MAGNETIC SURVEY OF JAPAN FOR THE EPOCH 1895.0. 165 



Fi<ï. 12 



Fi"'. 12 shows the lines drawn ^ with those zenith distances 

 at each point, the different planes of ö's being made coincident 

 by turning then about the vertical line through the epicenter. 

 We observe that those lines form a kind of caustic which ap- 

 parently converge below the epicenter found by tracing azimuths 

 from each point. 



Were there a real source below the level, the greater per- 

 meability of the earth's crust compared with that of the air, 

 will cause magnetic refraction making the position of the 

 image deeper than that of the source. As a trial, several 

 values of permeabilities //. between 1 and 3 were taken and lines 

 were drawn with increased zenith distances ç' = tg-'(/^-tgc) ; but 

 none of the values gave satisfactory focus. Though it is an easy 

 matter to bring those lines to a focus by assigning suitable perme- 

 abilities to different layers, such an artificial procedure will Ije 

 altogether a superfluous refinement Avith the present data. 



