the Displacement of Sliore-Unes. 407 



The contents of this memoir are essentially as follows : — 

 The precession of the equinoxial lines causes the summers in 

 about 10,500 years to be longer, and in the following 10,500 

 years shorter, than the winters. The conditions are opposite 

 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 continents under high latitudes in the winter 

 produces a diminished pressure of air over the sea. This low 

 pressure draws air from lower latitudes. For this reason, in 

 the Atlantic south-west winds prevail. Thus, in the winter, 

 the south-west winds of the North Atlantic are on an average 

 three times as strong as in the summer, in consequence of 

 the great refrigeration of the mainland. In the semi-period 

 when the winter falls in aphelion the average annual wind- 

 force is consequently greater. Now it is the prevalent wind 

 that produces the powerful marine currents, such as the 

 warm current in the Atlantic Ocean. The strength of the 

 marine currents is dependent upon the average wind-force for 

 the last great time-period. Now, as this average wind-force 

 is periodically variable in consequence of the precessions, the 

 strength of marine currents and the temperature of the sea 

 must also be subject to a periodical variability. For about 

 10,500 years the warm sea-current will increase, to diminish 

 in the next similar period, and so on constantly through all 

 time. When the winter falls in aphelion, the difference 

 between the littoral and inland climates will increase. The 

 propelling force of currents in the sea will increase and 

 diminish by 1—5 per cent, upon their total annual value 

 according as the winter falls in aphelion or perihelion, and 

 according as the eccentricity of the earth's orbit is small or 

 great. 



Such an alteration in the strength of the marine currents 

 will produce an alteration of the climate, which, however, will 

 not be very important, but which will nevertheless be great 

 enough to leave its traces in the deposits. During colder and 

 drier seasons the streams are fed in great part by spring 

 water. This water has drained slowly through the beds and is 

 charged with dissolved materials ; but the small quantity of 

 water and the feebler stream carries less clay, sand, and 

 gravel. During rainy seasons the rain carries down quantities 

 of such materials, but it flows off rapidly, and, as it for the 

 most part runs only over the surface, it has not time to 

 dissolve so much. Although the springs flow more abundantly 

 during rainy seasons, their water only mingles with the rain- 

 water. The streams are therefore poorer in dissolved material, 



