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 it is not powerful enough to cause the great climatic variations at the close of the Middle ages to be repeated. This cannot be expected to happen till the lapse of 1,000 years. But the sea is so sensitive 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 fish life and the ice conditions 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 ice cold or ice covered surface must have cooled the atmosphere above it. The climate 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 E.H. 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 [1] to attempt a reconstruction of the climate at Örseund in the 15t^thcentury. Like myself, Ekholm assumes it to be true that the Baltic was frozen in severe winters.

Ekholm points out that our cold winters now are always characterized by numerous cyclones, which pass south of Sweden. They generally come from the west and probably originate in our area fed by the Gulf stream.

Ekholm also points out that P. La Cour has found a considerable divergence in the wind direction during the winter months of Tycho Brahe's time from the present one.

In consequence of this low pressure belt the cyclones now generally pass across the south or middle part of the Scandinavian peninsula, bringing south westerly winds in Öresund. But the eastern or southeastern wind in 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 nowadays except in the early spring months and in exceedingly severe winters. “The opinions I have expressed in my paper on climatic variations in historic time coincide with Ekholm's representation in the chapter bearing the same title in his papers of 1899 and 1901. The impression to be got from our joint descriptions of the chief features in the climatic variations of the Middle ages 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 east coast of Greenland. Notes about ice hindering navigation in these seas are very rare. The sailing route to Greenland from Iceland ran straight west over the Gunbjörnsskär group of islands off Cape Dan, thence along the east coast to the straits north of Cape Farewell, which were then navigable and free from ice. Nor was navigation hampered by the ice in Davis Sound or in the sea between Greenland and North America. Ice was first met within the far north at the fishing places of Baffin 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 Gulf stream.

It is the drift ice of the Labrador Current which attacks the Gulf Stream south of Newfoundland and compels its warm water to spread eastwards towards the submarine base of the Azores and the coastal bank of Europe. There it is joined by the warm outflow from the Mediterranean, which the Danish expedition on Thor in 1910 found extended as far as the west coast of Ireland, and forms the vast area of warm water over which the great low pressure belt south of Iceland forms in winter. When no ice was carried by the Labrador Current and this current consequently played a less conspicuous part in the oceanic circulation, the Gulf stream “could take a more westerly course”, as Ekholm says. Then the ice melting took place in higher latitudes, in Baffin Bay, in the Arctic sea and even in the Polar basin.

Western and North western Europe then possessed a more continental climate with colder and calmer winter weather.

At the end of the 13^th century the first signs of an ice blockade of Iceland appeared and at the close of the 14^th century the drift ice outside East Greenland had developed so far, that the sailing route had to be altered. At the close of the 13^th century the invasion of Greenland by the Eskimos appears to have begun, and in the 14^th century came the destruction of the West Greenland settlement, followed by that of the eastern settlement and the loss of communication over sea. In 1200-1300 Iceland was again and again subjected to ice blockades. The ice drift from the polar sea steadily increased towards the time of the maximum of the tide generating force.

In the Baltic the effect was to increase the influx of salt water which carried the herring shoals into Öresund and gave rise to an abundant fishery which attained its maximum from 1100 to 1500 and then declined or was removed to northern Cattegat and the North Sea. The swelling of the bottom layer compelled the surface water to escape and to spread as a thin coating over Skagerak and Cattegat which there, as well as in the Baltic proper, became excessively heated in summer and cooled in winter, when there was a tendency for anticyclonic development over the snow covered Scandinavian peninsula surrounded as it was by a broad brim of ice cold or frozen Baltic water. Cyclones originating in the low pressure belt south of Iceland could as a rule not pursue their present course over Skagerak and southern Sweden, which then was the center of the anticyclone.

