ON THE iMOVEMENTS OF THE EAUTIl's CRUST, 331 



Iloworth aucl Siiess have theiei'orc both come to the same result. But 

 their explauatious are directly opposite. Howorth thiuks that it is the 

 land which has arisen under higher hititudes ; that the earth, as it were, 

 swells up towards the poles and contracts under the tropics. Suess, 

 who will not admit any other elevations than those which are the con- 

 sequences of foldings, is of the opinion that it is the sea which has flowed 

 towards the lower latitudes. He indicates as a possible explanation 

 changes in the length of the day and the centrifugal force. But this 

 change should then only have acted upon the sea, and therefore, since 

 the sea has flowed towards the equator, the day should have been con- 

 siderably shorter in the last geological period. We shall see hereafter 

 that there is no known cause which could have produced such a short- 

 ening of the sidereal day as would serve to explain what Suess wants 

 to explain. The old theory of refrigeration is scarcely fitted to explain 

 these conditions indicated by Howorth and Suess. Even Suess, who is 

 a zealous adherent of the theory of contraction, is obliged here to seek 

 for another explanation. 



Another theory however has come forth in our day, a theory which, 

 no doubt, is destined to play a great ])art in geology. It is derived 

 originally from the celebrated philosopher J. Kant, In 1754 he wrote 

 a memoir entitled " Untersuchung der Frage: ob die Erde eine Veriin- 

 derung ihrer Achsendrehung erlitten habe?" In this it is shown that, 

 by reason of the attraction of the moon and sun, the sea is constantly 

 in a movement opposite to the daily revolution of the earth. The fric- 

 tion of the tidal waves against the bottom and coasts of the sea dimin- 

 ishes the force of the axial revolution and works constantly in the same 

 direction, so that the sidereal days must for this reason alwiiys become 

 longer and longer. The moon always turns the same side towards the 

 earth 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 toward 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 Kobert Mayer, Helmholtz, and W. Thomson. 



There are certain peculiarities in the moon's movements which astron- 

 omers 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. Soe. Glasgoic, ISGS, 

 vol. HI. pp. 1 et seq.) 



In their " Natural Philosophy," Thomson and Tait treat the problem 



* Is it possible that the great abundance of old volcanoes in the moon may be ex- 

 plained by the great change which its axial rotation, and tiierefore probably also its 

 compression, has nndergone ? 



