360 THE DEPTHS OF THE SEA. [CHAP. VI11. 
by contact with the surface of the crust of the 
earth, the inevitable conclusion seems to have been 
early arrived at that, if such temperatures existed, 
they must be due to a general oceanic circulation,— 
to surface currents of warm water passing towards 
the poles, and compensating counter-currents of cold 
water from the poles towards the equator. Hum- 
boldt states that he showed, in 1812, “that the low 
temperature of the tropical seas at great depths could 
only be owing to currents from the poles to the 
equator.” ! 
D’Aubuisson, in 1819, also attributed the low 
temperature of the sea at great depths at or near 
the equator to the flow of currents from tie poles.’ 
But although the fact of the existence of currents 
lowering the temperature of deep water in equa- 
torial regions was admitted by various authorities 
in physical geography, little light was thrown upon 
the causes of this circulation. Latterly, the whole 
subject became obscured by the very general adop- 
tion of the doctrine already referred to of a perma- 
nent temperature of 4° C. all over the world beyond 
a certain depth; and it was not until the publi- 
cation of Captain Maury’s fascinating book on the 
‘Physical Geography of the Sea’ had given an extra- 
ordinary stimulus to the study of this department 
of science, that the question was again raised. 
It was natural from its geographical position, and 
from the much greater opportunity which it offered 
for the accumulation of the almost infinite number 
1 Fragments de Géol. et de Climatol. Asiat., 1831. 
Geological Society of London, 1571, 
