546 RESEARCHES IN SOUND. 



in hearing- the sound distinctly on approacliing the station at the distance 

 of from six to four miles, then losing it through a space of about three 

 miles, and not hearing it again until withi]i about a quarter of a mile of 

 the instrument, when it becomes suddenly audible in almost full jiower. 

 This i)heuomenon is always noticed when the vessel is api^roaching the 

 signal from the southwest, and the wind is in the same or in a southerly 

 direction, and therefore opposed to the direction of the sound from the 

 station, Avhich is the case during a fog. Commander Picking, having 

 frequently received complaints from masters of vessels as to losing the 

 sound at this i)lace, concluded to verify the facts by his own observation. 

 For this purpose he embraced the oppormnity of an inspection-tour in 

 July, 1877, to approach the station from the southwest during a fog. 

 In his own words, he heard the sound distinctly through a space of from 

 six to four miles, then lost it and could hear nothing until within a 

 quarter of a mile of the station, when the blast of the whistle burst 

 forth in full sound. The wind at this time was from the southward, or 

 against the sound. This cessation in the hearing of the sound could 

 not have been due to the failure of the instrument to emit sound, since 

 its operation is automatic when once started, and in this case the fog so 

 lifted on nearing the station as to admit the observation of the pufts of 

 steam emitted at each blast of the whistle. 



On a previous occasion General Duane and Mr. Edwards on api3roach- 

 ing the same signal from the southwest heard the sound at about six 

 miles distance, then lost it, and did not again hear it until withiu about 

 a quarter of a mile. The wind in this instance was also the same as that 

 in the observation of Commander Picking, namely, from the southwest. 



So well established was this j^henomenou that General Duane at- 

 tempted to remedy the evil by elevating the duplicate w^histle (with 

 •which every station is provided) to a height twenty-two feet above the 

 level of the other whistle, by placing it on the upper end of a tube. 

 But this arrangement produced no beneficial effect. 



In the morning of September 4, 1877, on which we commenced our ex- 

 periments, the weather was clear, the wind west-southwest, the velocity 

 from ten to twelve miles, remaining nearly constant during the day. 

 Our first object was to verify by direct observation the several features of 

 the phenomenon, and for this purpose we steamed to the southward, or 

 directly to the windward, from the station through the region in which 

 the abnormal phenomena had been noticed. The i^ressure of the atmos- 

 phere, as indicated by an aneroid barometer, was 28.9 inches. The tem- 

 perature of the air was 67^ Fahrenheit ; that of the water at various 

 points along our course was 58°, except at two i^oints where the ther- 

 mometer indicated 57°, This difference was too small to have any i^er- 

 ceptible effect on the density of fhe rapidly moving air which was 

 passing over the surface of the water. As we increased our distance 

 from the signal the sound slightly diminished in loudness until the dis- 

 tance was between a quarter and half a mile, when it suddenly ceased 



