PHYSICAL OCEANOGRAPHY 261 



They had been carried 1080 nautical miles in 246 days, that is, 

 4,4 miles per day on an average. 



Information about the currents is also obtained from objects 

 found drifting along with them. At Lofoten golf-balls have 

 been found which must have come across from Scotland. In 

 the Norwegian Sea drift-wood from Siberia is occasionally met 

 with ; once we came across the trunk of a Siberian tree thickly 

 covered with littoral diatoms, which had thus travelled right 

 through the polar sea, so that the log had come from the 

 northern coast of Asia with the same current that carried the 

 " Fram " through the northern waters. 



In order to study the currents, drift-bottles have often been Drift-bottles. 

 employed, in which are enclosed slips of paper with directions 

 to the finder to send the note to the address given, with ■ 

 information about when and where it was found. Fig. 176 

 shows the results of some of the bottle-experiments made in the Fulton's 

 North Sea by Fulton, who has in this way been able to give a ^"-p^"'"^" ^• 

 more complete account of the currents of the North Sea than 

 was previously possible. In this case the method gave quite 

 trustworthy results, because there were shores all round where 

 it was comparatively easy to recover the bottles within a short 

 time. As regards the great oceans, the method often gives 

 rather doubtful results. In the first place, one cannot know the 

 route followed by the bottle from the time it was thrown over- 

 board till the time it was found, and then it may lie for years 

 on the shore before it is found, so that no one can tell how long 

 it has been on its journey. 



These methods give a certain amount of information about 

 the motion of the superficial layers, but none about the deeper 

 currents. We can also study the set of the water-masses 

 by means of their physical or chemical qualities, especially 

 temperature and salinity and gaseous contents. It is, for 

 instance, known that the Gulf Stream carries much salt water 

 (with a salinity above 35 per thousand) from the Atlantic into the 

 Norwegian Sea, and the course of this salt water can be traced 

 farther north ; it forms a band along the coast of Norway, and 

 branches off in several places. The position of this salt water 

 indicates the course of the current itself, not at the surface only, 

 but also in the deeper layers. 



From a study of the distribution of salinity and temperature 

 the average direction of the drift of the water-masses may be 

 deduced, and an idea of the velocity obtained by calculation, ^j^j^,^ 

 Mohn, and more recently especially Bjerknes, have greatly Bjerknes. 



