1870.] 



Deep-sea Researches, 



219 



may be very rapid. Hence the average of 65° assumed by Mr. Croll on 

 the basis of observations made at considerable intervals of depth is alto- 

 gether unreliable. 



VIII. That the most recent and trustworthy observations indicate 

 that the edge of the Gulf-stream to the north-east of the i Eanks of 

 Newfoundland, is so thinned out and broken up by interdigitation with 

 Polar currents, that its existence as a continuous current beyond that 

 region cannot be proved by observations, either of Temperature or Move- 

 ment. ! 



IX. That the Gulf-stream and other local currents put in motion 

 by the Trade-winds or other influences acting on the surface only, will 

 have as their complement in a horizontal circulation return surface- 

 currents; and that the horizontal circulation of which the Atlantic Equatorial 

 Current and the Gulf-stream constitute the first part, is completed — so far 

 as the Northern Hemisphere is concerned — partly by the direct return of 

 one large section of the Gulf-stream into the Equatorial Current, and, 

 as to the other section, by the superficial Polar currents, which make 

 their way southwards, the principal of them even reaching the commence- 

 ment of the Gulf- stream. 



132. In conclusion it may be added that the doctrine of a General 

 Vertical Oceanic Circulation is in remarkable accordance with the fact now 

 placed beyond doubt by the concurrent evidence of a great number of 

 observations, that whilst the Density of Oceanic water, which is lowest in 

 the Polar area, progressively increases as we approach the Tropics, it again 

 shows a decided reduction in the Intertropical area. It has been thought 

 that an explanation of this fact is to be found in the large amount of rain- 

 fall, and of inflow of fresh water from great rivers, in the Intertropical 

 region; but it is to be remembered that the surface-evaporation also is 

 there the most excessive ; so that some more satisfactory account of the 

 fact seems requisite. Such an explanation is afforded by the doctrine here 

 advocated, the Polar water which flows towards the Equator along the 

 bottom of the ocean-basins being there (so to speak) pumped up and brought 

 to the surface *. And it is not a little confirmatory of the views advanced in 

 this Report, that in a recent elaborate discussion of the facts relating to 

 the Comparative Density of Oceanic Water on different parts of the Earth's 

 surface, the doctrine of a General Vertical Circulation is advocated as 

 affording the only feasible rationale of them f. 



* That water of a lower should thus underlie water of a higher degree of Salinity, 

 in travelling from the Pole to the Equator, is not difficult to account for, when the rela- 

 tive Temperatures of the two strata are borne in mind. 



t " Densite Salure et Courants de l'Ocean Atlantique," par Lieut. B. Savy, Annales 

 Hydrographiques, 1868, p. 620. 



