THE DEEP SEA AND ITS CONTENTS. 337 



towards the Tropic of Capricorn. This anomaly had been re- 

 marked by Lenz fifty years previously : but the valuable series 

 of temperature-observations which he took in Kotzebue's second 

 voyage was strangely overlooked by those who ranked as the 

 highest authorities on the physics of the earth, until recently 

 disinterred by Professor Prestwich. 



Not only is the stratum of above 40° Fahr. exceptionally deep 

 in the North Atlantic, but it is exceptionally warm, especially on 

 its western side, where a stratum of water having a temperature 

 above 60° Fahr, was found by the Challenger to range to a depth 

 of nearly 400 fathoms. Taking all circumstances into account, 

 I entertain no doubt that Sir Wyville Thomson is right in regard- 

 ing this stratum as the reflux of the northern division of the great 

 Equatorial Current, from the coast of the West India Islands and 

 of the peninsula of Florida, added to that of the Gulf Stream 

 proper. In consequence of the evaporation produced by its 

 prolonged exposure to the tropical sun, this water contains such 

 an excess of salt as, in spite of its high temperature, to be specifi- 

 cally heavier than the colder water which would otherwise occupy 

 its place in the basin, and consequently substitutes itself for the 

 latter by gravitation, to a depth of several hundred fathoms. 

 Thus it conveys the solar heat downwards in such a manner as 

 to make the North Atlantic between the parallels of 20° and 40° 

 a great reservoir of warmth, the importance of which will presently 

 become apparent. 



The Challenger investigations have now, I think, afforded 

 the requisite data for the final solution of a question which has 

 been long under discussion — what, namely, the Gulf Stream (or 

 Florida Current) does, and what it does tiot, for the amelioration of 

 the climate of North-western Europe. All the best hydrographers, 

 both of this country and of the United States, agree in the con- 

 clusion that the Florida current dies out in the mid-Atlantic, losing 

 all the attributes by which it had been previously distinguished — 

 its movement, its excess of warmth, and its peculiarly deep colour; 

 and that it then degenerates into a mere surface-drift, the rate 

 and direction of which depend entirely upon the prevalent winds. 

 But, on the other hand, most conclusive prooi has been obtained 

 by the systematic comparisons of sea and air temperatures along 



