32 F. L. Ekman, 



that the salt under-current in the Bahic does not flow constantly but be- 

 fits; also the proximity to the surface, which the ocean-water attains in 

 the Skagerack, is different at different times. These disturbances are 

 assuredly not merely accidental, but also of a regular and periodic cha- 

 racter. They must in the first place be dependent on the diflerent supply 

 of river-water in different seasons; in proportion as the outflowing 

 river-water is more abundant, the under-current will be more power- 

 ful, and the changes of saltness at different depths more sudden. The 

 effect of the variation of the rivers on the waters level must however 

 in these great basins l>e slow and easily compensated by the more 

 rapid effects of the winds. In the summer-time westerly winds pre- 

 vail in the Skagerack, wliich in a certain degree dam up the surface- 

 current between the Swedish and Norwegian coasts. But if the water 

 is dammed up in the Skagerack, it must also rise in the Baltic, and 

 throughout the whole system of basins the thickness of the superficial 

 fresher stratum is then increased. The under-current can under these 

 circumstances be diminished, even though the supply of river-water be 

 increased, and by diffusion and other causes the difterent strata of water 

 will be more and more mixed. But in spring easterly winds are more 

 prevalent, whereby the passage of the surface-stream out of the Ska- 

 gerack is facilitated. By this the hydrostatic surplus-pressure of the 

 superficial stratum is raised throughout the whole system of basins; the 

 fresher water flows out -more easily and the under-current accordingly 

 may assume a livelier motion. At this time of the year the surface both 

 in the Baltic and in the Kattegat is lowest, and the oceanic water in 

 Skagerack is then found nearer the surface than at other times, a cir- 

 cumstance, which already in 1870 I explained as caused by the winds. 

 Furthermore the course of the streams at any time of the year, 

 may be modified by variation of atmospheric pressure and by storms. 

 Simply by variation of atmospheric pressure, which disturbs the order 

 of the hj^droslatic pressures down to the bottom, the direction of the 

 streams may, without the cause being easily discovered, be reversed, and 

 the double streams exchanged for single. Suppose for instance that 

 over the whole of the Baltic the barometer sinks 30"""', this must have 

 the same effect on the difi'erences of pressure on the two sides of a ver- 

 tical section of the Sound, as if the water in the Kattegat had risen 4 cen- 

 timètres. Streams would then under circumstances set in trough the Sound 

 both on the surface and at the bottom. Again, if we suppose the water- 

 strata to have assumed their normal position, so that differences of spe- 



