356 SCHLESINGER— VARIATIONS OF LATITUDE. [April 24, 



pointed out that Euler had made a certain assumption regarding the 

 interior of the earth that had in the meantime been universally dis- 

 carded ; his period of ten months applies in fact only to a perfectly 

 rigid and unyielding earth. Newcomb showed that if the earth 

 yields to deformation to the same extent as though it were composed 

 throughout of steel, then Euler's period would be lengthened to 

 about fourteen months. Here we have the first dependable deter- 

 mination of the rigidity of the earth, a result that has since been 

 confirmed in several ways, particularly by a measurement of "bodily 

 tides " in the earth. 



The fourteen-month term (or the modified Eulerian term as it 

 is now called) has been under accurate observation for a quarter of 

 a century. The period can probably (though not certainly) be re- 

 garded as constant. This is what we should expect, for a change 

 in this period would call for a sensible alteration in the distribu- 

 tion of the material within the earth, or a change in the rigidity of 

 the earth. The amplitude of this term presents a very puzzling 

 problem. Its usual value is about ©".27, but twice in recent years it 

 has jumped to about o".40. Such a change could be accounted for 

 by supposing that the earth had received a severe blow or a succes- 

 sion of milder blows tending in the same direction. We are re- 

 minded that both Milne and Helmert have suggested that there 

 might be a direct connection between latitude variations and earth- 

 quakes. This suggestion was originally made by Milne very early 

 in this century when the astronomical data necessary to test it were 

 still very meager. It is to be hoped that the question will be taken 

 up again in the light of the information that has been added dur- 

 ing the past ten or twelve years. 



Though the Eulerian term is the largest part of the latitude 

 variation, it is by no means the only important one. We have next 

 an annual term with a maximum amplitude of about o".20. We 

 may say with some confidence that this term is seasonal and meteoro- 

 logical in 'its origin, but at present no more definite statement would 

 be warranted. It was early suggested that ocean currents might 

 cause this variation. These currents would have to vary greatly 

 with the season, either in the volume or the speed of the flow, or in 



