the Displacenient of Shore-lines. 421 



under an increasing, and more and more slowly under a dimin- 

 ishing eccentricity. 



As has been stated, there prevails, even among physicists, 

 a disagreement as to how far the earth will change its form, 

 in case the centrifugal force varies. Thomson is most inclined 

 to believe that it will not; Darwin is of opinion that it will. 

 And among other physicists whom I have consulted a similar 

 divergence prevails upon this point. One thinks that a 

 lengtheniug of the day even by several hours will be incapable 

 of altering the form of the solid earth ; another believes that 

 the solid earth will probably change its form just as easily as 

 the sea. And with regard to the rapidity with which the 

 sidereal day lengthens, opinions are just as much divided. 

 Darwin regards as possible variations much greater than 

 those wliich agree with the action of the tidal waves 

 calculated by Thomson for recent times. It is therefore clear 

 that this problem can hardly yet be finally solved ^ and that 

 different hypotheses will be for the present admissible. We 

 will therefore select that which is best fitted to explain the 

 facts, assuming that the variation of the tidal wave with the 

 eccentricity of the orbit may possibly be the cause of the 

 periodical displacement of coast-lines. But we put forth this 

 hypothesis with all possible reserve. Divergences of opinion 

 between the most esteemed physicists upon this matter, and 

 the neat manner in wliich the hypothesis is supported by 

 many facts, alone give us the courage to put forward con- 

 jectures which many vdW probably regard as not only bold, 

 but even improbable. 



The motive force of alterations in the form of the earth 

 should therefore be periodically variable with the ecceutricity 

 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 Reen, it becomes a matter of time. Even small 

 forces can produce an effect^ if they only have time to work 

 in. It is therefore probable 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 move- 

 ments 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 



