— 78 — 



The periods of the upper stratum and of the inner lower stratum corre- 

 spond more directly with the annual variation in the supply of fresh water; 

 the double period of the outermost parts of the lower stratum are again de- 

 pendent on the conditions of the inflow of salt water, which again are closely 

 connected with weather conditions and the doubly periodical course of the water- 

 level — that is to say the mass of water ^ in the Baltic. As these last 

 conditions vary considerably from year to year, the conditions of salinity also 

 change; in the outermost parts, these changes, which are generally considered 

 as accidental, are greater than the periodical variations. A considerable part 

 of these accidental variations however, are due to dislocations of the several 

 water-layers, and not to direct alteration of their composition. The supply of 

 fresh water has, in relation to the determining factors above mentioned, a far 

 more distinct annual course, with maximum in spring, and correspondingly, 

 the annual variations in the inward parts are more conspicuous ; here also, 

 however, the accidental changes can at times predominate over the regular 

 ones. As an example of these variations may be mentioned that 49 series 

 from August 1902 to August 1911 in the Bornholm Deep give a total scope 

 of fluctuation at 40 m. depth of 0,8 "/oo, at 60 m. of 6,6 %o. at the bottom of 

 3,0 "/oo; in the Gothland Deep, below 200 m. the total variability is l%o; and 

 in the Bothnian Sea, at abt. Lat. 62 N. — 20 E. according to 42 measurements 

 from Sept. 1898 to May 1912, the surface sahnity varied between 5.17 and 

 5.86<'/oo; at depths between 120m. and 149m, according to 33 bottom observa- 

 tions, between 5.93 %o and 6.51 "/oo- 



b. Temperature. 



In the covering layer of the Main Basin, the temperature has an annual 

 course dependent upon the varying meridian-altitude of the sun. The 

 minimum occurs at about the same time everywhere throughout the whole of 

 the covering layer; i. e. in February or thereabouts; whereas the highest tem- 

 peratures in the intermediate covering layer appear some three months later 

 than at the surface: at 40m. for instance in November. Finally, in the "deep 

 water", the minimum occurs in or after May, and the maximum in or after 

 November. By way of illustration, the mean temperatures for the period 

 August 1902— May 1907 at a German Station (D. Ostsee 10) south-east of 

 Bornholm, are here given. 



Feb. May Aug. Nov. 



m 2°5 4°7 16°7 8°8 



20 — 2°7 4°4 15°4 8°8 



40 — 2°6 3°6 4°8 8°9 



50 — 3°9 3°2 3°9 6°8 



Bottom (abt. 60 m). 5°9 4°1 4°3 6°0 



