180 V. WALFRID EKMAN. 



tions I a, II, III, and V. It is noted in the journal that it was 

 calm. Fig. 18, PI. XIX, shows the surface-current at 0'5 m. at the 

 various stations, the observations at Stations III and V being inclu- 

 ded in one circle, because they cannot be separated from each other. 

 It may naturally be supposed that it is the river-water that finds 

 its way southwards past the promontory at Stat. II, and here 

 causes the southward-flowing surface-current. As regards the 

 northward-flowing surface-current at the outer stations, it seems 

 to me to be simplest, or even necessary, to regard it as a part 

 of a great, horizontal vortex, comprising the surface-water strata of 

 the whole Lysaker Fjord, which are set in motion by winds or sur- 

 face-currents beyond the edge of the ice. It may certainly seem 

 difficult to suppose that a vortex of so great an extent, in a water- 

 stratum of only about one metre in depth, would be able to keep 

 itself in existence when deriving no force from above or below, but 

 only from the far away, narrow margin. But in reality a comparison 

 of the above-mentioned currents at depths of respectively 0*5, 14, 

 and 2'2 metres, gives a forcible, and in itself interesting proof of 

 how easily v\^ater-strata lying one upon another move quite freely 

 and independently of one another at their respective depths, if, at 

 least, by a sufficient difference in density, eddies are prevented 

 at their boundaries. 



On the same plate, XIX, diagrams (figs. 19 to 27) from various 

 depths at the several stations are also drawn, which help to 

 illustrate what has previously been said. 



As the influence of the river-water on the movement of 

 the lower layers did not appear to extend to any great depth, 

 and therefore could not have any effect worth considering on 

 the circulation in the fjord, it was natural to suppose that the 

 wind, through the mediation of the currents stirred up by it 

 in the ice-free Christiania Fjord, should be the main-spring also 

 of the currents under the ice. 



As quite a strong wind was blowing on the morning of the 

 13th April (changing in the course of the morning from ENE. 



