Dr. C Davison on Earthquake- Sounds. 



61 



earthquake-series. But this does not seem to be the case, so 

 far as we can judge from the limited materials at our disposal. 

 The series of the Mino-Owari after-shocks recorded at 

 Grifu is the most valuable and complete of any so far published. 

 Prof. Omori gives the number of earthquakes for every day 

 from Oct. 28, 1891, to Dec. 31, 1893, classifyiug them under 

 the heads violent shocks, strong shocks, weak shocks, feeble 

 shocks, and sounds. From this catalogue the curves in 

 fig. 4 have been constructed. Those for the violent and 



Fiar. 1. 



strong shocks have been omitted, on account of their numerical 

 inferiority. The three curves, continuous, broken, and dotted, 

 represent respectively the percentages (with regard to the 

 total number of earthquakes each month) of the weak shocks, 

 feeble shocks, and sounds, beginning in Nov. 1891. During 

 the first three months, the weak shocks rapidly decline in 

 relative frequency, while there is a corresponding increase in 

 that of both feeble shocks and sounds. But, at the end of 

 five months, the two latter curves begin to diverge, the feeble 

 shocks being subject to a fluctuating increase, and the sounds 

 to a fluctuating decrease, in relative frequency. 



Very similar, though less exact, is the evidence furnished 

 by Drummond's Comrie record from Oct. 1839 to Dec. 1811, 

 the principal shock being that which occurred on Oct. 23, 

 1839 *. In this series the earth-sounds greatly predominated, 



* Phil. Mag-, vol. xx. 18.42, pp. 240-247. 