They bent their course to the south, south of Hven, passing over the coast of north Germany where, in conjunction with the high water at spring tide, they caused the destructive storm floods of the winters and the torrential rains and inundations of the summers described in the chronicles. This is proved by yet another circumstance. The Volga is the only considerable tributary to the Caspian Sea. Brückner and Huntington have shown (see later) that the Caspian Sea had an exceedingly low water level in the 11^th century. This must mean that he conflux from the Volga decreased in consequence of the continental meteorological conditions in Europe at this time. These conditions must have altered entirely at the end of the 12^th century for then the level of the Caspian sea again rose according to Huntington:

It was the Volga, which had increased its water supply, its sources feeding on the rain brought by the Atlantic cyclones which, as Ekholm says, passed over Europe south of the island of Hven. From the Atlantic cyclones were sent out, partly to the Norwegian Sea and the coast of Norway, partly over the North Sea and the coast of Middle Europe. Sweden, Denmark and the Baltic countries generally appear to have suffered less from cyclones, according to Tycho Brahe's observations, owing to the fact that the Baltic in winter formed part of the anticyclone area which probably at the time was joined to the great Siberian-Russian anticyclone belt.

These climatic conditions had a bad effect on the harvest and economic conditions of Iceland, Greenland and north Europe. As regards Greenland and Iceland I refer to a previous chapter of this paper.

As to southern Europe I here reproduce the following description from the 14^th century contained in Jessop's Historic Essays p. 175:

All through the 14^th century earthquakes occurred continually all over Europe (which was the case in Iceland also). The earthquake in Jan. 1348 became famous because it occurred in Avignon simultaneously with the outbreak of the plague (in January according to Guido de Chauliac).

It does not seem strange that the maximum of the tide generating force which so greatly influenced the ocean's movements should be accompanied by earthquakes. The Committee for earthquake statistics appointed by the B.A. has found a periodicity of 18 years in this phenomenon, which seems to hint a connection with moon periods. Also in Scandinavian countries the climatic variations appear to have brought bad years.

Of corresponding events in Iceland during the 14^th century Professor P.A. Munch relates: (2)

On examining the historic dates from the last centuries of the Middle ages, Dr. Bull of Chistiania has come to the conclusion that the decay of the Norwegian kingdom was not so much a consequence of the political conditions at that time, as of the frequent failures of the harvests so that corn for bread had to be imported from Lübeck, Rostock, Wismar a. s. o. The Hansa union undertook the importation and obtained political power by its economic influence. The Norwegian land owners were forced to lower their rents. The population decreased and became impoverished. The revenue sank 60-70%. Even the income from the Church property decreased.

In 1367 corn was imported from Lübeck to a value of ½ millllion kronor. The trade balance inclined to the disadvantage of Norway whose sole article for export at that time was dried fish. Dr. Bull draws a comparison with the conditions described in the Sagas when Nordland produced enough corn to feed the inhabitants of the country. At the time of Asbjörn Selsbane the chieftains in Trondenäs grew so much corn that they did not need to go south over to buy corn unless three successive years of dearth had occurred. The province of Throndhiem exported wheat to Iceland a.s.o. Probably the turbulent political state of Scandinavia at the end of the Middle ages was in a great measure due to unfavourable climatic conditions, which lowered the standard life, and not entirely to misgovernment and political strife as has hitherto been taken for granted.

I have already pointed out that these conditions must be due to cosmic causes. But the influence of cosmic agents is not purely local, it is felt all over the earth. If the absolute maximum of the tide generating force in the beginning of the 15^th century really affected the climate of the earth, then its effects should be traced also outside the area whose climate is dominated by the Atlantic.

