302 



SCIENCE. 



[N. S. Vol. XXIV. No. 610. 



nection with the larger problem of the cause 

 of earthquakes, the formation of mountains 

 and other phenomena connected with the 

 physics of the earth, the discussion of Dr. 

 Thomson has interested me, but I find it diffi- 

 cult to believe that either Mallet's original 

 view or this ingeniously modified form of it is 

 well founded. Dr. Thomson says : " A truly 

 solid interior seems to be demanded by the 

 accepted great rigidity of the body of the 

 earth * * *." This view is very frequently 

 expressed, but in my recent paper, ' Eesearches 

 on the Eigidity of the Heavenly Bodies,' pub- 

 lished in the Astronomische Nachrichten, No. 

 4104, July 10, 1906, it seems to be proved 

 clearly and directly that the rigidity of the 

 earth is not inconsistent with a fluid interior. 

 And in another paper, not yet published, I 

 think I have proved conclusively that a fluid 

 substratum underlies the earth's crust. Ac- 

 cordingly, although I am an astronomer, and 

 have shown in the paper above cited that the 

 rigidity of the earth exceeds that of steel, and 

 perhaps closely approaches that of nickel steel 

 used in armor plate, it seems to me that geol- 

 ogists have adopted a mistaken course in con- 

 ceding the solidity of the earth demanded by 

 astronomers, for two reasons: (1) Eigidity 

 does not really disprove internal fluidity; (2) 

 the geological evidence of the existence of a 

 fluid substratum is overwhelming, and this 

 latter result is confirmed by my unpublished 

 investigation on the cause of earthquakes. 



In my paper on the rigidity of the heavenly 

 bodies, above cited, the argument respecting 

 the internal state of the earth is expressed as 

 follows : 



It is, perhapa, worth pointing out that as a 

 molten earth, in which the density follows La- 

 place's law, would have a mean rigidity of its 

 layers equal to that of wrought iron, the hypo- 

 thetical liquid interior would be much less easily 

 deformed by tidal forces than has been generally 

 supposed; so that reaction upon the enclosing 

 crust probably would not be very conspicuous. 

 The amount of this reaction would depend essen- 

 tially upon the difference between the rigidity of 

 nickel steel and of wrought iron, which is about 

 one fourth of the rigidity of the whole earth as 

 now constituted. Even if one supposed the in- 

 terior of the earth to be liquid, the pressure to 



which it is subjected is so great that the tidal 

 surgings of the nucleus, tending to deform the 

 crust, would be comparatively ineffective; and if 

 the crust of solid rock like granite be moderately 

 thick, it is doubtful if the yielding would be 

 sufficient to reduce sensibly the theoretical height 

 of the fortnightly tides of the ocean. Accord- 

 ingly it appears probable that the argument 

 drawn from the tides against the fluidity of the 

 earth's nucleus may in reality be somewhat less 

 conclusive than the most eminent mathematicians 

 have supposed. But from the accordance between 

 the value of the earth's rigidity obtained from 

 the theory of gravity with those found by Darwin 

 from observations of the fortnightly tides, and 

 by Hough from the prolongation of the Eulerian 

 period for the variation of latitude, it seems im- 

 possible to escape the conclusion that the rigidity 

 of our globe as now encrusted probably ap- 

 proaches that of nickel steel. 



It is scarcely necessary to add that the tra- 

 ditional theory long held by geologists that the 

 earth's interior is a mass of mobile liquid in 

 which currents still persist (cf. Fisher's 'Phys- 

 ics of the Earth's Crust,' second edition, pp. 246, 

 305, et seq.) when viewed from the gravitational 

 standpoint is, therefore, found to be inadmissible. 

 The great effective rigidity or viscosity of the 

 matter within the earth makes any supposed mo- 

 tion of the imprisoned fluid quite inconceivable. 



Thus, on the one hand, I have shown by 

 strict mathematical reasoning from recognized 

 data admitting of little dispute or uncer- 

 tainty, that the rigidity is not inconsistent 

 with internal fluidity, while on the other, I 

 have made it clear that the free circulation 

 of currents within the fluid nucleus would be 

 impossible, owing to the rigidity depending 

 on pressure. In fact the matter within the 

 globe, as Arrhenius has also pointed out, may 

 well be gaseous, simply condensed by pressure 

 till it has an average rigidity exceeding that 

 of steel. There are forces, however, which 

 may produce motion just under the crust, 

 which give rise to earthquakes. 



Dr. Thomson's view that ' the flexures 

 taking place in the earth's crust or in the 

 outer portion of its mass may bring to bear 

 upon deep-seated and, perhaps, already heated 

 solid rock masses a sufficient pressure to cause 

 them to readjust their positions,' and thus 

 give rise to volcanic action, is in accordance 



