ii APPENDIX. 



r 

 air sui'charged with moisture with colder air, and nowhere on the 

 surface of the earth do more favorable conditions exist for pro- 

 ducing fogs than on both our Atlantic and Pacific coasts. On 

 the Atlantic the cold stream of water from the polar regions in 

 its passage southward, on account of the rotation of the earth, 

 passes close along our eastern coast from one extremity to the 

 other, and parallel to this but opposite in direction, for a con- 

 siderable distance is the great current of warm water known as 

 the Gulf stream. Above the latter the air is constantly surcharged 

 with moisture, and consequently whenever light winds blow from 

 the latter across the former, the vapor is condensed into fog, and 

 since in summer along our eastern coast the southerly wind pre- 

 vails, we have during July, August, and September, especially on 

 the coast of Maine, an almost continuous prevalence of fogs so 

 dense that distant vision is entirely obstructed. 



On the western coast the great current of the Pacific, after 

 having been cooled in the northern regions, in its passage south- 

 ward gives rise to cold and warm water in juxtaposition, or, in 

 other words, a current of the former through the latter, and hence 

 whenever a wind blows across the current of cold water, a fog is 

 produced. 



From the foregoing statement it is evident that among the aids 

 to navigation fog-signals are almost as important as light houses. 

 The application, however, of the science of acoustics to the former 

 is far less advanced than is that of optics to the latter. Indeed, 

 attempts have been made to apply lights of superior penetrating 

 power, as the electric and calcium lights, to supersede the imperfect 

 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 is subject to fogs, renders the investigation of the 

 subject of fog-signals one of the most important duties of the 

 Light House Board. 



In studying this subject it becomes a question of importance to 

 ascertain whether waves of sound, like those of light, are absorbed 

 or stifled by fog ; on this point, however, observers disagree. At 

 first sight, from the very striking analogy which exists in many 

 respects between light and sound, the opinion has largely pre- 

 vailed that sound is impeded by fog. But those who have not 

 been influenced by this analogy have in some instances adopted 

 the opposite 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 



(46) 



