554 EESEAECHES IN SOUND. 



of the siren, because tlic study of it has been considered to offer tlie 

 easiest access to the solution of the question as to the cause of all the 

 abnormal i:>hcnomena of sound, and also because it is in itself an object 

 of much scientific interest. 



In my iDrevious notice of this phenomenon, in the report of the light- 

 house board for 1874, 1 suggested that it might be due to the reflection 

 from the crests of the waves of the ocean ; but as the phenomenon has 

 been observed during all conditions of the surface of the water, this ex- 

 planation is not tenable. 



Another hypothesis has been suggested, that it is due to a flocculent 

 condition of the atmosphere, or to an acoustic invisible cloud, of a 

 density in diflerent parts differing from that of the general atmosphere 

 at the time. To test this hypothesis experimentally, the large trumpet 

 of the siren was gradually elevated from its usual horizontal position to 

 a vertical one. In conception, this experiment appears very simple, but 

 on account of the great weight of the trumpet, it required the labor of 

 several men for two days to complete the arrangements necessary to the 

 desired end. The trumpet, in its vertical position, was sounded at in- 

 tervals for two days, but in no instance was an echo heard from the 

 zenith, but one was in every case i)roduced from the entire horizon. 

 The echo appeared to be somewhat louder from the land portion of the 

 circle of the horizon than from that of the water. On restoring the 

 trumpet to its horizontal position, the echo gradually increased on the 

 side of the water, until the horizontal position was reached, when the 

 echo, as usual, ax)i)eared to proceed from an angle of about twenty de- 

 grees of the horizon, the middle of which was in the prolongation of 

 the axis of the trumx^et. A similar experiment was made with one of 

 the trumijets of the two sirens at Little Gull Island. In this case the 

 trumjiet was sounded in a vertical position every day for a week with 

 the same result. On one occasion it hajipened that a small cloud passed 

 directly over the island on which the light-house is erected, and threw 

 down on it a few drops of rain. At the moment of tlie passage of this 

 cloud the trumpet was sounded, but no echo was produced. 



From these experiments it is evident that the phenomenon is in some 

 way connected with the plane of the horizon, and that during the contin- 

 uance of the experiment of sounding the trumpets while directed toward 

 the zenith no acoustic cloud capable of producing reflection of sound ex- 

 isted in the atmosphere above them. 



Another method of investigating this phenomenon occurred to me, 

 which consisted in observing the effects produced on the ears of the ob- 

 server by approaching the origin of the echo. For this i^urpose, during 

 the sounding of the usual intervalpf twenty seconds of the large trumpet 

 at Block Island, observations were made from a steamer which proceeded 

 from the station into the region of the echo and in the line of the pro- 

 longation of the axis of the trumpet, with the following results : 



1. As the steamer advanced, and the distance from the trumi^et was 



