50 F. L. Ekman, 



respect to the course of the ocean-currents, we should arrive, if we en- 

 deavoured to form an idea of what motions those causes of currents, 

 which in the earlier part of this essay we have treated separately, would 

 produce in the free ocean. It here appeared, that only in the case, when 

 we take into account all the above named causes, these motions seemed to 

 correspond with those, presented by reality in a perfect segment of the 

 ocean, e. g. the Atlantic between the equator and the North-pole. As 

 now these forces are evidently of such a nature, that in a complete segment 

 of the Ocean they must all come into operation, this circumstance appears 

 to me to be a proof of the correctness of the view taken. I have also 

 taken the opportunity to point out the need of thoroughly investigating 

 certain data, which have hitherto been much neglected, as being neces- 

 sary for the quantitative determination of the causes of the streams, 

 which should be one of the principal problems of hydrography. I place 

 among these in the first place the amount of evaporation and, wherever 

 it can be carried out, the determination of the mean water-level in diffe- 

 rent portions of the ocean. 



If we take a general view of the effects, produced upon the ocean 

 by the different forces which originally set its water in motion, we find, 

 singularly enough, that each of these forces produces both of the kinds 

 of stream, of which I have been endeavouring to explain the essential 

 difference viz: a surface-stream, caused by a limited, and a deep- 

 stream by an unlimited disturbance of the equilibrium. Heat produces a 

 surface-stream by warming the water and a deep-stream by evapora- 

 ting it. Cold can produce an under-current by the contraction and still 

 more by the concentration of the water, but it then leaves at the sur- 

 face a tendency to disturbance of equilibrium, which shows itself in the 

 form of a surface-stream on the melting of the ice. Rainfall causes a 

 deep-stream as its immediate consequence and a surface-current of 

 lighter water as a secondary effect. Winds occasion directly surface- 

 currents, and deep-streams by changing the level. Hivers^ and similar 

 constant out-pourings of lighter water, give immediately rise only to 

 surface-streams, but produce under-currents by mechanical reaction; 

 streams of this latter kind often accompany even the above mentioned. 

 The specific gravity of the Vt^ater plays in all these cases a very varied, 

 but in some or other respect always important part. It usually deter- 

 mines the level of the water-strata, but not alwa3's. As to the origine 

 independent of the variations of specific weight are especially the 



