1874] WRITINGS OF JOSEPH HENRY. 371 



portance in connection with the light-house service of the 

 United States. The north-eastern coast of the United States 

 on the Atlantic, and the entire western coast on the Pacific, 

 included in our territory, are subject — especiall}' during the 

 summer months, to dense fogs which greatly impede naviga- 

 tion as well as endanger life and property. 



The origin of the fogs on our coast is readily explained by 

 reference to a few simple principles of physical geography. 

 In the Atlantic ocean there exists a current of warm water 

 proceeding from the Gulf of Mexico between Cuba and 

 Florida which flows along our coast to the latitude of about 

 35°, and turning gradually to the eastward crosses the At- 

 lantic and impinges against the coast of northern Europe. 

 Throughout its entire course, on account of the immense 

 capacity of water for heat, the temperature of the stream is 

 greater that that of the ocean on either side. In addition to 

 this stream the Atlantic ocean is traversed by another cur- 

 rent of an entirely opposite character, one of cold water, 

 which coming from the arctic regions down Davis's Strait, 

 is thrown by the rotation of the earth, against our coast, 

 passing between it and the Gulf-stream, and sinking under 

 the latter as it approaches the southern extremity of the 

 United States. These conditions are those most favorable to 

 the production of fogs, since whenever the warm air, sur- 

 charged with moisture, is blown from the Gulf-stream in 

 the Atlantic — over the arctic current along the coast, and 

 mingles with the cold air of the latter, a precipitation of its 

 vapor takes place in the form of fog. Hence, especially in 

 summer, when the wind in the eastern part of the United 

 States is from a south-easterly direction, fogs prevail. As 

 we proceed southerly along the coast, the fog-producing 

 winds take a more easterly direction. 



A somewhat similar circulation in the Pacific ocean pro- 

 duces fogs on the western coast of the United States. In this 

 ocean a current of warm water, starting from the equatorial 

 regions, passes along the shores of China and Japan, and 

 following the general trend of the coast, turns eastward and 

 continues along our shore. The northern part of this cur- 



