the Displacement of Shore-lines. 511 



cycle, and each cycle, in the calculated curve, is composed of 

 16 arcs. 



The tidal wave, v^^hich is the most important agent in 

 altering the sidereal day, and which makes it longer, rises and 

 falls to a certain extent with the eccentricity. It so pre- 

 dominates over the other forces which alter the length of the 

 sidereal day, that the day steadily lengthens on the average 

 more rapidly in the middle of the cycles when the mean 

 value of the eccentricity is greatest, and more slowly at the 

 boundaries between them, when it is least, and, as regards 

 the individual arcs, with increasing rapidity during rising, 

 and decreasing rapidity during falling eccentricity. 



The interior of the earth is plastic in consequence of the 

 great pressure. The surface or " crust,'' opposes the greatest 

 resistance to change of form. But as the sidereal day 

 lengthens and the equatorial parts of the earth increase in 

 weight, a constantly increasing strain acts outwards towards 

 higher latitudes, and this strain increases until the resistance 

 is overcome. It must also be remembered that forces which 

 are too small to effect any sudden alteration in a solid body, 

 may, nevertheless, produce a change of form when they act 

 for a long time. 



Hence the lengthening of the sidereal day does not act 

 only upon the sea, but also upon the form of the solid earth. 

 The earth constantly approaches more and more to the 

 spherical form ; but the solid earth, in its movements, lags 

 behind the sea, which accommodates itself at once to the 

 altered time of rotation. 



As the motive power of these movements of the sea and 

 the solid earth is periodically variable in accordance with the 

 eccentricity of the orbit, these movements also take place 

 periodically more rapidly and more slowly. And as the 

 sea always adjusts itself to the forces before the solid earth, 

 it is probable that the shore-lines oscillate up and down once 

 for each rising and sinking of the eccentricity of the orbit. 

 This applies both to the individual arcs of the curve and to 

 the cycles. In such a cycle " the mean level of the sea " 

 rises and falls once during 16 oscillations. 



According to Darwin the sidereal day has become several 

 hours longer. It is therefore probable that so great a strain 

 must have accumulated in the mass of the earth, that a 

 slight increase of the strain would suffice to effect changes of 

 form at the weakest points. It is also probable that these 

 partial changes in the solid body of the earth must occur 

 especially during great eccentricities, or some time after 

 them, when the motive power increases most rapidly. 



2M2 



