X74 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



tions of the frictional effect. But the changes we are considering are 

 enormously greater than the whole frictional effect and consequently 

 any variation in the latter is quite powerless to produce them. Indeed 

 theory goes a step farther and indicates that no force external to the 

 earth that we know of can produce a change in the angular momentum, 

 except in the manner just stated. 



We are thus reduced to asking whether there are any external 

 forces which can alter the distribution of matter in the earth. The 

 only such forces we know of are tidal actions by the sun and moon. 

 But such forces are quite regular in their action, and the changes we 

 observe are extremely irregular, so that this source must be ruled out. 

 There are in fact no known forces outside the earth that can produce 

 the observed results. 



Are there surface changes due to meteorological causes or other 

 conditions which might be effective? When these results were first 

 published it was suggested that accumulations of ice and snow at the 

 poles might account for the phenomena. But calculation shows that 

 in order to use this source, the requisite amount of frozen water 

 would change the average sea level all over the world by something 

 like a foot, and we should all know if this had happened in 1897. 

 I took the trouble to look up the records and, as I expected, found no 

 significant change in sea level near that date. 



Local changes of level or volcanic action are apparently far too 

 small to produce the required effect. DeSitter calculated that if the 

 whole group of the Hunalaya Mountains could be razed and placed 

 at the poles, it would produce only a fraction of what is needed. 

 No miaginable surface changes seem sufficient for our purpose. 



We are then driven to changes below the surface to account for it. 

 Here we must resort entirely to h3q)othesis, for there is nothing that 

 we actually know which is available for our purpose. The most 

 plausible start is to imagine some action which changes the radius of 

 the earth. The astronomer has already adopted some such idea to 

 explain the changes which take place in the light of certain variable 

 stars. He imagines that they pulsate bodily — perhaps it is a feature 

 of all stars to a greater or lesser degree. If so, there is no difficulty in 

 imagining a pulsating earth — the remnant perhaps of a time when 

 such changes were on a large scale. The amounts required seem quite 

 small. If there is a uniform dilatation and compression throughout 

 the whole mass of the earth, a change in the external radius of 5 inches 

 is sufficient to account for the maximum change of rate which has so 

 far been observed. If the change took place in a layer 50 miles below 

 the surface, the external radius would have to change by 12 feet to 

 produce the same effect. If a change of this character is the cause of 

 the phenomenon, something between these two extremes is indicated. 

 Another suggestion which has been made is the redistribution of 



