CHAP, vin.] THE GVLr-STREA-M. 361 



of data required for the consideration of such sub- 

 jects, that the basin of the North Atlantic should 

 be selected for investigation, more particularly as 

 peculiarities of climate seemed there to be limited in 

 space, and well defined and even extreme in character. 



It seems at first somewhat singular that there 

 should be any room for question as to the causes, 

 the sources, and the directions of the ocean currents 

 which traverse the ocean in our immediate neigh- 

 bourhood, and exercise a most important influence 

 on our economy and well-being. The investigation 

 is, however, one of singular difficulty ._ Some currents 

 are palpable enough, going at a rate and with a force 

 which make it easy to detect them, and even com- 

 paratively easy to gauge their volume and define 

 their path; but it seems that the great movements 

 of the water of the ocean, those which produce the 

 most important results in the transfer of tempera- 

 ture and the modification of climate, are not of this 

 character. These move so slowly that their surface 

 movement is constantly masked by the drift of vari- 

 able winds, and they thus produce no sensible effect 

 upon navigation. 



The path and limits of such bodies of moving 

 water can only be determined by the use of the 

 thermometer. The equalizing of the temperature 

 of bodies of water in contact with one another and 

 differently heated, by conduction, difl'usion, and 

 mixture, is however so slow, that we usually have 

 but little diflSculty in distinguishing currents from 

 different sources. 



Up to the present time little had been done in 

 determining the depth and mass of currents by the 



