22 



A. BLYTT. 



[No. 8. 



Southern Hemisphere on the other hand the winter is 5 days longer 

 than the summer. But these relations change through the pre- 

 cession of the equinoxes, the period having a mean duration of 

 21,000 3^ears. Thus, 10.500 j^ears ago the conditions were the 

 reverse of what they are at present, and the same will be the 

 case 10,500 years hence. The winter at the Northern Hemi- 

 sphere will then fall when the sun is furthest from the earth, 

 and last longer than the summer, and in the Southern Hemi- 

 sphere conditions will be the reverse. 



But the orbit of the earth is also subjected to periodical 

 clianges, inasmuch that it differs more from the circular some 

 times than at others. The further it deviates from it the greater 

 becomes the difference between the length of wdnter and sum- 

 mer, and this difference may even amount to more than 30 days 

 every year. The length of winter and summer varies, therefore, 

 in the course of 10,500 years, and the difference increases the 

 more the earths orbit deviates from the circular. During the 

 10,500 years in which the winter is longer than the summer, 

 there will be several thousand more winter days than summer 

 ones, and in the second half cycle there will be as many thou- 

 sand less. Even at present, when the orbit deviates but little 

 from the circular, the excess of winter or summer days for each 

 half cycle is more than 50,000, and when the deviation is 

 greatest it amounts to nearly 220,000 days, or some 600 years. 



As the cooling of the continents contributes to preserving 

 the low atniospherical pressure over the oceans, and this directs 

 the prevailing winds and currents at sea, the winds, thus directed, 

 as, for instance, the south-west winds of the Atlantic, 7nust he 

 stranger in ivinter than in summer. And this is indeed the case. 

 The weather conditions differ in summer and winter. Of course 

 south-westerly winds blow predominantly in the North-Atlantic 

 and West-Europe all the year, but they predominate more in the 

 winter. According to Prof. Mohn their force in the North-Atlantic 

 is about three times as great in winter as in the summer, and 

 similar conditions prevail in the Pacific Ocean. In the southern 

 temperate seas, north-west winds, which correspond to south-west 



