418 WRITINGS OF JOSEPH HENRY. [1874 



to Saint John, the fog-signal shall be sounded every day on 

 which the steamboats from these ports pass the station, both 

 in clear and foggy weather, the pilots on board these vessels 

 having, for a small gratuity, engaged to note the actual dis- 

 tance of the boat when the sound is first heard on approach- 

 ing the signal and is last heard on receding from it. The 

 boats above mentioned estimate their distance with consid- 

 erable precision by the number of revolutions of the paddle- 

 wheel as recorded by the indicator of the engine, and it is 

 hoped by this means to definitely decide the point in ques- 

 tion. We think it probable that fog ma}^ very slightly dimin- 

 ish the penetrating power of sound, or in other words pro- 

 duce an effect analogous to that on the propagation of light. 

 But when we consider the extreme minuteness of the parti- 

 cles of water constituting the fog as compared with the 

 magnitude of the waves of sound, the analogy does not hold 

 except in so small a degree as to be of no practical impor- 

 tance, or in other words the existence of a fog if a true — is 

 we think a wholly insufficient cause of diminution of sound ; 

 a view borne out by the great distance at which our signals 

 are heard during a dense fog. 



Another cause of the diminution of the penetrating power 

 of sound (also probably a true one) is the varying density 

 of the atmosphere — from heat and moisture, in long dis- 

 tances. The effect of this however would apparently be 

 to slightly distort the wave of sound rather than to oblite- 

 rate it. However this may be, we think from all the obser- 

 vations we have made, the effect is small in comparison with 

 another cause, viz., that of the influence of wind. During a 

 residence of several weeks at the sea-shore, the variation in 

 intensity of the sound of the breakers at a distance of about 

 a mile, in no case appeared to be co-incident with the varia- 

 tions of an aneroid barometer or a thermometer, but to be in 

 every instance affected by the direction of the wind. 



The variation in the distinctness of the sound of a distant 

 instrument as depending on the direction of the wind is so 

 marked, that we are warranted in considering it the princi- 

 pal cause of the inefficiency in certain cases of the most 



