170 



A. TANAKADATE. 



all over the region and increases numerically toward the mean 



point of intersections of these arcs, or pericenter in the nomenclature 



f)H' 

 now adopted ; that of the horizontal force —^ — is also negative in 



the sense measured from the pericenter (practically southward), 



and is numerically maximum somewhere between the Stations I 



and VII in Great Britain. 



9Z' 9H' 



The equality of signs of — y— and — g — within the critical 



value of 6, 35° i, shows that the image must be placed below the 



level ; and their negative signs show that its sign must be positive. 



We thus find for each of the points taken. 



fa 



O 



< 



Figs. 14 and 15 show these azimuths and zenith distances. 

 This value of mean depth makes the value of -^-=0.7 which gives 

 by (18) the circle of minimum variation (numerically maximum) 

 of the horizontal force — ^ — at 11. °2 from the epicenter; it passes 

 therefore through the north of England and Ireland, roughly agree- 

 ing with the observed position. 