The areas that can come under consideration are North America and Central Asia. In these parts of the world an intense research work is carried on which I can only slightly touch upon here. Professor E. Huntington (3) has studied the climatic variation both in Asia and America from a geological point of view. In America ha has obtained results of considerable interest by measuring the annual growth of the big Californian fir trees Sequoia gigantea some of which are 2000-3000 years old. I reproduce here Huntington's diagram (Fig 20) of the growth pro decade of their annual rings (i.e. the width of 10 annual ring in mm corrected for the difference in growth in the different ages of the tree. Fig 20. The growth, of course depends on the climate. Huntington writes:

Judging from the diagram such climatic conditions have prevailed during the 13^th and 14^th centuries. To me it seems probable that the abundant precipitation which favoured the rapid growth of the Californian pine trees was due to the influence of the tide generating force of the circulation in the Pacific. In any case it is interesting to find evidence of exceptional climatic conditions at the end of the Middle ages on the Pacific coast as well as on the Atlantic. In the interior of Asia these variations were still more fateful. The Asiatic steppe lakes alternately expanded and dried up causing destruction to the districts and cities on their shores which compelled the population to migrate. Huntington's investigation of ancient ruins and shorelines bring him to the following conclusion: “All the lakes appear to have experienced a period of contraction in the early part of the Christian era followed by expansion in the Middle ages and by renewed but less marked contraction in modern times.”

The expansion of the steppe lakes in Asia, which indicates a wet period occurred, according to Huntington, simultaneously with the moist period in California or about 1400 a.C. It was preceded by a dry and warm period which turned the kingdoms of central Asia into deserts and caused the invasion of Europe by the steppe people under Genghis Kahn a.o. According to Huntington several such periods of drought have occurred since the beginning of the Christian area both in America and central Asia. Huntington's opinion has recently been confirmed by Brückner who is a great authority on this matter; Brückner writes: “ebenso ist der Mongolinvasion eine Klimaverschlechterung in Asien vorausgegangen. Für diese liegt ein sicheres Sympton vor: das Kaspiche Meer hatte, nach Bauwerken an seiner Uferzone deren Alter sich bestimmen liess, in XII Jahrhundert einen Tiefstand wie niemals nachher und wie lange Zeit nich vorher. Ein silcher Tiefstand kann nur durch grosse Trockenheit hervorgerufen worden sein. Diese Trockenheit kann aber nicht lokal gewesen sein sondern muss, wie das ganze Gebiet der Wolga, so auch weite Gebiete Zentral-Asien mitbetroffen haben.

Brückner also points out how the effects of these wet or dry periods differ according to the geographical position of the countries. A period of draught which turns vast areas in Asia and North America into desert land may bring fertility and opulence to the countries on the Atlantic shores:

Bruckner's investigations concerning the 35-36 years variation of the climate, the rise and fall in the amount of precipitation, the prices of corn, and the emigration within the last century indicate, that the problem has once more become actual as stated by Ehrenheim in his paper “Om climaternas rörlighet” has once more become actual.

The very extent of these variations which encompass both America and Central Asia indicates that they are of cosmic origin. I have suggested that the one of their causes may be the variations in the tide generating force which in the first place depends on the changes of the moon periods. To be valid this explanation must be shown to apply to the great periodic variations encompassing 1800 years as well as to the smaller changes which are accomplished within a month. It is possible that by using the harmonic analysis in a less summary manner than the hitherto we may be able to distinguish between the effects of heat and gravity upon the meteorological variation of short periods. Mr. Strömberg who assisted me in the calculations of astronomic values has brought forward a new hypothesis which is the subject of a special paper and which shows the influence of the variation in the tide generating force on the changes of the mean temperatures in the course of a month. It is a common belief that the moon influences the weather. This belief is founded on the actual experience of mankind through countless ages.

Before starting weather forecasts of long range, however, it would be well to see how far the experience of the past bears out the theory here advanced as to the connection between climatic changes and the variation in the tide generating force. The greatest of the periods I have indicated, the period for the occurrence of the absolute maxima, attained its latest epoch in 1434 and has a periodicity of about 1800 years. The maximum previous to that of 1434 must have occurred about 366 b.C. In the interval there must have been an absolute minimum of the period, some where about 500-600 a.C. What happened then?

As already mentioned, an absolute maximum of the tide generating force occurred 3 or 4 centuries before the Christian area. We must now if this maximum also had effects resembling those in the 14^th century a.C. e.g.; the devastating storm floods on the North Sea and the Baltic coasts, the inundations and cold periods in the northern countries with their consequences, famine and migration of the population.

