1886.] ON VARIATIONS OF CLIMATE IN THE COUESE OF TIME. 



21 



the consequence is that south-west winds blow in this sea sum- 

 mer as well as winter. 



The opinion held by Croll, Zoppritz etc. that the tvin ds are 

 the chief caiise of sea-currents is now generally accepted by 

 savants. The winds set the surface of the sea in motion and 

 by frictional resistance the movement is conveyed to lower 

 depths. It depends on the force and the duration of the wind 

 how deep the action will have effect. The main current runs 

 in the direction of the wind prevailing* and its speed is depen- 

 dent on the average one of the surface. Winds of short dura- 

 tion are only capable of changing the direction of the current 

 on the surface, but through the predominance of such winds 

 through thousands of years, great currents are created. Their 

 strength may vary, but their direction is independent of tem- 

 porary changes of the wind. For the upper net of currents 

 which alone affects the climate and which reaches to a depth 

 of a couple of hundred fathoms (Mohn), the average direction 

 and force of the wind during the last great epoch are determinal. 



Such a great stream is the warm North-Atlantic current. 

 It softenes the winter even at higher latitudes. As the surface 

 imparts heat to the air, the heat lost is replaced from lower 

 depths, and as long as there is a store of heat below, the sea 

 will always yield heat to the air. The mild climate of Norway 

 is, therefore, dependent on this warm current. It runs predomi- 

 nantly in a north-easterly direction, and, thus, it must, in conse- 

 quence of the general laws for currents and winds have run 

 through untold ages, or, as long as sea and land have been 

 divided as at present, 



We will now see if the force which guides this current is 

 periodically changeable. As we know, the orbit described by 

 the earth round the sun is not circular but elliptical, so that 

 the distance between the two bodies varies according to seasons; 

 when there is winter in the Northern Hemisphere the earth is 

 nearest to the sun, and the nearer the earth approaches the 

 sun, the quicker it runs, so that the winter in the North is 

 shorter than the summer. The difference is 5 days. In the 



