438 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



it is possible to disentangle it from the apparent cliaos of oscillations. 

 If thereupon we compare the size of the real flood with that which 

 might be exjDected according to mathematical theory, with that which 

 according to calculation should result from the moon's attraction, 

 we find that the globe is certainly at least as rigid (riege) as steel. 

 Let me explain the word- " riege," which is somewhat unusual. You 

 know that even steel is flexible; every knife blade, every steel pen 

 proves it. The less yielding a material is in this sense, under the 

 influence of the same force, the more rigid (riege) it is. Steel is one 

 of the most rigid bodies that we know. Now, the behavior of the 

 earth as regards the tides proves that in rigidity it is at least equal to 

 steel. This remarkable result is confirmed and defined with greater 

 precision by an examination of what are called " polar oscillations." 



As you are aware, the " geographic latitude " of a place is measured 

 by degrees of an angle. By this we mean the angle which the plumb 

 line forms at the place in question with the axis of revolution of the 

 earth. This latitude is of fundamental importance in all astronomic 

 measurements, and hence it receives the most careful attention in all 

 observatories. Now it was noticed that the observations did not 

 always give exactly the same latitude. At first it was suspected that 

 this arose from local disturbances of observations, against which the 

 observer has to struggle constantly in all scientific researches. But 

 when the thousands and tens of thousands of observations were system- 

 atically tested, it became apparent that the cause was to be sought 

 not in local but in cosmic influences. It appeared probable, and it 

 was afterwards demonstrated by observatories specially erected for 

 the purpose, that the cause is to be sought in continual disj^lacements 

 of the axis of revolution of the earth. In other words, the earth 

 does not turn steadily on the same axis, but changes its axis of revo- 

 lution constantly within certain limits. Imagine that you are sta- 

 tioned at the place where the imaginary axis emerges from the globe ; 

 that is to say, at the " poles " of the earth in the astronomic sense, 

 and you will have to conceive these " centers of revolution " as being 

 located not at an invariable point of the earth's surface, but as shift- 

 ing their positions. According to the observations thus far made, 

 they migrate around certain definite mean positions, from which they 

 depart at times as much as 10 meters. 



What mean these migrations of the poles of revolution ? That they 

 can take place at all is not surprising to the physicist, for the earth 

 is essentially a spinning top, and such " pole oscillations," that is to 

 say, such displacements of the axis of revolution, may be observed in 

 every top. Mathematical theory teaches that they must occur when- 

 ever the axis of revolution does not coincide accurately with the 

 " axis of figure." It is safe to assume that the rotation of the earth 

 does not take place precisely around the axis of figure, nor is this 



