ASTRONOMY AND GEOLOGY EDDINGTON 201 



Had the earth a solid crust at the time the catalysm happened? 

 I cannot tell at all. But, if it suits geological theories, I can see 

 no objection whatever to the hypothesis that the earth had a solid 

 crust at the time. No cohesion of the crust would seriously resist 

 the enormous forces involved when the resonant vibration got 

 started. It would not be appreciably more difficult than the dis- 

 ruption of a molten earth. The view that the Pacific Ocean is the 

 hollow left at the place where the moon broke off seems tenable 

 unless geologists find objection to it ; and in that case we may sup- 

 pose that the water now collected in the hollow formerly covered 

 the earth— or most of it. This change of condition of the earth may 

 (or may not) have happened within geological times. When the 

 earth was covered with water there would be no landlocked seas 

 and no appreciable tidal friction from the sun (the moon being not 

 yet born) . so that we can allow a long previous history during which 

 the length of day was nearly constant at 3 or 4 hours. That rather 

 helps to make the whole theory self-consistent. 



These speculations stand very much as they did when Darwin put 

 forward his theory. But I am tempted to add further speculations 

 arising out of the location of the frictional dissipation. (I am 

 taking advantage of the great opportunity for speculation which 

 this address affords. Ordinarily I am restrained, because people 

 would ask, What facts can you produce in support of your specula- 

 tions? But here I am asking the question, Have you any facts 

 which seem to support them? If not, by all means let them drop.) 

 The frictional dissipation acts as a brake on the earth's rotation, 

 and we now feel confident that the brake is a surface brake applied 

 at certain points on the earth's surface where the favorable condi- 

 tions exist. The retarding force is transmitted to the earth's in- 

 terior, and so delays the rotation as a whole ; but unless the material 

 is entirely nonplastic there will be a tendency for the outer layers 

 to slip on the inner layers. I do not know how much the material 

 a few hundred miles below the surface would be expected to give 

 under the strain; it may be inappreciable, but I will assume that 

 though small it has some effect. 



We have then the whole crust slipping from east to west over the 

 main part of the interior. Probably it would go very stickily, some- 

 times arrested by a jamming which would hinder it for a time and 

 then going on more easily. That is helpful in explaining certain 

 astronomical observations. There are irregularities in the motions 

 of heavenly bodies, noticed particularly in the swift moving moon, 

 but shown also on a smaller scale in the sun and planets, which 

 appear to indicate that our standard timekeeper, the earth, is a 

 little irregular. Now, of course, it is the rotation of the surface 

 of the earth which determines our standard time. I find it diffi- 

 cult to believe that there can be irregular variations in the angular 



