lxXXvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I9O9,. 



ingenuity suggest doubts as to their validity. "Wiechert, 1 on the 

 other hand, definitely asserts that observations made with a vertical 

 seismometer show ' that the movements of the first phase are 

 longitudinal, and of the second phase transverse, waves.' Obser- 

 vations of a less direct kind made by Omori also confirm Oldham's 

 view. Thus our conclusion is fully sustained. But our knowledge 

 concerning the internal state of the earth does not rest on these 

 arguments alone ; recently, by means of observations made on hori- 

 zontal pendulums, a method which was unsuccessfully employed by 

 Sir George Darwin many years ago, Hecker 2 has obtained a measure 

 of the amount of deformation suffered by the earth under the 

 influence of the sun and moon. This is an achievement which 

 may be counted as one of the greatest triumphs of the present 

 century. From December 1st, 1902, to April 30th, 1905, a con- 

 tinuous record was obtained of the movements of a pair of hori- 

 zontal pendulums mounted in a closed chamber excavated from the 

 side of a well at a depth cf 25 metres below the surface of the 

 ground. Observations could be made outside the chamber and at 

 some distance from it. The curve constructed from the records 

 is shown in fig. 1, p. lxxxvii. If the interior of the earth were 

 fluid, the area of this curve would have been nil : if it were abso- 

 lutely unyielding, the curve would have coincided with the outer 

 ring. As it is, the actual movement of the pendulums is about 

 two-thirds of the amount that it would have been if the earth 

 were absolutely rigid. Hecker concludes from this that the body 

 of the earth behaves very much as it would do if it consisted 

 entirely of steel. 



Again, the terrestrial poles, as is now well known, suffer a slight 

 displacement, and describe a closed curve about 20 metres in 

 diameter, in a period of 427 days. Astronomers, assuming an 

 absolutely rigid earth, have calculated this period and arrived at 

 305 days as the result. It was suggested by Newcombe that the 

 discrepancy might be due to a defect of rigidity ; and, on repeating 

 the calculations, with this point in view, Hough 3 found an elastic 

 yielding such as would result if the earth consisted of steel. 



Finally Love, 4 combining the data afforded by this phenomenon 



1 E. Wiechert, ' Die Erdbebenforsehung, ihre Hilfsmittel & ibre Resultate 

 fiir die Geopbysik' Pbysikalische Zeitsehrift, 1908, pp. 36^7. 



2 O. Hecker, ' Deobachtungen an Horizontal Pendeln ii. d. Deformation des 

 Erdkorpers, &c.' Yeroffentl. d. k. Preuss. Geodatisch. Inst. n. s. No. 32, 1907. 



3 S. S. Hough, ' The Potation of an Elastic Spheroid ' Phil. Trans. Roy. Soc. 

 ser. A. vol. clxxxvii (1896) pp. 319-44. 



Love Proc. Roy. Soc. ser. A. vol. lxxxii (1909) pp. 73-88. 



