1892.] President's Address. 309 



splendid agreement with those of the European observatories: 

 Berlin, Prag, and Strasbourg. They prove beyond all question that 

 between May, 1891, and June, 1892, the latitude of each of the three 

 European observatories was a maximum, and of Honolulu a mini- 

 mum, in the beginning of October, 1891: that the latitude of the 

 European observatories was a minimum, and of Honolulu a maximum, 

 near the beginning of May, 1892 : and that the variations during the 

 year followed somewhat approximately, simple harmonic law as if for 

 a period of 385 days, with range of about ^ sec. above and below the 

 mean latitude in each case. This is just what would result from 

 motion of the north and south polar ends of the earth's instantaneous 

 axis of rotation, in circles on the earth's surface of 7'5 metres radius, 

 at the rate of once round in 385 days. 



Sometime previously it had been found by Mr. S. C. Chandler that 

 the irregular variations of latitude which had been discovered in 

 different observatories during the last 15 years seemed to follow a 

 period of about 427 days, instead of the 306 days given by Peters' and 

 Maxwell's dynamical theory, on the supposition of the earth being 

 wholly a rigid body. And now, the German observations, although 

 not giving so long a period as Chandler's, quite confirm the result 

 that, whatever approximation to following a period there is, in the 

 variations of latitude, it is a period largely exceeding the old estimate 

 of 306 days. 



Newcomb, in a letter which I received from him last December, 

 gave, what seems to me to be, undoubtedly, the true explanation of 

 this apparent discrepance from dynamical theory, attributing it to 

 elastic yielding of the earth as a whole. He added a suggestion, 

 specially interesting to myself, that investigation of periodic varia- 

 tions of latitude may prove to be the best means of determining ap- 

 proximately the rigidity of the earth. As it is, we have now, for the 

 first time, what seems to be a quite decisive demonstration of elastic 

 yielding in the earth as a whole, under the influence of a deforming 

 force, whether of centrifugal force round a varying axis, as in the 

 present case, or of tide-generating influences of the sun and moon 

 with reference to which I first raised the question of elastic yielding 

 of the earth's material many years ago. 



The present year's great advance in geological dynamics forms the 

 subject of a contribution by Newcomb to the ' Monthly Notices of the 

 Royal Astronomical Society,' of last March. In a later paper, pub- 

 lished in the ' Astronomische Nachrichten,' he examines records 

 of many observatories, both of Europe and America, from 1865 to the 

 present time, and finds decisive evidence that from 1865 to 1890 the 

 variations of latitude were much less than they have been during 

 the past year, and seeming to show that an augmentation took place, 

 somewhat suddenly, about the year 1890. 



