1877] WRITINGS OP JOSEPH HENRY. 523 



From the report of the observers at the station it appeared 

 that as the vessel passed into the distance but one blast was 

 heard during its whole course. In this case (as in the pre- 

 ceding experiment of sailing to the south west) the sound 

 moving against the wind was refracted upward, and as the 

 whistle was but six inches in diameter it did not give suffi- 

 cient volume to again reach the earth by spreading. 



In experiments of previous years the fact has been shown 

 that the sound is heard under certain conditions better when 

 moving against the wind than in the opposite direction. 

 This was notably the case in the experiments made at Sandy 

 Hook in September, 1874, during which a sound from the 

 west was heard at first ivitJi the wind about three times as far 

 as a sound from a similar source was heard from the east, 

 or against the wind ; then the same sound was heard from 

 the west three times as far as from the east after the wind 

 had settled to a calm ; and in a third observation the same 

 phenomenon was observed after the wind had changed to a 

 direction opposite to that of the sound, and had increased to 

 a velocity of ten miles an hour from the east. These effects 

 were afterwards shown to be connected with the fact that 

 the upper wind during the whole day was blowing strongly 

 from the west, and that the apparent changes of the wind 

 were due to currents at the surface, and thus a sufficient ex- 

 planation was given to the phenomena observed. 



It would appear however from the investigations of last 

 summer that the wave of sound which has been refracted 

 upward may descend at a greater distance from its origin 

 than even that at which sound moving with the wind can 

 be heard, probably involving a peculiar case of undulating 

 or compound refraction ; but this requires further investiga- 

 tion. 



Each series of observations gives rise to new questions, and 

 indicates that the subject is one which is rich in new results. 

 Unfortunately however the observations can only be made 

 by the aid of steamers; and these — in the Light-House service, 

 can only occasionally be employed in the rare intervals of 

 more imperative duties. 



