1877] WRITINGS OF JOSEPH HENRY. 497 



experiments of 1875, the sound of the whistles of the steamers 

 was heard for a certain distance, then ceased to be heard for 

 a considerable interval, after which it was heard again. 

 Furthermore the pilots of the steamboats from New York to 

 Boston report that the sound of the automatic buoy is found 

 to be intermittent, being heard at a distance, then becoming 

 inaudible, and heard again as the steamer approaches the 

 source of sound. 



From all the facts gathered on this subject, I think it 

 highly probable that in all cases in which sound moving 

 against the wind is thrown up above the head of the ob- 

 server, it tends to descend b}' the lateral spread of the sound- 

 wave and to reach the earth again at a distance ; the con- 

 ditions however for the actual production of this effect are 

 somewhat special, and will depend upon the amount of the 

 initial refraction and the quantity of the sound-waves. Be- 

 sides the lateral spread of the sound-wave there are two other 

 causes sufficient (in certain cases) to bring a portion of the 

 sound-waves which have been elevated in the air — back again 

 to the earth : the first is when an upper current of wind is 

 moving in an opposite or approximately opposite direction 

 to that at the surface of the earth, in which case an opposite 

 or downward refraction would take place ; and the second is 

 the case in which the surface-wind is terminated above by 

 Si stratum of comparatively still air ; in this case also, a reverse 

 refraction (but of less amount) would take place, which would 

 tend to bring the sound-wave downward. 



We can readily imagine that a solitary island, cooled by the 

 radiation of heat at night, would every morning send a cur- 

 rent of cold air in all directions from its centre. In this case, 

 the sound from a whistle placed in the centre of the island 

 would be inaudbile in a space entirely surrounding it, and 

 thus give rise to a condition mentioned by General Duane, 

 in which a fog-signal appeared to be surrounded by a belt 

 of silence. 



September 5, 1877. — The next experiment was made on the 

 morning of the 5th, on leaving the station. In this case we 

 proceeded along the direction of the same line in which the 

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