We possess a few historic notes from which we may infer something relating to the climate. The best proofs however the study or archaeology and quaternary geology will furnish. The 3^rd and 4^th century b.C. denote the earliest epoch of the Iron age or rather the transition from the Bronze age to the Iron age. During the later Stone age and the first stage of the Bronze age, which, according to Montelius, may be put at about 1600 b.C., the Littorina epoch still lasted. The oceanic water had then freer access to the Cattegat and the Baltic, partly because the headland of Skagen was not yet formed, partly because the great depression with which the Littorina epoch commenced had made Öresund deeper than it is now by some 5 or 6 meter. The salt water then entered the Baltic in larger quantities and the North Sea fishes found the same conditions of life in the middle of the Baltic as nowadays in southern Cattegat (see Munthe's map p. 24).

To the geologist of the quaternary period the threshold of the Baltic in the Belts and the Sund constitute a point of vantage in estimating the influence of post glacial changes. The numerous careful measurements of the fossil deposits on the coast of Scania and Sjaeland have given the following results.

During the epoch of the Littorina sea Öresund was 5 or 6 meter deeper than now. The salt oceanic water could then enter the Baltic more easily than now, and this influenced the hydro graphic state and the animal life of the Baltic, which then differed greatly from the present. From the former and present spread of the mollusk fauna Munthe has mapped out the salinity limit (the isohalines) of the surface layer of the Baltic at the Littorina period. A comparison between Munthe's surface map and that of the present time which I have compiled from the F.L. Ekman's observations in 1887 shows the influence on Baltic hydrography caused by a livelier interchange with the waters of the ocean due to the lowering of these thresholds of the Baltic. The state of the Baltic in the neighbourhood of Gothland during the Littorina period corresponds to that of southern Cattegt at the present time.

The salt water had, consequently, a far freer access to the interior of the Baltic, and with the salt water came the sea fish whose migrations at the time extended far into the Central and Northern Baltic. Munthe has been able to reconstruct the hydrographical conditions existing during the Littorina period, in a surface map of the salt percentage existing at that time. If we compare Munthe´s chart of the salt proportion in the Baltic during the Littorina period, i.e., for about 5,000-6,000 years ago, with a similar map for the present day we shall find that the limit for 100% salt, which now lies at the Sund and Gedser, during the Littorina period lay in the gulf of Bothnia, north of Åland. The Annual herring migration from the North Sea, which now extends to the neighbourhood of the Sund, extended during the Littorina period through the whole of the Baltic and into de Gulf of Finland and the Gulf of Bothnia.

The same sate of things must have existed as regards the other sea fish: plaise, flounder, cod and sprat etc. Southern forms of molluscs, too, such as Tapes, Littorina, Ostrea, came with the Atlantic waters into the Baltic. Oyster banks existed everywhere in Isefjord, and around the Danish Islands. This was the period of the “kitchen middens”; the great fish migration period, when the gates of the Baltic stood wide open to the fish from the Atlantic. All the species of fish found in the Baltic, except the eel, sea spike, and mackerel, are to be considered as relics from the Littorina period, at the close of which began to rise of the land which restores the depth conditions of the Sund and the Belt to about their present position. This geological alteration, which was completed about 3,000 years ago, had a far reaching influence on the fish species of the Baltic, which are now in a greater or less degree separated from their relations in the ocean and live under other hydrological conditions which have gradually altered their physiological life conditions and have even commenced to set their stamp on the bodily and the exterior form of the fish.

The expression “relict” must, however, be understood relatively. The ancient relicts from the days of the Polar Sea consist, at present, of some lower animal forms, mostly inhabitating the great depths and including too the arctic gray seal, which still lives in the Baltic and in the lake Ladoga in Russia. These are relicts in the original sense of the word, from a period dating 50,000-100,000 years back. Amongst them there is only one certain arctic species of fish, the Cottus quadricornis.

