1874] WRITINGS OF JOSEPH HENRY. 417 



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 entirel}'' obstructed. 



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

 having been cooled in the northern regions, in its passage 

 southward gives rise to cold and warm water in juxta- 

 position, or in other words a current of the former through 

 the latter, and hence whenever a wind blows across the cur- 

 rent 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 of the science of acoustics 

 to the former, is however 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 a mile or two in thickness, is sufficient to 

 obscure the image of the sun, the intensity of the light of 

 which is far greater than that of any artificial light, it 

 must be evident that optical means are insufficient for ob- 

 viating 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 important duties of 

 the Light-House Board. 



In studying this subject it becomes a question of impor- 

 tance to ascertain whether waves of sound (like those of light) 

 are absorbed or stifled by fog; on this point however ob- 

 servers disagree. While from the striking analogy in many 

 respects between light and sound, the opinion has largely 

 prevailed that sound is impeded by fog, the opposite opinion 

 has been adopted by some observers — that sound is in some 

 instances better heard during a fog than in clear weather. 

 To settle this question definitely the Light-House Board has 

 directed that at two light-houses on the route from Boston 

 27 



