310 EAKTHQUAKES. 



The relative positions of the images were in part 

 governed by the state of the tide. Altogether the move- 

 ments were so strange that M. d'Abbadie did not venture 

 any speculations as to their cause, but he remarks that 

 the cause of the changes he observed were sometimes 

 neither astronomical nor thermometrical. These observa- 

 tions, the principal object of which was to determine 

 changes in level rather than earth vibrations, were carried 

 on between the years 1868 and 1872.^ 



Observations at Gainhridge. — Another instructive set 

 of observations were those which were made in the years 

 1880-1882, by George and Horace Darwin, in the Caven- 

 dish Laboratory, at Cambridge. The main object in these 

 experiments was to determine the disturbing influence of 

 gravity produced by lunar attraction. The result which 

 was obtained, however, showed that the soil at Cambridge 

 was in such an incessant state of vibration that whatever 

 pull the moon may have exerted upon the instrument 

 which was employed was masked by the magnitude of 

 the effects produced by the earth tremors, and the ex- 

 periments had, in consequence, to be abandoned. 



The princi^^le of this instrument was similar to one 

 devised by Sir William Thomson, and put up by him in 

 his laboratory at Glasgow. As erected by the brothers 

 Darwin, at Cambridge, it was briefly as follows : A 

 pendulum, which was a massive cylinder of pure copper, 

 was hung by a copper wire, about four feet long, inside a 

 hollow cylindrical tube rising from a stone support. A 

 small mirror was then hung by two silk fibres, one 

 of which was fastened to the bob and the other to the 

 stone basement. A ray of light sent from a lamp on to 

 the mirror was reflected to a scale seven feet distant, and 

 by this magnification any motion of the bob relatively to 

 ^ G. H. and H. Darwin, Bejyorts of British Association, 1881. 