Among the relicts of the Littorina period we reckon, in the first place, the strömling or small herring, the indigenous herring race of the central and northern Baltic, which now lives isolated from the North Sea herring, whose migrations nowadays do not extend past the portals of the Baltic Sea. From the first, the strömling was a local race –a relic of the great herring migrations of a couple of thousand years ago, which has survived and gradually differentiated physiologically and even morphologically into a new species.

After the maximum of the Littorina epoch, which occurred before the beginning of the Bronze age, in the time of the “Kjökkenmöddinger” (ancient refuse deposits) 4000-5000 years ago, and upheaval of the land surface occurred which lessened the depth of the Öresund to something like the present. This elevation of the land was almost complete at the close of the Bronze age 600-500 b.C. From then, for the last 2500 years the bottom of the Öresund has remained nearly constant, its elevation increasing only some 0.25 meter since that time. The atmospheric conditions also changed into the cold climate of the early Iron–age. The transition from the warm climate of the Bronze age was according to Sernander accomplished in a few centuries, 650-400 b.C., at the last stage of the Bronze age. The temperature of this epoch (“the Fimbul winter” of the Sagas) must have been considerably lower, for in the peat layers from that time we find deposits of sub arctic forms. From the end of the Bronze age a gradual elevation of the land has been and is still in progress. Sernander estimates the elevation for the last 2,000 years to:

In the environment of Upsala ……………. 10 meter
On the West coast of Sweden …………… 5 m.
In Öresund ………………………………………..... 0,24 m.

On the Bohuslän shore are found sub fossil remains of Ostrea edulis, Tapes decussates a.d. which formed the ancient oyster beds of the Littorina epoch, but which now are either extinct or survive only as relicts in a few protected localities. They bear testimony both to the climatic deterioration and the upheaval of land during and after the Bronze age.

In Greenland, Spitzbergen, Franz Josef Land and on the coast of North America sub fossil deposits are found of molluscs which must once have lived in warmer waters. This proves that the land elevation and the deterioration of climate have passed over the entire North Atlantic coast. I will give some examples in proof of this.

The Swedish expeditions found in numerous laces on Spitzbergen sub fossil deposits of Mytilus edulis (1). Among such sub fossil relicts from a more temperate sea found on Spitzbergen there are, beside Mytilus, also Cyprina islandica, Littoria littorea, and Anomia squamula, all of which cannot live in the fjords of Spitzbergen under the present conditions.

On the shores of the southeast coast of the Disco bight on Greenland, A.S. Jenesen (2) has found fossil shells of Anomia squamula and Zirphea crispata. The present limit of these molluscs is the south coast of Labrador and the St. Lawrence bight, which shows that during some part of the post glacial time a warmer climate must have prevailed than now.

The chief representative of this warmer postglacial period in the mollusc fauna of East Grönland and of Franz Josef Land is Mytilus edulis (3). On Iceland it is represented by Purpura lapillus (4) a. o.

It is probable that all these southern species of molluscs lived simultaneously in the North Atlantic ocean and its ramifications during this warm postglacial period. For the more distant parts of the ocean this view still must be regarded as a hypothesis but in Sweden and Denmark was have archaeologic finds excavated from the shore deposits which enable us to discern between the warmer and the colder period and to fix the time limit with some approach to certitude. The southern mollusc species are found together with remains from the Bronze age. As to the next period, the rarity of archaeological finds in the graves from the early Iron age about 400 b.C to 100-200 a.C. shows that the high stage of civilization in the Bronze age had for some reason or other declined and that the population had decreased and was less prosperous.

1. G. Andersen found Mytilus also in King Charles island about 40 m. above the sea level: the Norwegian geologist Staxrud found the same mollusc at 60 m. on Spitzbergen. V. Nordmann: Anomia squamula som Kvartae fossil paa Spitzbergen. Meddel. Dansk Geol. Forening Bd 4 Köbenhavn 1912.
   
