MAGNETIC SURVEY OF JAPAN FOR THE EPOCH 1895.0. 149 



when the depth of ahoiit 0.18 R is reached, the value of the 

 maximum becomes zero at about 35° i from the epicenter. 

 Beyond this limit, there will be two circles on which the 

 variation vanishes with a circle of maximum variation between 

 them. The surface is now divided into three regions by those 

 circles ; on the epicenter and antipode sides the force will de- 

 crease upward, but in the middle zone it will iîicrease upward, 

 the effect due to the change of direction accompanying the in- 

 crease of height being greater than the opposite effect due to 

 the increase of distance. In this zone, if the variation of vertical 

 force alone be considered, it would appear as if there lies a dis- 

 turbing source of opposite kind below. 



As the source approaches closer to the surface, the circle 

 of no variation on the epicenter side as well as that of the maxi- 

 mum variation will shrink round that point, while the circle of the 

 minimum variation fades toward the antipode. The other circle 

 of no variation takes the assymptotic position at 70° i from the 

 epicenter, dividing, in the limit, the spherical surface into two 

 parts, the region of upward increase on the epicenter side, and 

 the region of upward decrease on the antipode side. 



When the source is above the level, the vertical force still 

 decreases upward in regions directly below the source. This 

 apparent paradox will be easily cleared, if we reflect that when 

 the positive source is below the level, the force is in positive 

 sense being directed upward, and it decreases in positive sense : 

 but, when it is above, the force is in negative sense, being 

 directed downward, and the upward increase of this negative force 

 is algebraically equivalent to a decrease of positive force. Were 

 there no other magnetic force except that due to the disturbing 

 source such as now described, it may be an easy matter to tell 



