EESEAECHES IN SOUND. 491 



and calcium lights, to supersede the imi)erfect fog-signals in use. When 

 however we consider the fact that the absorptive power of a stratum of 

 cloud, which is but a lighter fog, of not more than two or three miles in 

 thickness, is sufficient to obscure the image of the sun, the intensity of 

 the light of which is greater than that of any artificial light, it must be 

 evident that optical means are insufficient for obviating the difficulty in 

 question. 



The great extent of the portions of the coast of the United States 

 which are subject to fogs renders the investigation of the subject of fog- 

 signals one of the most imiiortant duties of the Light-House Board. 



In studying this subject it becomes a question of importance to ascer- 

 tain whether waves of sound, like those of light, are absorbed or stifled 

 by fog; on this point howe^^er, observers disagree. At first sight, from 

 the very striking analogy which exists in many respects between light 

 and sound, the opinion has largely prevailed that sound is impeded by fog. 

 But those v.ho have not been influenced by tbis analogy have in some 

 instances adopted the oi)posite opinion — that sound is better heard during 

 a fog than in clear weather. To settle this question definitely the Light- 

 House Board have directed that at two light-houses on the route from 

 Boston to Saint John the fog-signals 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 distance of the boat when the sound 

 is first heard on approaching the signal and is last heard on receding 

 from it. The boats above meutioned estimate their distance vrith con- 

 siderable precision by the number of revolutions of the i)addle-wheel as 

 recorded by the indicator of the engine, and it is hoped by this means 

 to definitely decide the point in question. We think it probable that 

 fog does somewhat dimiuish the jienetrating power of sound, or in 

 other words, produce an eflect 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 de- 

 gree as to be of no practical importance, or, in other words, the exist- 

 ence of a fog is a true, but, we think, a wholly insufficient cause of di- 

 minution of sound, which view is borne out by the great distance at 

 which our signals are heard diu-ing a dense fog. 



Another cause, w^hich without doubt is a true one, of the diminution 

 of the penetrating power of soun<l is the varying density of the atmos- 

 phere, from heat and moisture, in long distances. The effect of this, 

 however, would apparentlj^ be to slight!}' distort the wave of sound 

 leather than to obliterate it. However this may be, we think, from all 

 the observations 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 



