ee 
CHAP. VIII. ] THE GULF-STREAM. 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, diffusion, and 
mixture, is however so slow, that we usually have 
but little difficulty 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 
