— 76 - 



covering layer" is of importance. In autumn, the vertical circulation seeks to 

 render the two layers homogenous, but although these may occasionally for a 

 few months melt into a single covering layer, the division into two may occa- 

 sionally also remain unbroken throughout the whole year. In the transition 

 from shallower to deeper and more extensive regions, the covering layer is 

 transformed into a single upper stratum, the "intermediate covering-layer" 

 becoming gradually thinner, and finally disappearing; and the deep-water is 

 reduced to a single under-layer. In the shallow areas no "hollow-water" is 

 found. The following table gives a view of the fourfold division in the Goth- 

 land Deep in July 1907. 



M 10 20 25 30 40 50 60 70 80 90 100 125 150 175 200 220 

 at 4,914, 93 5,10 5,55 5,67 5,77 5,85 5,8 7 6,25 7,20 7,65 8,17 8,73 9,06 9,27 9,29 9,3 4 



Covering- Intermediate Deep-Water. Hollow:Water. 



Layer. Covering-Layer. 



These forms of division can be recognised throughout the Baltic, more 

 or less distinct according to the varying geographical conditions. In the 

 Deeps or the parts of the sea, where the boundaries of these strata occur, 

 a geodetically defined point may fall in one stratum at one time, and at another 

 time be found in another layer. 



a. Salinity. 



At the surface, the salinity decreases from the outer parts inward. It 

 amounts, on an average, between Scania and Rügen, to 8"/oo, in the Bornholm 

 Deep 7— 7V2 7oo, between Dago and Aland to 6— 6V2O/00, in the Bothnian Sea 

 to about öo/oo, and in the Bottenvik to abt. So/oo, in the middle of the Gulf 

 of Finland to about 4^/2 "/oo, while in the innermost parts, to the North and 

 North East it is below 0,5 "/oo- As a rule, the salinity on the eastern, resp. 

 southern, coasts is greater than that of the western, resp. northern, coasts. 

 At the bottom, the salinity in the Bornholm Deep averages somewhat over 

 16 »/oo> in the Gothland Deep abt. 12 »/oo, in the Bothnian Sea abt. 6 «/oo- In 

 the averages for many years we can everywhere observe an annual period in 

 the salinity; in the lower stratum of the Arkona Basin, the Bornholm Deep, 

 and probably also of the Main Basin, this period is a double one. In the 

 outermost parts, the maxima fall in winter and summer, as in the Kattegat 

 and Belt Sea; proceding inwards, they are gradually retarded by up to three 

 months. In the upper stratum, and in the lower strata of the Gulfs of Fin- 

 land and of Bothnia, the period is single. In the covering layer, the minimum 

 falls in spring-summer; in the intermediate covering layer we find it three 

 months later, while in the lower stratum the period is opposite. 



