ON SEISMOLOGICAL INVESTIGATIONS. 



81 



of which It is assumed have not been lost to history or tradition. In the 

 next column we find the sum of earthquakes of values III. and II., 

 •whilst in the last column is given the sum of earthquakes having an 

 intensity of Classes III., II., and I. The destructivities of Classes I., 

 II., and III. have already been defined. 



These figures are also shown as three curves (fig. G). An inspection 

 of these will show that they have a striking similarity. Between the 

 years 1150 and 1250 each indicates an increase in seismic activity during 

 that period. The next increase commences about the year 1650, since 

 which records even of the most destructive type of earthquake have 

 during successive periods each of fifty years rapidly increased in number. 

 Inasmuch as all these records refer to a European area, it is difficult to 

 imagine that the great increase in numbers from the middle of the 

 seventeenth century should be accounted for by a sudden stimulus to keep 

 better records from that period. Earthquakes of Class III. may be 

 regarded as events of historical importance ; and, if this is so, it would 

 seem that records of the very destructive disturbances between a.d. 1300 

 and 1600 should approximate in their correctness to those which occurred 

 subsequently to the latter date. My own idea is that about 1650 there 

 was a very marked increase in seismic and, I may add, volcanic activity. 

 Whether this has any connection ^s-ith the observations of geologists or 

 those who study magnetics or other branches of earth physics must be left 

 for their consideration. 



IX. On a Seismogram obtained in London on October 16, 1907. 



The accompanying seismogram, Plate II., was recorded in the engineer- 

 ing workshops of Mr. R. W. Munro, South Tottenham, where an instru- 

 ment had been set up for examination before shipping for South Australia. 

 Vibiations due to machinery are marked, while at night the instrument 

 yielded a fairly straight line. This steadiness at night, notwithstanding 

 casual traffic, suggests that a seismograph might find a sufficiently steady 

 site in a London square or park. The seismograph is identical with those 

 first adopted by the British Association, the record -receiving surface, 

 Jiowftver, instead cf only moving at 60 mm. per hour now moves at four 



1908. o 



