336 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



the investigation of the difference between the observed period of 427 

 days and that of 305 called for by theory, findin^^ the explanation 

 in the slight yielding of the earth, and have deduced the result that 

 the earth as a whole must possess an effective rigidity a little greater 

 than that of steel. In confirmation of these results, tidal students 

 have found evidences, though very slight, of a minute tide with a 

 period of 430 days. And in still another field these results may 

 possibly prove of interest. No less an authority on earthquakes than 

 Prof. Milne has expressed the opinion that earthquakes are more 

 frequent at those epochs when the axis of the earth is farthest from 

 its mean position, though this theory is not accepted by most seis- 

 mologists. 



In the wide field of stellar photometry a very large proportion 

 of our knowledge of the southern stars comes from the results of 

 the Harvard photometric expeditions and particularly from its sta- 

 tion at Arequipa, Peru. Through the visual results secured at Cor- 

 doba, the photographic magnitudes as given by the Cape Photo- 

 graphic Durchmusterung and the long series of exact visual 

 estimations made with the meridian photometer at Arequipa, we may 

 say that, except in certain special studies on the fainter stars, the 

 state of our knowledge of the relative brilliancy of the stars of the 

 Southern Hemisphere is not inferior to that of the Northern 

 Hemisphere. 



From this station, too, has come far the largest proportion of 

 what is known to-day with reference to the variable stars in the 

 more southerly regions of the sky. Epoch making in this branch is 

 the discovery by Prof. Bailey of a very large number of variable 

 stars in clusters. The Magellanic Clouds and other clusters in the 

 Southern Hemisphere have alone given about 2,000 new variable 

 stars ; the determination of the periods of all these and the study of 

 the peculiarities in their variation will in itself furnish work for 

 many years to come. Much remains to be done as well on the 

 brighter variable stars of the Southern Hemisphere. 



Through the excellent work at Arequipa, also. Harvard's exten- 

 sive spectographic surveys have been extended to the South Pole. 

 While it is certain that future studies with spectrographs of higher 

 dispersion will bring forth many new facts with regard to stellar 

 constitution, there is no doubt that Harvard's extensive surveys of 

 the entire sky in the photometric and spectrographic fields will for 

 decades be to the astrophysicist what the Bonn Durchmusterung has 

 been to the worker in the astronomy of position. 



As in the surveys just mentioned, the spectrograph was at first 

 employed solely to determine the constituent elements of the sun and 

 the stars, but the application of the Doppler-Fizeau principle to 

 the determination of a star's velocity in the line of sight from its, 



