What then was the state of the Baltic at that time when it 

 happened in cold winters that its surface became frozen over? The 

 Swedish researches have furnished an answer to this question. 



Fig. 15 gives the position of the the isohalines in the western 

 Baltic as found by F. L. Ekman in August 1877. We see that the 

 mixed salt water, which is found in the Cattegat and the Belts, imme- 



Gotlcuxd 



Djup 



Fig. 15. 



diately upon entering the Baltic flows along its bottom to the big 

 basin east of Bornholm. Nowadays the undercurrent cannot fill 

 this basin to a higher level than some 40 — 50 meters from the sur- 

 face n« h and east of Bornholm and 60 — 70 meters below the sur- 

 face in the big basin east of Gothland. 



If the undercurrent increased so as to raise the level of the 



bottomlayer some 9—10 meters in the neighbourhood of Bornholm, 

 the Baltic would freeze as it did in the 13th or 14th century. 

 The following tables contain the results of 3 soundings at the 

 station Se east of Bornholm where the water is 95 meter deep 

 and the surface layer of uniform salinity is 50 meter in thick- 

 ness. 



Depth 



m. 



10 » 



20 » 



30 » 



40 » 



50 » 



75 » 



95 » 



15 august 1904. 

 55° 21' Lat. N. 15° 39' Long E. 



Salinity 

 7.29 o/ 00 

 7.25 » 

 7.25 » 

 7.25 » 

 7.32 » 

 10.21 » 



Temperature 



15°.09 C. 



14°. 61 » 

 14°. 60 

 13°.40 



4°. 70 



3°. 52 



3°. 64 » 

 3°.52 » 



16.04 

 17.02 



5 november 1904. 

 55° 20' Lat. jST. 15° 30' Long. E 

 Temperature 

 10°.3 C. 

 10°.23 » 

 10°.23 » 

 10°. 23 » 

 10°. 23 » 

 7°. 02 » 



4°.25 » 

 3°.70 » 



15 march 1905. 

 55° 23' Lat. X. 15° 30' Long. E. 



Salinity 



Temperature 



Salinity 



7.29 % 



2°.2 C. " 



7.39 °/ 00 



7.27 » 



2°. 12 » 



7.39 » 



7.28 » 



2°.08 » 



7.43 'i 7 



7.29 » 



2°. 15 » 



7.56 » 



7.29 » 



2°. 20 » 



7.67 » 



9.42 » 



3°. 40 » 



9.15 » 



16.06 » 



5°. 33 » 



15.61 » 



16.58 » 



4°. 59 » 



16.03 » 



It is easy to calculate from the tables that the surface-layer 

 has given off some 30.000 calories to the atmosphere from 

 August to November. From November to the middle of March 

 the surface of the sea had lost 367500 calories pro square- 

 meter. The temperature of the surface-layer had in the meantime 

 sunk from an average of 9.°7i C. to 2.°36 C. or with 7.°35 degrees. 

 Had the surface-layer been some 35.7 meter thick instead of 50 



6 november 1904. 



meter and given off the same amount of heat to the atmosphere,, 

 then its temperature would have sunk to the freezing point = 

 — 0.°5 G. and the open Baltic east of Bornholm would have been 

 covered by ice in March 1905, The same calculation can be 

 made for the Baltic north of Bornholm wherefrom we possess sound- 

 ings from Nov. 1914 and March 1905. The Swedish station Ss- 

 is situated in the strait between Scania and Bornholm. 



15 march 1905. 





55° 26' Lat. N. 



14° 46' Long. E. 



55° 26' Lat. N. 



14° 46' Long. E. 



Depth 



Tenrperature 



Salinity 



Temperature 



Salinity 



m. 



10°.6 G. 



7-54 %0 



2°.3 C. 



7.50 °/ 00 



10 » 



10°. 59 » 



7.54 » 



2°.31 » 



7.56 » 



20 » 



10°.59 » 



7.52 » 



2°. 26 » 



7.65 » 



30 » 



10°.63 » 



7.67 » 



2°.5 » 



7.80 » 



40 » 



11°. 81 :> 



11.56 :> 



1°.91 » 



8.04 » 



60 » 



11°.25 » 



14.52 » 



r.75 » 



13.14 » 



68 » 



8°. 87 » 



15.61 » 



■ 1°.71 » 



13.87 » 



Here the surface layer, which measures 40 m., had emitted 

 some 340000 calories pro square-meter from Nov. 6th 1904 to March 

 15th 1905. The temperature had sunk from 10°.6 to 2°.i6 C. 

 If the surface- layer had been 10 — 12 meter less thick the temperature 

 would have sunk somewhat under 0° by the same heat emana- 

 tion. The surface layer, however, is cooled farther in some winters. 

 In 1896 its temperature sank to 1°.3 C. If in such a winter the 

 bottom water had risen' 8 — 9 meters nearer to the surface a 

 continuous sheet of ice would have covered the middle part of 

 the Baltic proper. If the influx of salt water into the Baltic were so 

 strong that its level reached up to 30 instead of 40— 45 meters from 

 the surface in the strait of Bornholm, then the ice would have been 

 safe for communication between Scania and Bornholm in the 

 winter 1896—1897. But in the mild winter 1904—1905 the sea 

 would have been open and free from ice. 



In this way we may form an opinion as to how much the 

 Baltic has altered in the course of the last 5 to 6 centuries. 

 Fig. 15 is the outcome of such a calculation and shows a section of 

 the western and southern Baltic reconstructed after this method. 

 A larger supply of salt water would make the Baltic physically 

 resemble the fjords of the Cattegat which as we know freeze to 

 in cold winters; and the herring shoals from the North Sea would 

 then follow with the salt current to the gates of the Baltic as they 

 did every authumn in the Middle-ages. 



It would thus require no very considerable change in the 

 hydrographic conditions to bring about the freezing of the Baltic 



in severe winters or to make the herring return to its ancient 

 haunts in the Sound at the coast of Scania. Such conditions recur- 

 approximately even in our days in certain years and at certain in- 

 tervals. It all depends upon the intensity of the watersupply from 

 the ocean through the undercurrent. In other papers I have shown 

 that this undercurrent varies under the influence of the tides. 

 It possesses semidiurnal, diurnal, monthly, annual, multiannual 

 and secular periods according to the variation in the tide-generating 

 force. The daily pulsation we discovered in 1907 and succeed- 

 ing years in the Great Belt, the monthly in the Gullmarfjord. 

 The annual period has been traced in many ways by its effects; 

 higher water-level in authumn, maximum of the influx through the 

 Great Belt and Oresund in December, the seasonal migrations of 

 the herring which now generally stops at the point where the Catte- 

 gat channel shoals out S. of Gothenburg, but in certain authumns 

 sends contingents down to Oresund and the Belt in such 

 a number as to suggest to certain authors the idea that the great 

 Hanseatic fishery of the Middle-ages was no other than the herring 

 fishery which is carried on nowadays in Oresund. 



Finally there is the wellknown fact, that in certain winters 

 the driftice appears in the Sund, the Baltic and the Cattegat in 

 such quantities as to block the Sund and the Cattegat harbours 

 for a couple of months. Such winters appear to occur periodically. 

 In 1809 — 1814 there occurred such winters, the drift-ice getting, 

 crammed into the strait between Scania and Bornholm. 



— 18 — 



