of the strait, and then continues its inward course along the bottom 

 of this sea. in the form of a so-called »solitary wave». 



The exchange of water between the Ocean and the North Se,a 

 thus obtains a pulsating character, in consequence of the influence 

 exerted by the tide-water on the under-current. In consequence 

 of these cascades of salt water, the deep depressions of the Baltic 

 are filled one after the other, and when one basin is filled it poors 

 its superfluous water in another cascade into the next, so that, in 

 proportion as the salt water enters the Baltic, the surface water is 

 driven out. 



Such is the influence of the daily and semi-daily tide-water on 

 the circulation in the Baltic sounds. 



Beside these tidal waves in the boundary between the upper 

 current and the bottom current we have the great waves of longer 

 period described in the foregoing which come in from the 

 North Sea moving slowly through the whole of the Skagerak and 

 the Cattegat like great cascades of water which carry with them the 

 shoals of herring from the eastern regions of the North Sea into the 

 Cattegat, which acts like a funnel or a fish-trap in the narrow in- 

 nermost end of which the immigrating herrings are crowded together. 

 There are several such tracts where the shoals of herring are crow- 

 ded into close masses — »herring-mountains », as they are called 

 by Swedish fishermen — in consequence of the small breadth of the 

 channel, or the irregular contour of the bottom, which acts like a 

 filter that admits the water-current but stops the progress of the herring. 

 In such places the herring-fishery is carried on with seines ( »snorp- 

 vadar»). In places where the channel is wider, such as around An- 

 holt and off Varberg, fishing is best carried on with driftnets as the 

 shoals of herring are more widely spread there. The last and the nar- 

 rowest passage that the water-current has to pass before it enters 

 the Baltic is formed by the Sund and the Belts. 



Relief -chart of Skagerak and Cattegat. 



Here it is that the greatest sea-fishery of the Middle Ages — 

 the so-called Hanseatic herring fishery — was pursued, the centre 

 of which was near Skanor and Falsterbo, and was richest during the 

 13th, 14th and 15th centuries, since which period it has diminished 

 to the humble proportions it now has. What could have been the 

 cause of this decline in one of the principal fisheries of Scandinavia 

 within historic times? It cannot have been any geological alteration 

 diminishing the exchange of water with the Baltic as was the case after 

 the close of the Littorina period, for during the last 2,000 years the 

 bottom of the Sund has not risen more than % meter at most. 



The reason does not lie in a geological, but in a hydrographical 

 alteration in the intensity of the circulation of the water, which has been 

 caused by cosmic influences. It can be shown that the tide-producing 

 powers of the moon (and the sun) were considerably greater during 

 the centuries mentioned above than they are now, in consequence 



of the relative position of the orbit of the moon in respect to the 

 earth and the sun at the period of the winter solstice. At certain 

 occasions during that time of the year the sun, earth and moon app- 

 roached each other more nearly than usual. Such constellations return 

 at intervals of about 1,800 years and are distinguished by an increased 

 intensity in the circulation of the sea, and of ebb and flood, by great 

 variations in the climate and in solar activity. The increased inten- 

 sity in the circulation of the water drove the waters of the North Sea 

 into the Cattegat, the Sund and the Baltic. During the Middle 

 Ages, as now, this did not occur in one even continuous stream but 

 cascade-like, by means of great under-water waves that pressed the 

 salt water into the Baltic much more powerfully than now. These 

 pressings-in were strongest in the autumn, just as now, and with 

 the watermasses or »water-mountains » that the tide-producing power 

 of the moon drove into the Baltic went the great herring shoals, 

 »the mountains of herring » of which Saxo and other chroniclers 

 from the 13th and 14th centuries have astonishing tales to tell. This 

 hydrographical circumstance gave Skanor and Falsterbo a period of 

 extraordinary prosperity during the last centuries of the Middle Ages. 



