360 THE DEPTHS OF THE SEJ. [chap. viii. 



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." l 



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 the poles. 2 



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 9 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 Geol. et de Climated. Asiat., 1831. 



2 Traite de Ceognosie. — Quoted in the Anniversary Address to the 

 Geological Society of London, 1871. 