2. Ad. S. Jensen: On the Mollusca of East Greenland I. With an introduction on Greenland fossil Mollusc fauna from the quaternary time. Medd. Om Grönland. Bd 29. 1909 (Reprint 1905)
   
3. A.G. Nathorst. Bidrag till nordsöstra Grönlands geologi. Geol. Föreningens Förh. Bd. 23. Stockholm 1901.
   
4. G. Bardarson, Purpura lapillus i haevede Lag paa Nord Kysten af Island. Vidensk. Meddel. Naturhist. Fören. I Köpenhavn 1906-1907.

This decline begins already in the last stage of the Bronze age, which according to Montelius occurred 600-500 b.C. About his time another significant alteration took place. Montelius (1) has proved that the amber trade then took up a new route. During the Bronze age amber was chiefly brought from the North Sea coast and transported on the western trade routes, the Elbe, the Weser a.s.o. to the Mediterranean countries. From 700 b.C the western route for this trade was exchanged for the eastern trade route along the Vistula a.s.o. which indicates that the supply of amber was thenceforth derived from the Baltic instead of from the N. Sea. The cause of this was, according to my opinion, the following:

The climatic deterioration which set in towards the end of the Bronze age resembled that which occurred 1800 years later at the end of the Middle ages in so far that violent storm floods devastated the coasts of the North Sea whereby the districts in which amber was found: Friesland, the west coast of Jutland and Schleswig, were destroyed. I here recall the fact that there are statements in literature that connect the invasion by the Teutons and Cimbrians into Gallia and later into Italy with a big inundation of the Sea which destroyed their homesteads in Jutland (the Cimbrian peninsula).

These catastrophes, which probably began as early as the 6^th century .bC., struck directly the Scandinavian tribes who invaded the German and later the Gallic countries. Italy first felt the shock when the Gallic tribes began to rally the country. All the stages of this transmigration of the northern tribes, which was the first to shake the power of Rome, can now be traced thanks to the archaeological discoveries which were discussed by the archaelogic and geologic Congresses in Stockholm 1908 and 1910. Kossina of Berlin proved that a transmigration of the tribes from the neighbourhood of the Vistula began about 600 b.C. and Lineau of Lünenburg traced the same movements in the western provinces along the Weser. Next comes the proof to be gathered from ancient writers of History (the ancient Roman writers of History, Florus, a.o.) who tell us of the Cimbrians invading Gallia and Italy threatening the Roman Republic by their great victories at Noreia, where Papirius Carbo was defeated in 108 and Arausio in 109, until Marius by his victory at Aquae Sextiae averted the consequences of this first transmigration of the German tribes from the countries on the Mediterranean.

This was the primary cause of the first transmigration in the first millennium b.C. which commenced with the decline of the Bronze age civilization through catastrophes in Nature which forced the inhabitants of the North Sea countries to emigrate.

If we inquire into the physical causes of the alterations which compelled the tribes of Northern Europe to emigrate we find that a maximum in the tide generating force of the sun and moon must have occurred about the 3^rd and 4^th century b.C. This caused disturbances in the oceanic circulation such as the storm floods and the inundations of the coast and deterioration of the climate along the borderland of the North Sea and the Baltic. Its effect on the Atlantic coast of Spitsbergen, Iceland and Greenland is traceable by the lowering of the sea temperature which caused the extermination of the delicate southern species of molluscs, the Tapes, Anomia, Zirphaea, a.o. Very likely outbursts of polar ice also occurred as happened 18 centuries later about 1300-1400 a.C.

In the Littorina epoch there were oyster beds along the coast of Sjaelland and the Scandinavian coast offereing great opportunities for the maintenance of a population both with regard to agriculture and fishery. The people of that time traded with the British Isles as well with the Mediterranean countries (1). Besides fur and slaves, amber was the chief article for barter. The supply of amber came then from the North Sea coast. The principal trade route was along the Elbe up to the Danube and then over the Brenner pass into Italy.

