2 F. L. Ekman, 



of the ocean, and every motion in one part must give rise to motion in 

 other parts, every ocean-current strictly speaking depends on many such 

 causes. The business of the hydrographer should therefore be rather to 

 discover what part each of them has in the formation of the separate 

 streams, than to carry out any particular theory grounded upon any 

 special cause. A quantitative determination of this kind can however at 

 present only be placed among things to be wished for, as the amount of 

 observations, necessary for its carrying out, does not as yet exist, and 

 the calculation of the effects moreover presupposes both the number and 

 nature of the cooperating causes accurately kiiown. The opposite views 

 that even up to the present time are entertained on this subject, seem 

 to indicate that the preparation of the question is not yet sufficientlj^ 

 advanced, and it is the object of the present work to contribute in some 

 means to the supplying of this deficiency. 



For our purpose I divide the above mentioned causes into two 

 groups, namely A) such as produce a change in the icater's level and thus 

 indirectly give rise to streams; and B) such as immediately j^roduce a stream 

 in consequence of the meclianical action, ivhich a medium already in motion 

 exercises on the loater. To this last category, (which must itself also pro- 

 duce a change of level) belong winds and the efflux of rivers ; further- 

 more every ocean-current, however it may have originated, immediately 

 becomes itself such a cause. To the former category belong especially 

 variation of temperature in the seawater, evaporation and the meteoric 

 rain-fall. 



A. CAUSES, WHICH INDIßECTLY PRODUCE OCEANIC STREAMS 

 THROUGH CHANGE OF LEVEL. 



The form of a sphéroïde of revolution, which the surface of the 

 ocean would otherwise assume, is subject to small perturbations. In addi- 

 tion to the periodical disturbance produced by the Moon's attraction, we 

 have in the first place that, which results from the unequal distribution 

 of the atmospheric pressure. According to the mean barometric pres- 

 sure in the neighbourhood of the Atlantic in the northern hemisphere, 

 as determined by Schouw and Poggendorff, and the barometerobserva- 

 tions in the southern hemisphere, collected by Maury, the surface of the 

 ocean at the equator appears to stand about 11 centimètres higher than 

 at the tropics; in consequence of the diminishing of the barometric 



