1870.] 



Deep-sea Researches. 



215 



be continually flowing off, a constant motion from above downwards will 

 continue to take place in the entire column, so long as a fresh stratum is 

 continually being exposed to the influence of surface-cold. 



127. On the other hand, the agency of Equatorial Heat, though directly 

 operating on only a thiu film of surface-water, will gradually pump-up (so 

 to speak) the Polar water which has reached its area by creeping along 

 the deepest parts of the intermediate Oceanic basins. For since, as already 

 shown, an indraught of the upper stratum surrounding the Polar basin must 

 be continually going on, the place of the water thus removed must be sup- 

 plied by w r ater drawn from a still greater distance ; and thus the movement 

 will be propagated backwards, until it affects the upper stratum of the 

 Equatorial area itself, which will flow off Pole-wards, bearing with it a 

 large measure of Heat. The cold and dense Polar water, as it flows in at 

 the bottom of the Equatorial column, will not directly take the place of 

 that which has been draughted off from the surface ; but this place will 

 be filled by the rising of the whole superincumbent column, which, being 

 warmer, is also lighter than the cold stratum beneath. Every new arrival 

 from the Poles will take its place below that which precedes it, since its 

 temperature will have been less affected by contact with the warmer water 

 above it. In this way an ascending movement will be imparted to the 

 whole Equatorial column, and in due course every portion of it will 

 come under the influence of the surface-heat of the Sun. This heat will 

 of course raise the level of the Equatorial column, without augmenting its 

 absolute weight ; and will thus add to the tendency of its surface-stratum 

 to flow towards the lowered level of the Polar area. But as the super- 

 heating extends but a short way down, and as the temperature of the 

 water beneath, down to the " stratum of intermixture " (§ 80), is very mo- 

 derate, whilst the water below that stratum is almost as cold as that of 

 the Polar basin, it is evidently in the latter that the force which maintains 

 this vertical circulation chiefly originates. 



128. Here, then, we have a. vera causa for a General Oceanic Circulation, 

 which, being sustained only by the unequal distribution of Solar Heat, 

 will be entirely independent of any peculiar distribution of Land and 

 Water, provided always that this does not prevent the free communication 

 between the Polar and Equatorial Oceanic areas, at their depths as well as 

 at their surface. That this agency has been so little recognized by 

 Physical Geographers, we can only attribute to the prevalence of the 

 erroneous idea of the uniform deep-water temperature of 39°, of which 

 the Temperature-observations made in our Expeditions of 1868 and 

 1869 have shown the fallacy. Until it is clearly apprehended that Sea- 

 water becomes more and more dense as its temperature is reduced, and that 

 it consequently continues to sink until it freezes, the immense motor 

 power of Polar Cold cannot be apprehended ; but when once this has 

 been clearly recognized, it is seen that the application of cold at the 

 surface is, in the case of Sea-water, precisely equivalent as a moving force 



