402 THE DEPTHS OF THE SEA. [CHAP. vin. 



depressions in the surface isotherms, the balance of 

 probability seems to be in favour of the view that the 

 conditions of temperature and the slow movement of 

 this vast mass of moderately cold water, nearly two 

 statute miles in depth, are to be referred to an 

 Antarctic rather than to an Arctic origin. 



The North Atlantic Ocean seems to consist first of 

 a great sheet of warm water, the general northerly 

 reflux of the equatorial current. Of this the greater 

 part passes through the Strait of Florida, and its 

 north-easterly flow is aided and maintained by the 

 anti-trades, the whole being generally called the 

 Gulf-stream. This layer is of varying depths, ap- 

 parently from the observations of Captain Chimmo 

 and others, thinning to a hundred fathoms or so in 

 the mid- Atlantic, but attaining a depth of 700 to 800 

 fathoms off the west coasts of Ireland and Spain. 

 Secondly of a ' stratum of intermixture ' which ex- 

 tends to about 200 fathoms in the Bay of Biscay, 

 through which the temperature falls rather rapidly ; 

 and thirdly, of an underlying mass of cold water, 

 in the Bay of Biscay 1,500 fathoms deep, derived as 

 an indraught falling in by gravitation from the 

 deepest available source, whether Arctic or Antarctic. 

 It seems at first sight a startling suggestion, 

 that the cold water filling deep ocean valleys in the 

 northern hemisphere may be partly derived from 

 the southern ; but this difficulty, I believe, arises 

 from the idea that there is a kind of diaphragm at 

 the equator between the northern and southern ocean 

 basins, one of the many misconceptions which follow 

 in the train of a notion of a convective circulation in 

 the sea similar to that in the atmosphere. There is 



