500 MEMORIAL OF JOSEPH HENRY. 



at the base of the cliff on which stands the light-house. The wind 

 was blowing seven miles an hour. The vessel sounding its steam 

 whistle steamed away from the light, going in the direction towards 

 which the wind was blowing. The listener on the top of the light- 

 house heard the sound four times longer than the observer on the 

 beach ; but when the vessel ran away from the light-house against 

 the wind, the sound disappeared first to the observer on the top of 

 the light-house. 



It was also observed that sometimes on approaching a fog-horn 

 from a distance the intensity of its sound would gradually increase, 

 then die down quite rapidly and become inaudible through a space 

 of from three to four miles, and often would not reappear till the 

 vessel was within a mile of the fog-horn. Often when the sound 

 came to the listener against a moderate wind the fog-horn would 

 become inaudible at a distance of three or four miles, while on other 

 days, when the wind was going with the sound, the listener had to 

 sail away 25 miles before the horn ceased to be heard. Observa- 

 tions made at Block Island and Point Judith showed this fact in 

 the following manner: The distance between these fog-horns is 

 seventeen miles, and the sound of one can be distinctly heard at the 

 other when the air is quiet.and homogeneous ; but if the wind blows 

 from one towards the other the listener at the station from which 

 the wind blows is unable to hear the other horn. 



The most remarkable series of Henry's observations was made at 

 Whitehead Station, Maine, situate on a small island about one mile 

 and a half off the coast. The vessel was approaching the station 

 from the south and with the wind. "The belt of silence" was 

 reached and traversed, and then the sound reappeared again. This 

 happened whether the vessel was steaming towards or away from 

 the station, the wind remaining all the while southerly. But during 

 these observations on the vessel the sounds of the steamer's whistle 

 were heard without interruption at the station. Now the steamer's 

 course was directed to the other side of the station ; and steaming 

 away from the fog-horn and against the wind the whistle at the 

 station was constantly heard by those on the vessel, but those at the 

 station now perceived the steamer's whistle to go into and out of 

 " the belt of silence." 



