PHYSICAL OCEANOGRAPHY 267 



is here indicated by the thick arrows. Two measurements 

 were made at 1830 metres (Nos. I. and IV. in the figure), and 

 two at 915 metres (Nos. II. and III.), and at the same time 

 observations were made at 10 metres with another apparatus. 

 The time by the watch is noted in the figure. The arrows 

 in V. show the currents thus found at 10 metres after allowing 

 for the assumed drift of the vessel, and it is seen that the 

 variations both in velocity and in direction are large. This 

 method is, however, uncertain so long as the currents in deep 

 water are unknown ; if these are considerable, the thick arrows 

 in Fig. 178, v.. do not give the actual currents at 10 metres, 

 but only the relation between these currents and those in deep 

 water. Still one thing is at least clear from the figure : the 

 thick arrows alter their direction regularly, and the change is 

 counter-clockwise. A continuous alteration of set is one of 

 the characteristics of tidal currents, and the conclusion is in all 

 probability admissible that our measurements at Station 49 C 

 prove the existence of tidal currents in the Atlantic Ocean, 

 even where it is very deep. 



Tidal motion in the sea is due to the attraction exercised Tides and 

 by the sun and moon on the water-masses, which varies from ^'^^^ currents. 

 place to place. It would take us too far to enter into the 

 theories of the tides here, and besides, we have not yet a clear 

 solution of the problem, because, among other reasons, we have 

 no observations from the open sea, but only those from the 

 coasts. The rise and fall of the surface, known as tides, are 

 accompanied by currents, and the study of these currents in 

 the open sea would be of great importance for the comprehen- 

 sion of tidal phenomena. In the "Michael Sars " Expedition, 

 as mentioned above, we made a number of current-measure- 

 ments, the principal object being to find out if it were possible 

 to make trustworthy observations of the veldcity and direction 

 of tidal currents in the ocean. This has not been done 

 before in deep water. Buchanan in 1883 made some interest- Buchanan. 

 ing measurements on the Dacia Bank, off the west coast of 

 Morocco, and found marked tidal currents during the couple 

 of hours the observations lasted. Afterwards R. N. Wolfenden Woifenden. 

 discovered tidal currents on the Gettysburg Bank. Beyond 

 these and a few other observations, we have no observations 

 from the open ocean far from land and none at all in deep water. 



We usually figure to ourselves the attraction of the moon Tidal waves, 

 and the sun producing a tidal wave which can develop freely 

 in the Southern Ocean, where a zone of water encircles the 



