258 THE ATLANTIC. [chap. v. 



torn temperature near Fernando Noronha, almost under the 

 equator, is 0°-2 C, close upon the freezing-point ; it is obvious 

 that this temperature was not acquired at the equator, where the 

 mean annual temj)erature of the surface-layer of the water is 21° 

 C, and we may take the mean normal temperature of the crust 

 of the earth as not low^er, at all events, than 8° C. The water 

 must, therefore, have come from a place where the conditions 

 were such as to impart to it a freezing temperature; and not 

 only must it have come from such a place, but it must be con- 

 tinually renewed, however slowly, for otherwise its temperature 

 would gradually rise by conduction and mixture. Across the 

 whole of the Xorth Atlantic the bottom temperature is consid- 

 erably higher, so that the cold water can not be coming from 

 that direction ; on the other hand, we can trace a band of water 

 at a like temperature, at nearly the same depth, continuously to 

 the Antarctic Sea, where the conditions are normally such as 

 to impart to it its low temperature. There seems, therefore, to 

 be no room for doubt that the cold water is welling up into the 

 Atlantic from the Southern Sea ; w^e shall, however, discuss this 

 more fully hereafter. 



The investigation, by this indirect method, of the movements 

 of the water of the oce'an, w^as one of the points to which our 

 attention was very specially directed ; and it was prosecuted 

 throughout the voyage with great care. The method of taking 

 temperature sections w^as first systematically employed, so far as 

 I am aware, by the American Coast Survey in their examination 

 of the Gulf-stream, and some modifications, extending its use to 

 deep water, were devised during the cruises of the Lightning 

 and Porcupine ; and the instructions to the Challenger were 

 chiefly based on our experience in the preliminary trips. (See 

 " The Depths of the Sea," p. 284 et seq.) 



The observing stations were fixed as nearly as possible in a 

 straight line, if possible either meridional or on a parallel of lat- 

 itude ; the bottom temperature was carefully determined by the 



