from the crests 

 of the waves of the ocean ; but it having been stated that the 

 same phenomenon 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 reflection from an invisible 

 cloud on the verge of the horizon, the trumpet of the large 

 siren on Block Island was gradually elevated from a hori- 

 zontal 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 



