CIECULATION OF THE OCEAN. g^Y 



evidenced by the rise of the bathymetrical Isotherms, by the keeping 

 down of the Surface Temperature, and by the low (Polar) Salinity of 

 Equatorial surface water. 3. The movement of the upper stratum of 

 oceanic water from the Equatorial region towards each Pole, as the neces- 

 sary complement of the deep Polar under-flow. 4. The dependence of 

 this double movement upon the disturbance of hydrostatic equilibrium 

 constantly maintained by Polar cold and Equatorial heat. The opposition 

 which has been raised to this doctrine of a thermal circulation has mainly 

 rested on one or both of two pre-conceptions : — 1. The origination of all 

 oceanic movements in the surface-action of Wind. 2. The sufficiency of 

 the Gulf Stream to produce the amelioration of the climate of north- 

 western Europe. 



Sir Wyville Thomson makes the following remarks on this subject :* 



«* All the facts of Temperature distribution in the Atlantic appear to 

 favour the view that the entire mass of Atlantic water is supplied by an 

 indraught from the Southern Sea, moving slowly northward, and inter- 

 rupted at different heights by the continuous barriers which limit its 

 different basins ; but this involves the remarkable phenomenon of a vast 

 body of water constantly flowing into a cul de sac from which there is no 

 exit. "When I suggested this view some years ago, I was asked, very 

 naturally, how it was possible that more water could flow into the Atlantic 

 than flowed out of it, and at that time I could see no answer to the 

 question, although I felt sure that a solution must come some day. Now 

 it seems simple enough ; but in order to understand the conditions fully, 

 I would ask my readers to recall the appearance of the Atlantic, and of 

 the Pacific also, which is under exactly the same conditions. On the 

 globe one sees much more clearly than on a map that the Atlantic is a 

 mere tongue, as it were, of the great ocean of the water hemisphere 

 stretching up into the land. The Arctic Ocean, with which it is in con- 

 nection, is again a very limited sea, and nearly landlocked. The North 

 Pacific is another gulf from this ' water-hemisphere,' but one vastly wider 

 and of greater extent ; while the South Pacific is included within the 

 ' water hemisphere.' 



"Although from the meridional extension of the continents to the south- 

 ward, the water of the Atlantic is, as I have shown, directly continuous, 

 layer for layer, with the water of the Antarctic basin, it must be looked 

 upon not as being in connection with that basin only, but as being a por- 

 tion of the great ocean of the water-hemisphere ; and over the central 

 part of the water-hemisphere, precipitation is certainly greatly in excess 

 of evaporation, while the reverse is the case in its extensions to the north- 

 ward. The water is therefore carried off by evaporation from the northern 

 portions of the Atlantic and of the Pacific, and this vapour is hurried down 

 towards the great zone of low barometric pressure in the southern 

 hemisphere ; the heavy, cold water welling up from the southward into 

 the deepest parts of the northward extending troughs, to which it has free 



Voyage of the Challenper ; the Atlantic," vol. ii., pages 324—326. 