The climate at the culmination of the Littorina epoch was Atlantic, insular, but with the beginning of the great land elevation it passes over into a warm and dry “subboreal” stage which lasted the greater part of the Bronze age up to 600-400 b.C. when the climate deteriorated greatly and the civilization of the Bronze age perished with the postglacial warm period.

The land elevation during the Littorina epoch was greatest on the Bothnian coast. North of Hernösand it attained its maximum of 288 meter. At Öresund and the Belts it was at its lowest, about 5-6 meter, which however was sufficient to change the conditions of the fish life in the Baltic in the manner already described.

In the Bronze age, 1600-650 b.C., the Scandinavian climate was warm: all boundaries of vegetation in Sweden were then on the average 3º further north than now. In proof of this B. Sernander(3) relates the following facts.

In Sjaellan millet was grown. Trapa natans, now extinct in Sweden, grew in the lakes of northern Sweden.

The hazel tree grew in higher latitudes, and at greater height over the sea on in the Swedish Nordland.

The boundary of the pine forests lay in higher altitudes on the mountain slopes. (2) Stipa, now extinct safe in the neighbourhood of Falköping in Vestergothland, was then quite common.

According to Sernander (3), the climate during the Bronze age was warm and dry. The land elevation brought large areas of the present Baltic coast of Sweden, f. inst. part of Upland, above the sea level and numerous lakes and marshes were formed, which dried up in the dry climate. Sernander claims to have discovered a “drying zone” in the bottom deposits of these lakes contrasting sharply with the succeeding layers which consist of clay and sand brought thither by the rivers during the next period which was markedly colder.

We have of course no report as to the state of the Greenland Sea at this remote period, but we know something about the sea north of Iceland from Pytheas journey in 330 b.C. He speaks of the sea as “mare pigrum” a sluggish and congelated sea. It is curious that ice and ice hindrance are mentioned just then as being typical for the northern seas and that 18 centuries afterwards, at the next maximum epoch. The Icelandic annals also report new and powerful ice blockades of the coasts of Iceland and Greenland, whereas in the intervening time in the 8^th and 9^th century a.C. the Vikings appear to have met with no hindrance at all from ice in their journeys to Iceland, Greenland and America. Apparently a warm and iceless period, which favoured agriculture and shipping, and allowed the Scandinavian races to expand in the powerful manner which characterizes the Viking age, must have occurred between the intervals of the two maxima of 400 b.C. and 1400 a.C. This prosperous epoch then corresponds to the former postglacial heat period or the Kjökkenmödding and the earliest stage of the Bronze age.

The remembrance of the bygone civilization two thousand years earlier lived in the myths if the German race and found its expression in the Edda. According to Viktor Rydberg, the myths of the Edda centre about a great catastrophe in Nature, the Fimbul winter, or “Gotterdämmerung” when frost and snow ruled the world for generations.

One relic of the lost civilization appears to have outlived the “Fimbul winter”. By the ancient temple of Upsala stood an enormous tree, perennially green, which is thus described by Adam von Bremen:

Sven Nilsson expressed the opinion that the holy tree of Upsala was a yew tree. Fritz Läffler in a recently published paper “Det evigt grönskande trädet vid Upsala Hednatempel” (i Fästskrift till H.F. Teilberg), has expressed the same opinion founded on a large collection of proofs. Adam von Bremen's statement: “no one knows what kind of tree it is” has caused some difficulties because nowadays the yew tree can be grown in Upland and is still found in western Norway although it requires a more temperate climate than ours.

In the Swedish museums also prehistoric vessels made of yew are preserved. Conventz, who examined these finds, which belong to the period 800 b.C to 900 a.C., reports however, that vessels made of this wood material are very rare in the ancient provinces of Sweden. This Schübeler corroborates. Nathorst reports that no fossil or sub fossil remains of the yew have been found within the precincts of ancient Sweden. The vessel in the museum of Stockholm was found in Bohuslän and the two vessels in the museum of Lund are probably from Danish territory. Läffler says: “ancient Sweden would this be devoid of finds of antiquity made of the wood of the yew tree. This would sufficiently explain why the holy tree of ancient Upsala was said to be of a kind unknown to the inhabitants of that place in 1100”.

