ON THE MOVEMENTS OF THE EARTh's CRUST. 339 



The motive force of alterations in the form of the earth shoukl there- 

 fore be periodically variable with the eccentricity of the orbit. The sea, 

 which is fluid, adjusts itself at once in accordance with the smallest 

 change in the length of the day. But the solid earth offers resistance'; 

 and the day lengthens slowly and imperceptibly. With such small 

 forces, as we have already seen, it becomes a matter of time. Even 

 small forces can produce an effect, if they only have time to work in. It 

 is therefore i)robable that the solid earth will be behind the sea in its 

 movements. Some time will elapse before the "crust" and the inner 

 plastic mass begin to yield. The ground under a building often begins 

 to give way only when the building has stood for some time. If then 

 the solid body of the earth lags behind the sea in its movements, and 

 the movements both of the sea and of the solid earth occur periodically 

 more strongly and more feebly, because the motive force is stronger and 

 weaker according as the eccentricity of the orbit increases or diminishes, 

 it was conceivable that the coast-lines would come to be displaced up 

 and down once for every time that eccentricity increases and dimin- 

 ishes. For there must be the greatest probability that the solid earth 

 ma3' yield at one place or another when the tension in the interior be- 

 comes strongest. 



It is important now to examine whether the action of the tidal wave 

 and variations in its strength are great enough to explain the displace- 

 ment of the coast-lines. This is a mathematico-physical problem, and 

 it is not for me to solve it. I put it as a question for the decision of 

 competent men, and shall confine myself to the following remarks: 



If the sidereal day has once been several times shorter, and the earth 

 at the time was a solid body, the tension and pressure in its interior 

 will increase with the length of the sidereal day, until finally the tension 

 becomes so great that the earth begins to yield. It will then accommo- 

 date itself, if not in its entirety, at least partially, until the tension is 

 equalized, at any rate in part. Perhaps then a state of repose will 

 occur, during which a new tension will accumulate, which may introduce 

 a new change of form. And these spasmodic changes of form in the 

 body of the earth when strained to the limit of its power of resistance 

 would occur precisely when the eccentricity had approached its highest 

 value, and the tension increased most rapidly, or sometime afterwards. 

 Under such circumstances, possibly, the small variation which the tidal 

 force undergoes with the eccentricity would turn the scale, and deter- 

 mine the time for the changes of the solid earth. 



Thomson says ( Trans. Geol. ISoc. Glasgoiv, 18G8) that it is still ho[)e- 

 less to attempt to solve the question of how rapidly the sidereal day 

 lengthens, by means of tidal action. By way of trial he calculates (/. c. 

 p. 26) the action of the existing tidal wave to be so great that the earth 

 in one hundred years should be retarded one hundred and eighty scc;- 

 onds, with which corresponds a lengthening of the day of 0.01 second ; 

 '4>nd if we take this retarding power, for the sake of simplicity, as con- 



