1875] 



WKITINGS OF JOSEPH HENRY. 



467 



apart with equal velocity would in two minutes be separated 

 by a space represented by 4, which would indicate the audi- 

 bility of the sound moving against the wind, and for the 

 same reason the other vessel, hearing the sound seven min- 

 utes longer, would have the additional space represented by 

 14, and adding to this four spaces, we have 18 to represent 

 the audibility of the sound in the direction approximating 

 that of the wind. 



The following table exhibits at one view the results of the 

 foregoing experiments, which relate to sound moving against 

 the wind and with the wind, reduced to miles: 



These results are in accordance with those of all the direct 

 observations on the effect of wind on sound, which had pre- 

 viously been made by the Light-House Board, with the excep- 

 tion of those at Sandy Hook in September, 1874, as given in 

 the last report, in which the sound was heard from a steamer 

 farther against the wind than in the direction of the wind. 

 This anomaly was explained by the existence of an upper 

 current of air moving in an opposite direction to that at the 

 surface, in accordance with the hypothesis of the refraction 

 of sound. 



It will be observed that four of the experiments give ex- 

 actly the same distances to represent the audibility of sound 

 with and against the wind. This co-incidence was not ob- 

 served until after the notes were collated for discussion, and 

 (if not accidental) was due to the equal velocity of the wind, 

 and the general conditions of the atmosphere on the two 

 days. 



To give a definite idea of these relations, we have plotted 



