522 WRITINGS OF JOSEPH HENRY. [1877 



found fertile in new results in previous investigations of the 

 same subject. To test this and to ascertain the dependence 

 of the phenomenon on the wind, the position of the focus or 

 the origin of the sound was changed. For this purpose the 

 whistle of the steamer was sounded while a portion of the 

 observing force was placed at the station ; by this arrange- 

 ment it was found that while the vessel, in reference to the 

 sound of the signal at the station, passed through a region 

 of silence, the observers at the station who gave attention to 

 the sound from the steamer heard no interruption of the 

 signal. This experiment was repeated each way, going to 

 and coming from the station. 



From this result it appears that the sound going with the 

 wind was heard at every point on its course, while the 

 sound moving against the wind was suddenly lost at a given 

 point and not recovered again until a distance of more 

 than a mile had been traversed by the vessel. This result 

 was in strict conformity with the theory of refraction ; in the 

 case of the sounds travelling against the wind, the upper 

 part of the wave would usually be more retarded than the 

 lower, and consequently the sound wave would be thrown 

 upward above the head of the observer. At a given altitude 

 this difference of velocity would cease, and by the general 

 tendency of sound to spread, the sound wave would again 

 reach the earth. 



But to test this still further, and to show that the locality 

 was not an essential condition of the existence of the interval 

 of silence, the experiment was repeated on the opposite side 

 of the station, so that the sound from the fog-signal would 

 move in the direction of the wind. Some of the observers 

 were placed at the station and the others remained on 

 board the vessel; both instruments were sounded, the one in 

 the intervals of the sounding of the other. 



In this case the sound from the fog-signal was continuous 

 to those on board of the vessel through a distance of over 

 four miles, and could probably have been heard many miles 

 farther, but the progress of the steamer in that direction was 

 stopped by the land. 



