the Displacement of Shore-lines. 413 



towards the e.arth because the earth's tidal action on the mass 

 of the moon while still fluid constantly rendered the axial 

 revolution of the moon slower, until at last the moon was 

 compelled to turn always the same side towards the earth*. 

 In this way also, at some far distant period, the earth will 

 come to turn the same side always to the moon. This opinion 

 of Kant's has been recognized as correct by the first physicists 

 of the present day, by men such as Robert Mayer, Helmholtz, 

 and W. Thomson. 



There are certain peculiarities in the moon's movements 

 which astronomers are inclined to explain by the assumption 

 that the sidereal day gradually increases by reason of the 

 friction of the tidal wave. But with regard to this we will 

 merely refer the reader to Thomson and Tait's ' Treatise on 

 Natural Philosophy,' and to a memoir by the first-named 

 author, " On Geological Time " (Trans. Geol. Soc. Glasgow, 

 vol. iii. 1868, pp. 1 et se<jq.). 



In their ' Natural Philosophy,' Thomson and Tait treat 

 the problem of the earth's axial rotation. They state that 

 there are various forces which may be efficient in altering it — 

 some make the sidereal day shorter, others make it longer. 

 The latter are preponderant, and among them, again, the tidal 

 wave plays the greatest part ; so that for this reason in the 

 course of time the sidereal day becomes always longer and 

 longer. Refrigeration is the most powerful force which con- 

 tributes towards the shortening of the sidereal day, but its 

 action is calculated by Thomson (Trans. Geol. Soc. Glasgow, 

 I. c. p. 2S} at only q-(}qq of the tidal wave ; and this last 

 action cannot be annulled by any of the other forces, which 

 act sometimes in one, sometimes in another direction (trans- 

 port of material from higher to lower latitudes, or vice versa, 

 accumulation of ice at the poles, &c.), and which in course of 

 time cease to act, the tidal wave acting always, for millions of 

 years, in the same direction (Thomson, "Geological Dynamics," 

 Trans. Geol. Soc. Glasgow, vol. iii. part 2, 1869, p. 223). 



In this way, therefore, the sidereal day must in course of 

 time always become longer and longer. Now, what influence 

 has this upon the earth ? If this were fluid throughout, it is 

 clear that it must at once change its form. According 

 as the sidereal day became longer and the centrifugal force 

 diminished, its compression must have decreased. But the 

 old theory of a fiery fluid interior is now rejected by physicists, 



* Is it possible that the great abundance of old volcanoes in the moon 

 may be explained by the great change which its axial rotation, and there- 

 fore probably also its compression, has undergone ? — A. B. 



