ON SEISMOLOGICAL INVESTIGATIONS. 51 



HI. On the Jiecords /ro7n two similar Seismographs at Kew. 

 From the National FJtysical Laboratory. By Charles Ciiree. 



A Milne seismograph, No. 31, intended for Coimbra, was set up for 

 examination at the National Physical Laboratory on October 30, 1900, 

 its pendulum being at the same level and having the same orientation as 

 that of the seismograph No. 9 belonging to the Laboratory. The points 

 of suspension of the two pendulums were about 1 1 feet apart. At first 

 the supports of No. 31 rested simply on the stone floor, while those of 

 No. 9 passed through the floor down to a cement bed. After a month's 

 trial, however, the seismographs were interchanged, with a view to elimi- 

 nating the difference, if any, between the supports. The instruments 

 were adjusted to nearly the same sensitiveness (assuming identity of gauge) ; 

 they had very approximately the same period and the same rate of 

 subsidence of artificially produced vibrations. 



Seven considerable earth tremors were recorded by both instruments. 

 In the four largest the times of commencement of the ' preliminary 

 tremors' shown by the two traces were in excellent agreement, no difier- 

 ence exceeding 0'2 minute. In the other three cases the apparent times 

 differed by from 1-7 to 4-6 minutes, the diflPerence being greatest for the 

 smallest tremors. The times of commencement of the large movements 

 agreed better than those of the preliminary tremors. 



As will be seen by a comparison of figs. .5 and 6, there were conspicuous 

 differences in details in the records from the two instruments. This, 

 presumably, is mainly due to the supports. The instrument standing on the 

 floor had, as a rule, a lessened amplitude of vibration, the reduction ave- 

 raging some 30 per cent. There were, however, not infrequent exceptions 



Fig. 5. December 25, 1900. ' 



/ 5.27.4 5.55.1 



5.16.4 , ^ 



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Seismo. No. 31 ou Kew table. 



5.16.6 



5.'-".5 5.55.1 



-^^--''=-<^<>— ^^i:':^^ 



.*"■ 'I IlillllU'ifHllli HI — 



Seismo. No. 9 ou Coimbra table. 

 Fig. 6. January 7, 1901. 



0.38.3 



^^ 



Seism. No. 9 ou Coimbra table. 



Seismo. No. 31 ou Kew table. 

 E 2 



