248 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1916. 



phenomena which one should attempt to embody in other supple- 

 mentary hypotheses. 



I submit that in studying the earth it is a mistake to think that 

 there is any necessary conflict between the idea that the earth be- 

 haves as an elastic body and the idea that it is yieldiing in a viscous 

 manner. A body may behave in both ways at once. The earth is 

 probably acting largely as an elastic body under small forces which 

 change rapidly and at the same time is yielding in a viscous manner 

 to forces of larger intensity which are applied in one sense con- 

 tinuously for long periods. 



The object of this address will have been accomplished if it serves 

 in time to arouse the imagination and interest of some one and to 

 guide him to greater effectiveness in attacking the problems presented 

 by the earth as seen from the geophysical standpoint. 



IV. 



VARIATIONS OF LATITUDE: THEIR BEARING UPON OUR KNOWL- 

 EDGE OF THE INTERIOR OF THE EARTH. 



By Frank Schlesingeb. 



To review even hastily the contributions that astronomy has made 

 to our Imowledge of the figure and dimensions of the earth and the 

 constitution of its interior would consume more time than I can 

 fairly claim as my share. Let me therefore pass over those points 

 that are on accepted ground and are matters of general agreement 

 from the different points of view represented in this symposium; 

 and let me dwell instead upon certain recent developments espe- 

 cially in need of consideration, concerning which the astronomer 

 desires the criticism and help of the geologist, the seismologist, the 

 physicist, and the meteorologist. These developments have come to 

 us directly or indirectly through a study of latitude variations, so 

 that most of what I shall have to say will deal with this subject. 



Although variations of latitude are in a sense a very recent addi- 

 tion to our knowledge, yet, on the theoretical side, at least, we find 

 the beginning more than a century and a half ago. In 1755 Euler 

 considered "the rotation of solid and rigid bodies" in a memoir 

 that is now recognized as the foundation stone for our edifice. He 

 showed that if such a body is projected into space it will exhibit tAvo 

 kinds of rotation; the first of these is the familiar one that corre- 

 sponds to the day in the case of the earth ; the other is more subtle 

 and corresponds to the variation of latitude. By reason of this the 

 axis of the diurnal rotation is continually changing within the body, 

 progressing in a regular way, and coming back after a time to its 

 earlier positions. .An ordinary top gives us a simple example of this 



