1889.] THE DISPLACEMENT OF BEACH-LINES. 23 



its form with as much ease as the sea. And as to the speed 

 with which the sidereal day becomes prolonged there are quite 

 as greatly divided opinions. Darwin assumes many times as 

 great changes as Thomson does. It is clear, therefore, that 

 these problems can scarcely yet be finally solved, and that dif- 

 ferent hypotheses may still be discussed. We will choose the 

 one, then, that best explains the facts, whilst we assume that 

 the variation of the tidal-wave with the eccentricity of the 

 Earth's orbit may possibly be the cause of the periodical dis- 

 placement of the beach-lines. But we present this hypothesis 

 with every possible reservation. The different opinions held by 

 the most eminent physicists on these subjects, and the beautiful 

 manner in which the hypothesis is supported by so many facts, 

 alone gives us courage to express hypotheses that, probably by 

 many, will be considered to be not only bold but even improbable. 



The motive force of the alterations in the form of the Earth 

 therefore varies periodically with the eccentricity of the terre- 

 strial orbit. The seas being fluid accommodate themselves, in- 

 stantly, to the slightest alteration in the length of the day, 

 whilst the solid crust opposes a resistance. The day becomes 

 lengthened slowly and imperceptibly. The influence of weak 

 forces, we have seen above, is also dependent on the length of 

 the time in which they act. Even weak forces may therefore 

 exercise an influence, if they only have sufficient time to act in. 

 It is, therefore, probable that the dry land will be more sluggish 

 in its movement than the waters. There will elapse some time 

 ere the „crust" and the internal plastic mass will begin to yield. 

 The foundation of a building begins often, first to give way after 

 the building has stood for some time. If, now, the solid Earth 

 is slower in its movements than the sea, and the movements of 

 both the sea and the land take place periodically, with greater 

 and lesser force, because the motive force is stronger or weaker 

 according as the eccentricity of the orbit rises or falls; it 

 becomes quite imaginable that the beach-lines will come to 

 be displaced and oscillate up and down once, for every time the 

 eccentricity rises and falls; moreover there must be the greatest 



