— 74 — 



the inflowing under-current has its greatest speed somewhat above the bottom 

 (abt. 17 cm/sec. at 15 — 20 m.). The currents have a yearly period; at 2V2 m. 

 the outflowing current has maxima of over 20 cm/sec. in May and August and 

 is, during all the time from March to August, greater than its annual mean 

 value. Throughout the time from September — February however it is less 

 than its yearly average. On the other hand, the bottom current reaches a 

 maximum of abt. 32 cm/sec. in December; during the time from October — 

 April it is above its mean value, and in the time from May to September 

 below this. The periods for the upper and under currents are thus almost 

 opposite to each other. 



4. The Baltic. 



By the Baltic is understood the sea inside the submarine ridge which 

 joins Scania with Sealand, Meen, Falster and the German coast at Bars. The 

 region receives its hydrographical character 1) from the great supply of fresh 

 water (the greater part in the innermost areas), causing a corresponding out- 

 flow, through the Danish waters, on the surface; and 2) from the inflow of 

 salt water along the bottom. Thus we can, throughout the whole of the Bal- 

 tic, distinguish between an upper stratum of comparatively slight salinity and 

 a salter lower stratum. In the lower stratum we have a resulting, though not 

 absolutely continual inward movement. As, however, the Baltic consists of 

 several hollows, the Arkona Basin, the Bornholm Deep, the Main Basin, and 

 further hollows near Aland and in the Gulf of Bothnia, it happens occasionally, 

 when the inflowing water is especially salt, and thus heavy, that this very 

 salt water sinks down here, forming a relatively stationary "hollow-water" layer 

 beneath the stratum of regularly inflowing "deep-water", and being intermit- 

 tently renewed, chiefly in winter and summer. As it gradually becomes mixed 

 with the deep-water and hfted upwards by the inflow of new and heavier 

 "hollow- water", it thus contributes by and by to the formation of the "deep- 

 water" stratum. The hollows have thus a regulating effect. 



From the Bornholm Deep inwards, the surface stratum is of considerable 

 thickness, 40—60 m. The direct summer heating does not, however, as a 

 rule penetrate below abt. 20 m, and thus we have here, in spring and until 

 autumn, a thermal division of the upper stratum into two layers. And as the 

 supply of fresh water at these seasons is also restricted to the "covering layer" 

 thus formed, we obtain a corresponding division as regards salinity. Biologi- 

 cally and dynamically also (on account of the distinct division as to at this 

 division of the upper stratum into a "covering layer" and an "intermediate 



