336 NATURE AND MAN. 



The thermal condition of the North Atlantic, however, is very 

 different. Putting aside the extraordinarily low temperature of 

 29! revealed by the Porcupine temperature-soundings in the 

 stratum occupying the deeper part of the channel of 500 fathoms 

 between the Faroe and the Shetland Islands, which has been 

 since proved to be a southward extension of the true Arctic 

 basin, no lower bottom-temperature than 35 had been anywhere 

 met with in our earlier work, while we had found the thickness 

 of the warm stratum ranging from 40 upwards to range from 

 800 to 900 fathoms. This want of a truly glacial understratum 

 I attributed to the limitation of the communication between the 

 deeper parts of the Arctic and North Atlantic basins, preventing 

 the coldest water of the former from flowing out into the latter. 

 And this explanation has been borne out by the subsequent 

 temperature-soundings of the Valorous, which have shown the 

 existence of a ridge between Greenland and Iceland, lying at a 

 depth which allows water of 35 to pass over it, while keeping 

 back the deeper stratum of Arctic water. I had further predicted 

 than an Antarctic underflow would probably be found to range 

 to the north of the Equator, where it would be recognized by 

 the reduction of the bottom-temperature below 35 : and this 

 prediction was verified in the first temperature-section carried by 

 the Challenger obliquely across the Atlantic to St. Thomas s, the 

 bottom-temperature there falling a degree, and showing a still 

 further reduction as it was subsequently traced southwards to the 

 Equator, where it fell nearly to 32. 



But, further, I had ventured the prediction that the meeting 

 of the Arctic and Antarctic underflows under the Equator would 

 cause an uprising of cold water from the bottom towards the 

 surface, so that the plane of 40 would be found nearer the sur 

 face in the neighbourhood of the line than either to the north 

 or to the south of it ; and it was a great surprise to many on 

 board the Challenger to find, as they first approached the Equator 

 from the Tropic of Cancer, the plane of 40 rapidly rising from 

 a depth of 700 fathoms towards the surface, though the tempera 

 ture of that surface-stratum was itself becoming higher and higher 

 until water of 40 was found at a depth of less than 300 fathoms, 

 descending again to about 400 as the Challenger s course was laid 



