1889.] 



THE DISPLACEMENT OE BEACH-LINES. 



already made, in my Memoir on the Probable Cause of the 

 periodical Alteration of the Velocity of the Ocean currents 

 (Chria. Vid. Selsk. Forh. 14 Dec. 1883; Biolog. Centralbl. Er- 

 langen 1884 n. 2). 



The material substance of that Memoir is as follows: The 

 precession of the equinoxes causes the summer to be, for about 

 10,500 years, longer than the winter, but in the subsequent 10,500 

 years to be shorter. This relation is reversed in the northern 

 and southern hemispheres. The difference between the number 

 of winter and summer days increases with the eccentricity of 

 the Earth's orbit. 



The cooling of the Continents in the higher latitudes during 

 winter, produces low atmospheric pressure over the seas. These 

 low atmospheric pressures exhaust the air from the southern 

 latitudes and from this cause south-westerly winds prevail in 

 the North Atlantic Ocean. 



During winter the south-westerly winds of the North Atlantic 

 Ocean are, therefore, owing to the great reduction in the tem- 

 perature of the interior land, usually, thrice as strong as in 

 summer. In the semi-period, when the winter occurs in aphelion, 

 the mean aunual velocity of the wind becomes, consequently, 

 greater. Now, it is the prevailing winds that produce the grand 

 ocean currents, such as the warm stream of the North Atlantic 

 Ocean. The velocity of the ocean currents is dependent on the 

 mean velocity of the winds during the last great division of 

 time. As, now, this mean velocity of wind is periodically chan- 

 geable, owing to the equinoctial precession, therefore the velocity 

 of the ocean currents, and the temperature of the waters must, 

 consequently, be subjected to a periodical change. 



For about 10,500 years that warm current will increase in 

 velocity and temperature; and, again, in the following 10,500 

 years will diminish, and will continue to alternate thus through- 



When winter occurs in aphelion, the difference between 

 coast-climate and inland-climate will become more distinct. The 

 motive-power of the ocean currents will increase and diminish 



