— so- 

 in the "hollow-water" the temperature depends almost entirely upon 

 the intermittent inflow of new and salter masses of water, and the annual 

 course is faintly defined. East of Bornholm, at depths of more than 90 m., 

 the lowest and highest bottom temperatures found were respectively 3°.16 and 

 7°.21 (difference 4°.l); and east of Gothland, at depths of more than 210 m. 

 the corresponding figures are 3°. 32 and 5°.79 (difference 21/2°). At the last 

 named place, as in the "hollow-water" of the whole of the Central Basin, the 

 variability of the bottom temperature is on the whole but slight. In the 

 central part of the Bothnian Sea, the amplitude at 100 m. amounts to 

 scarcely 1^/2°- 



A very characteristic feature of the Baltic is the fact that in the great 

 majority of cases higher temperatures are found in the lower stratum than in 

 the lower parts of the intermediate covering layer, which is due to the move- 

 ment of the "deep-water". In summer, when the covering layer has a high 

 temperature, the lowest temperatures are thus found at the transition from 

 the intermediate covering layer to the "deep water". The following series 

 from the sea east of Gothland, 3 August 1907, is in many respects typical: 

 Om: 13°.80; 20m: 13°.22; 40m: 3°.68; 65m: 2°.34; 100m: 4°.15; 210m: 4°.74. 



c. Conditions of oxygen and current. 



Owing to the fact that the "hollow-water" of the Central Basin is only 

 renewed now and then, its contents of oxygen varies greatly, since in stag- 

 nating water, the quantity of oxygen decreases gradually, partly owing to the 

 consumption of oxygen by living organisms, partly on account of the oxydising 

 of organic matter. At the bottom of the Bornholm Deep, the quantity of 

 oxygen varies between less than 10 "/o to 70 "/o of the quantity which it would 

 contain if in free contact with the atmosphere (relative contents of oxygen). 

 At depths of more than 200 m. east of Gothland, the relative amount of 

 oxygen varies between abt. 5 and abt. 50 %; in the northern part of the 

 Central Basin (abt. Lat. 59° N.), at depths of more than 100 m, it amounts to 

 roughly between 20 and 50 %. At the bottom of the Gulf of Finland, also, 

 the quantity of oxygen in summer and autumn can in places fall to about 

 20 "/o, while the lower stratum of the Gulf of Bothnia, which, on account of 

 the ridge round Aland, is mainly composed of water from the intermediate 

 covering layer of the Baltic, always seems well aerated, having over 70 "/o 

 relative contents of oxygen. The covering layer is well aerated all through 

 the year, as also the intermediate covering layer, being thoroughly aired in 

 autumn and winter by vertical circulation; over 80 °/o relative contents of oxy- 

 gen there being, moreover, apparently very little consumption of oxygen by 



