every maximum of declination there lies a period of 18.6 years. When 

 the moon attains its highest declination, i. e. when it comes highest 

 and lowest in the skye it exercises its greatest tide-producing power 

 on our seas. I have examined all the information that exists respec- 

 ting the Swedish herring fishery for 150 years, during the herring- 

 periods of the 18th and 19th — 20th centuries, and have marked the 

 favourable fishing-years with max., and the unfavourable with nun. 



Wax. Wax. Max.* + 



Fig. 



When the tide-producing power diminishes most, the herring-migra- 

 tion stops at the first narrows of the submarine channel of the Catte- 

 gat, south of Gothenburg, or even farther to the north, so that, at 

 last, the herring has to be sought for off the Skaw (Skagen) in Den- 

 mark, as was the case this winter (1913—14). When the tide-gene- 

 rating power is not sufficient to carry the North Sea herring farther 

 in, an advance-guard of the herring-shoals is usually found during 

 December and January in the German North Sea bays and at the 

 mouths of the Elbe and Weser. That which is lost by the Cattegat 

 is gained by the North Sea fishery, just as happened this winter. 

 It is remarkable that the first growth of the herring fishery in the 

 North Sea known in history began in the 16th century, the period 

 when the great fishery in the Sund came to an end. 



We find that, without exception, the good fishery-years coincide 

 with the greatest lunar-declination and tideraising power, and 

 the bad fishing-years with the lowest declination 1 ). 



When the tideraising power of the moon last was greatest it 

 was able to carry the herring-shoals through the whole of the narrows 

 of the submarine channel of the Skagerak and the Cattegat all the 

 way into the Sund and the Baltic. That was 600—900 years ago. 



Max itax 



1837 1855 187* 18S13 IHll 



19. 



This may seem to present but cheerless prospects for Swedish 

 fishermen and, it is true of course, that the herring fishery is a dif- 

 ficult and dangerous pursuit. But of late years fishing-vessels and 

 fishery-methods have developed so that it is now possible to look 

 for the herring-shoals at other places and to catch them by other 

 means than a century ago, when the Swedish fishermen were conten- 

 ted with fishing in the fjords and sounds with standing-nets (»satt- 

 garn») and land-seines ( »land-vadar »). It is to be hoped too that 

 in the same degree that we are able to discover and study the causes 

 of the visits of the herring to the Swedish coast and its occa- 

 sional absences fro m our waters, it will be possible to calculate 

 in advance the prospects of the autumn and the winter fishing- 

 seasons. 



VII. 



The Climate of Northern Europe at the close of the Middle-age. 



The state of our surrounding seas varies on a lesser scale than 

 in the Middle-ages owing to the fact that the periodically acting 

 agent is less variable now than formerly, and is not power- 

 ful enough to cause the great climatic variations at the close of 

 the Middle-ages to be repeated. This cannot be expected to 

 happen till after the lapse of 1000 years. But the sea is so sensi- 

 tive to variations in the tide-generating force that the smaller 

 variations now observed serve as an index to estimate the 

 great changes which have occurred in the fishlife and the ice-con- 

 ditions of the Baltic. 



The changes must have reacted on the summer- and winter- 

 climate of the Baltic countries. In summer the thin surface-layer was 

 heated to a higher temperature and in winter the icecold or ice-co- 

 vered surface must have cooled the atmosphere above it. The cli- 

 mate must have been more continental with warmer summers and 

 colder winters than now. The observations made on Hven by 

 Tycho Brahe about 1590 compared with H. E. Hamberg's review 

 of the time 1750 — 1800 bear out this conjecture. It is not likely 

 that the spring and authumn climate would differ much from the 

 present. 



Tycho Brahe's observations have enabled Ekholm 2 to attempt 

 a reconstruction of the climate at Oresund in the 15th century. 

 Like myself Ekholm assumes it to be true that the Baltic was frozen 

 over in severe winters. 



»Although even now there is considerable formation of ice in 

 the Scandinavian seas, it is evident that the freezing of the Baltic, 

 the Sund, the Belts, Cattegat and Skagerak occurred on a 

 much larger scale in Mediaeval time, especially from about 1000 to 

 1400. What the conditions were before the year 1000 is not known, 

 because the Sagas mention nothing about iceconditions. It is difficult to 

 imagine what the weather could be like in a winter that covered 

 the entire southern Baltic, Cattegat and Skagerak with ice 

 strong enough to be used for communication.* 



Ekholm points out that our cold winters now are always cha- 

 racterised by numerous cyclones, which pass south of Sweden. 

 They generally come from the west and probably originate in or 

 are fed by the Gulfstream. 



»The most plausible explanation would be that the Gulf- 

 stream was weaker in Mediaeval time or took a more westerly 

 course then and that consequently the climate was more continen- 

 tal. In the latter case the climate of Iceland and Greenland must 

 also have been milder ». 



Ekholm also points out that P. La Cour has found a consi- 



derable divergence in the wind-direction during the wintermonths 

 of Tycho Brahe's time from the present one. 



»Nowadays the ruling wind-direction for the year is averagely 

 Southwest, during all months except April and May when the 

 South-east is more frequent. In Tycho Brahe's time South-east 

 was the average direction for the year; Southeast or East ruling 

 the 7 months January — May and October and December. In the 

 remaining months too the Southeast obtained a secondary maxi- 

 mum of which now scarcely a trace remains. This shows that 

 the distribution of pressure must have been very different then, 

 the low-pressure belt which now as at rule lies between Norway 

 and Greenland and in the Arctic sea must have been very little 

 developed in those days. 



In consequence of this low-pressure belt the cyclones now 

 generally pass across the south or middle part of the Scandinavian 

 penninsula, bringing southwesterly winds in Oresund. But the 

 eastern or southeastern winds of the winters in Tycho Brahe's 

 time show that the cyclones then took a course more to the south 

 passing as a rule south of Hven, f. inst. from the North Sea, through 

 southern Denmark to Germany. This course is almost never taken 

 by cyclones now except in the early spring months and in 

 exceedingly severe winters ». 



The opinions I have expressed in my paper on climatic var- 

 iations in historic time coincide with Ekholm's representation in 

 the chapter bearing the same title in his papers of 1899 & 1901. 



The impression to be got from our joint descriptions of the 

 chief features in the climatic variations of the Middleages is the 

 following: 



At the beginning of the Viking-age 500 — 600 a. C. the minimum 

 period of the tide-generating force occurred. From that time till 

 about 1100, or in the Viking-age proper, the climate of Iceland 

 and Greenland was comparatively mild, as there was no ice- 

 blockade of the north coast of Iceland and the eastcoast of Green- 

 land. Notes about ice hindering navigation in these seas are very 

 rare. The sailing-route to Greenland from Iceland ran straight 

 west over the Gunbjornsskar group of islands off Cape Dan, thence 

 along the eastcoast to the straits north of Cape Farewell, which 

 were then navigable and free from ice. Nor was navigation ham- 

 pered by the ice in Davis Sound or in the sea between Greenland 

 and North-America. Ice was first met with in the far north at 

 the fishing places of Baffins Bay. The powerful Labrador-current 

 carried no ice in those centuries, or in any case, nothing comparable 

 to the present quantity. This again reacted on the Gulfstream. 



1 In N:o 7, 8, 9 of the fishery journal »Der Fischerbote ». (Hamburg. 1911). I have published a detailed description of the vicissitudes of the Swedish herring- 

 fishery during the last 150 years. 



2 Ekholm. Quarterly Jcurn. R. Meteor, Soe. 1901 January. 



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