422 A. Bly tt on the prohahle Cause of 



would come to be displaced up and down once for every time thnt 

 eccentricity increases and diminishes. For there must be the 

 greatest probability that the solid earth may yield at one place 

 or another when the tension in the interior becomes 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 displacement of coast-lines. This is a mathema- 

 tico-physical problem, and it is not for me to solve it. 1 put 

 it as a question for the decision of competent men, and shall 

 confine myself to the following remarks : — 



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

 the earth at the time was a solid body, the tension and pres- 

 sure 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 accommodate 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 in- 

 troduce 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 eccen- 

 tricity had approached its highest value, and the tension 

 increased most rapidly, or some time afterwards. Under such 

 circumstanceSj possibly, the small variation which the tidal 

 force undergoes with the eccentricity would turn the scale, 

 and determine the time for the changes of the solid earth. 



Thomson says (Trans. Geol. Soc. Glasgow, 18(38) that it is 

 still hopeless to attempt to solve the question of how rapidly 

 the sidereal day lengthens, by means of tidal action. Byway 

 of trial he calculates (/. c. p. 2(j) the action of the existing 

 tidal wave to be so great that the earth in 100 years should 

 be retarded 1 80 seconds, with which corresponds a lengthening 

 of the day of 0*01 second; and if we take this retarding power, 

 for the sake of simplicity, as constant, the day, in 100,000 

 years (the time which is on the average occupied by nn oscilla- 

 tion of the eccentricity) should become 10 seconds longer. 

 Moreover, Thomson reckons only the marine tidal wave. To 

 this should now be added Darwin's " interior tide,'' his " bodily 

 tides," which I know no means of calculating. For many 

 millions of years, when the moon was nearer and the tidal 

 action considerably stronger, the day also increased more 

 rapidly. But nowadays its increase is undoubtedly much 

 slower, and we cannot expect great general changes of level 

 in a short time from this cause. 



To a lengthening of the day by 10 seconds (according to 

 Todd, /. c.) corresponds a shortening of the equatorial radius 