Läffler comes to the conclusion that during the Viking age the yew was extinct in Sweden, because it had been exterminated. He believes that the big tree in Upsala was a remnant left from a previous period, “it cannot be denied, that such a tree may have grown from a seed brought from abroad, or that the population may have protected and guarded this relic of a past vegetation.”

In this conjecture Läffler is probably right. The yew tree grows very slowly and may certainly attain an age of 1000-2000 years. We have every reason to believe that the holy tree in Upsala was more than 1000 years old and had survived from the Bronze age.

Great transmigrations of people occurred also in the second millennium b.C. Then too the direction of the movement was towards the Mediterranean countries and it is not impossible that its cause may also been unfavourable climatic conditions in northern Europe which, according to our theory, should have culminated about 2000 b.C. It should also be remembered that at the time of the melting of the inland ice in Sweden, cold and warm periods must have alternated.(1) The traces left in the stratified clays indicate that the melting process was not uniform the whole time through. It would be of great interest to ascertain whether some kind of periodicity can be traced in the ice melting process.

Going back as to the melting of the ice cover or about 9000 years ago, we have to take into account one more fact, i.e. the change in the eccentricity of the ecliptic and in the inclination of the Earth's axis against the ecliptic, which according to Ekholm and Charlier must have caused a warm period about 9000 years ago (and a cold period about 28,000 years ago).

If we keep to the theory that climatic variations are caused by cosmic agents, then we must conclude that the greater variations which accompany the geological epochs (the glacial period, the tertiary period a.s.o.) are ruled by changes in the ecliptic and in the position of the Earth's axis and the lesser variations, which are the subject of this investigation, are ruled by changes in the position of the orbit of the moon of which the longest periods encompass some 19 centuries with minor periods of 93, 18, 9, 4½ years a.s.o., the briefest, or those which influence the weather, measuring months or weeks.

Several maxima of the tide generation force have fallen in the interval between the melting of the inland ice and the end of the Littorina epoch. These maxima have been accompanied by catastrophes in Nature such as storm floods, inundations, a.s.o. On the coast of Scania there are remnants of such catastrophs in the shape of shingle banks, as e.g., the “Järabacken” which must have been formed in prehistoric time. It would be interesting to find out if its formation coincided with the critical epochs of Stone age and the Bronze age here described.

Sven Nilson held that Järabacken was formed by a violent uproar of the sea probably at the end of the Bronze age, which event marked the great emigration of the northern tribes 600-100 b.C Recent geologic and archaeological investigations make it probable that Järabacken belongs to a earlier period. De Geer who has traced this formation all round the coast of Scania believes that it marks the maximum limit of the sea during the Stone age.

It is a recognized fact that after the glacial period great variations of climate have occurred. My effort has been to show in this paper that these climatic variations are connected with variations of the tide generating x force. At the epochs when maxima and minima in the tide generating force occur, changes in the climate have occurred which in some cases have assumed a catastrophic character whereas at the time of minima the climatic conditions appear to have been more stable and uniform. I have shown that there is an interval of about 18 centuries between the absolute maxima, the last one occurred at the end of the Middle ages and the last minimum at the beginning of the Viking age. The diagrams in page 7 show the position of the lunar orbit in 1434, 1894, 1903, and in 1912. The diagrams show clearly the difference between our time and the time of absolute maximum 500 years ago. It is also evident that a secondary maximum of the type we call perihelion apside has approximately occurred within the last 18 years at the time of the winter solstice and the Earth's perihelion.

These constellations have shown their effects in certain changes of short periods in the climate, but chiefly in disturbances of the ice conditions in the Arctic and Antarctic seas and the polar currents.