MAGNETIC SURVEY OF JAPAN FOR THE EPOCH 1895.0. 143 



The vertical and horizontal forces at P will be 



/J = rr-COSC 



H' = „ sine 



with the geometrical relations 



>(1) 



% — r cosO 

 co?c = - 



r sin^ 

 sinc = 



\^) 



tr = v' + ;ï- — 2rz gos6 



or = (s - r)- + Arz sin--^- for numerical work 



Z' and H' denoting the vertical and horizontal components ; the 

 latter can again be resolved along any directions in the tangent 

 plane. 



Remembering that 



()n 1 f)c sine 



— ' - = cosr and — — = — — - 



f)% UZ {) 



the vertical variations of those forces are 

 9Z' m 



= ^^(2cos"r — sin-c) 



= — ^SlQC cose 



UZ [f 



>(3) 



or in terms of d 



dz 



ßH/ 3m rs'mdiU-rcosd) 



>(4) 



dz 



(' P- 