The powerful in-current during these centuries filled also the 

 deep channels and basins of the Baltic with salt water to a greater 

 height than now occurs. In consequence, the fresh surface-stratum 

 was shallower and during cold winters could be cooled down below 

 the freezingpoint so that, during certain winters, as for example 

 in 1306, 1321, etc. the Baltic became one great sheet of ice, making- 

 it possible to cross the ice from Skane and Denmark to Germany 

 and from Gothland to Estland in Russia. I have shown that it is 

 possible to calculate what the proportion must be between the surface- 

 water and the bottom-stratum for the Baltic to freeze again as during 

 the Middle Ages. In the fig. 15 the limits of such a proportion are 

 shown by dotted lines. If, in consequence of an increased intensity 

 of the undercurrent, the limit of the salinity, 9 °/ 00 , should 

 be altered so as to lie on a level with that shown by the dotted line, 

 i. e. about 8 — 10 met. higher at Bornholm and 15 — 20 met. higher 

 at Gothland, the southern Baltic would freeze as now takes place 

 in the Gulf of Bothnia when there is a cold winter. The same thing 

 would result if the Sund were made a couple of metres deeper; for 

 example, by dredging the submarine-channel called the »Flintran- 

 nan» to a greater depth. We should then once more have the rich 

 herring-fisheries of the Middle Ages at their old places but we should 

 also suffer from the cold winters of that era, with the Baltic Sea comp- 

 letely frozen over, and Skane, in the south of Sweden, would have 

 the cold continental winter-climate it had in the days of Tycho Brahe 

 (middle of 16th century). 



The great immigration of herring that nowadays takes place 

 in the autumn and winter does not, as a rule, extend farther than 

 the tract south of Gothenburg, between Nidingen and Tistlarne, 

 where the submarine channel of the Cattegat becomes narrower. 

 The advance-guard of the herring-shoals, which makes its appearance 

 as early as in August and September, swims higher up into the upper 

 water-strata and spreads over the broad expanse of the southern 

 Cattegat where the fish are caught by means of drift-nets; the her- 

 rings are finally once more squeezed into crowded shoals at the nor- 

 thern mouthpiece of the Sund and the Belts. In certain years, when 

 the sea-impulses are stronger, there is a fairly rich herring fishery 

 here which can be experienced even for some distance into the Baltic. 

 On these occasions there is repeated in our days, although on a di- 

 minished scale, the great herring fishery of Oresund that existed 

 in the Middle Ages, see the relief-chart 



The great period of the greatest possible tide-power and water- 

 circulation in the sea and the most intense solar activity will not 

 return before the lapse of more than 1,000 years. The last time it 

 occurred was at the close of the Middle Ages, and the time before 

 that, at the close of the Bronze Age, about 600—400 B. C. But 

 within these 2,000-year periods there are shorter lunar-periods of 

 the second, third and fourth rank with a length of 80—90 years, 18 

 years, 9 years, 4 years and 2 years, all of which influence the water- 

 circulation and the fish-life of the Baltic, Cattegat and Skagerak. 

 In the annual migrations of the herrings we have a very sensitive in- 

 dicator of the influence exerted by these lunar periods on the move- 

 ments in the sea. When the tide-producing power of the moon in- 

 creases the movements in the boundary stratum af the sea become 

 stronger and the current and the under-water waves carry the herring- 

 shoals farther into the Cattegat and gather them in more crowded 

 multitudes at the now-existing principal fishery-places, viz., the 

 coast-bank south of Gothenburg between Tistlarne and Nidingen. 

 If the lunar power is weaker the herring-shoals remain farther out, 

 on the western and eastern sides of the submarine channel of the 

 Cattegat, all of which acts on the results of the herring fishery. An 

 example of this is given by the following diagram. The undulating 

 line represents the declination of the moon, and the years when this 

 has been greatest are marked at the upper curve, and the years when 

 it has been least are shown at the lower arc of the curve. Between 



— 20 - 



