46 THREE CRUISES OF THE " BLAKE." 



As early as in Cook's second voyage, Foster attempted to ob- 

 tain the temperature of the ocean below the surface. As one 

 of the results obtained by Krusenstern's circumnavigating voy- 

 age, it was supposed that the temperature of the oceans at great 

 depths was uniform. But all the earlier observations were de- 

 fective from inaccurate soundings, and from the absence of 

 attempt to correct the temperature observations for pressure. 

 Among these we may mention those of the Arctic expeditions 

 of the Rosses, the Scoresbys, the Parrys, Franklin, and others. 

 As early as 1773, in Phipps's Arctic expedition, temperature 

 observations were taken at considerable depths. 



Lenz. on Kotzebue's second voyage, corrected his observations 

 for jnessure, and obtained a temperature of 3.05'' C. at a depth 

 of about 960 fathoms. He was the first to establish the fact 

 that at the equator the cold water (bathymetrical isotherms) was 

 much nearer the surface than in the more temjDerate zones either 

 to the north or south of it. Yet, until a comparatively recent 

 time the idea was prevalent that below a certain depth the ocean 

 preserved a uniform temperature of 4° C, although we possessed 

 the very definite observations taken by the United States Coast 

 Survey officers, which annually recorded lower temperatures, 

 taken with instruments improved for each year's work. It was 

 not until the Miller-Casella thermometer came into general use, 

 after being employed by the " Lightning " and " Porcupine," 

 that extensive thermometric observations were made sufficiently 

 accurate to serve as the foundation of oceanic temperature sec- 

 tions. We now have the broad outUnes of ocean tempera- 

 tures, thanks mainly to the observations made under the direc- 

 tion of the United States Coast Survey, and by the " Lightning," 

 " Porcupine," ^ the " Tuscarora," the " Challenger," and the 

 "Gazelle," with a number of other vessels of the Swedish, 

 Danish, Italian, Norwegian, EngKsh, French, and American 

 navies. 



1 Pouillet and Humboldt seem both to hypothesis of an interchange of water 

 have been struck with the importance of between the poles and equatorial regions, 

 some of the early observations on deep- depending on a difference of tempera- 

 sea temperature, and Pouillet distinctly ture ; but this seems to have made no 

 states that the phenomena of ocean tem- impression on geographers, 

 peratures could be best explained by the 



