46 THREE CRUISES OF THE “ BLAKE." 
As early as in Cook's socond 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 pressure, 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 temperate 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., aithough 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. 16 was 
not until the Miller-Casella thermometer came into general use, 
after being employed by the “Lightning” and * Porcupine,” 
that extensive thermometrie observations were made sufficiently 
accurate to serve as the foundation of oceanic temperature sec- 
tions. We now have the broad outlines 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 
4 Gazelle," with a number of other vessels of the Swedish, 
Danish, Italian, Norwegian, English, 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 
