MAGNETIC SUKVEY OF JAPAN FOR THE EPOCH 1895.0. 153 



the nodal circle of the vertical force, on the suhcenter side of 

 which the sign of the variation is to be changed. If the nodal 

 circle does not cut the ellipse, the whole space inside the ellipse 

 on the suhcenter side becomes region of upward increase and 

 that between the ellipse and the circle that of upward decrease, 

 the rest remaining the same ; if it cuts the ellipse, the space 

 within it as well as that of the zone is divided into two regions; 

 if it touches the ellipse at all, it must touch it at the pole, 

 when the discontinuity at the point becomes a cusp. 



The Variation of the West Component vanishes all over the 

 meridian whose plane passes through the source. When the 

 source is above, its sign within the nodal circle is to be changed, 

 as in the case of the north component, dividing the surface into 

 four regions of alternately positive and negative variations. 



The maximum and minimum of the variations of the north 

 component, are on the meridian circle whose plane contains the 

 source, and those of the west component either on the nodal 

 ellipse or the meridian which is at quadrature with the above. 



Tables XXVI and XXYII give the values of those vertical 

 variations for several values of î'/R , m and R being taken as 

 unity; and Figs. 7 and 9, their graphs in polar co-ordinates; the 

 values are positive outward and negative inward from the circum- 

 ference of the circle which represents a section of the sphere 

 through the source. This method is adopted for the easy appre- 

 hension of the various positions on the sphere although it has 

 the disadvantage of making the positive and negative magnitudes 

 appear unsymmetrical on account of the convergence of the 

 radial lines. Figs. 8 and 10 are the same for the case of the 

 plane surface, 



