538 



SCIENCE 



[N. S. Vol. XXVI. No. 669 



in saying that the earth is approximately 

 an oblate spheroid, but more nearly an 

 ellipsoid with three unequal axes, having 

 its surfaee furrowed according to the 

 formula for a certain spherical harmonic 

 of the third degree, and displaced relatively 

 to the geoid towards the direction of the 

 Crimea. 



As regards the amount of elevation and 

 depression in different parts, the agreement 

 of the theory with the facts is not so good. 

 The computed elevation is too small in 

 southern Africa, Brazil and the southern 

 part of South America, too great in the 

 Arctic regions, to the south of Australasia, 

 and in the Mediterranean region. There 

 are many reasons why we could not expect 

 the agreement to be very good. One is the 

 roughness of the method of harmonic 

 analysis that was used. But there is also 

 the fact that many causes must have con- 

 tributed to the shaping of our actual conti- 

 nents and oceans besides those which have 

 been taken into account in the theory. It 

 appears, however, that the broad general 

 features of the distribution of continent 

 and ocean can be regarded as the conse- 

 quences of simple causes of a dynamical 

 character: eccentric position of the center 

 of gravity, arising from a past state of in- 

 adequate resistance to compression, an in- 

 herited tendency, so to speak, to an ellip- 

 soidal figure, associated with the attraction 

 of the moon in a bygone age, the rotation, 

 and the interactions of these various causes. 



In attempting to estimate the bearing of 

 the theory on geological history we must be 

 guided by two considerations. The first is 

 that the earth is not now gravitational ly 

 unstable. From observations of the propa- 

 gation of earthquake shocks to great dis- 

 tances, we can determine the average resist- 

 ance to compression, and we find that this 

 resistance is now sufficiently great to keep 

 in check any tendency to gravitational in- 



stability. The eccentric position of the 

 center of gravity must be regarded as a 

 survival from a. past state in which the 

 resistance to compression was not nearly so 

 great as it is now. The second guiding 

 consideration is that, according to the 

 theory, the inequalities which are expressed 

 by spherical harmonics of the third degree 

 are secondary effects due to the inter- 

 action of the causes which give rise to in- 

 equalities expressed by harmonics of the 

 first and second degrees. We should ex- 

 pect, therefore, that the inequalities of the 

 third degree would be much smaller than 

 those of the firet and second degrees; but 

 the harmonic analysis shows that the three 

 inequalities are entirely comparable. We 

 must conclude that the harmonics of the 

 first and second degrees which we can now 

 discover by the analysis are survivals from 

 a past state, in which such inequalities were 

 relatively more important than they are 

 now. Both these considerations point in 

 the same direction, and they lead us to 

 infer that certain secular changes may 

 have taken place in the past, and may still 

 be going on. Sixty-nine years ago Charles 

 Darwin wrote : ' ' The form of the fluid sur- 

 faee of the nucleus of the earth is subject 

 to some change the cause of which is en- 

 tirely unknown, and the effect of which is 

 slow, intermittent, but irresistible. ' ' Forty- 

 two years later Sir George Darwin showed 

 that any ellipsoidal inequality in the figure 

 must be gradually destroyed by an irre- 

 versible action of the same nature as in- 

 ternal friction or viscosity. The same may 

 be said of a state in which the center of 

 gravity does not coincide with the center 

 of figure when the resistance to compres- 

 sion is great enough to keep in check the 

 tendency to gravitational instability. The 

 state would be changed gradually in such 

 a way as to bring the center of gravity 

 nearer to the center of figure. A symptom 



