GEOGRAPHICAL CO-ORDINATES. 



413 



438. INTRODUCTION OF GEOGRAPHICAL CO-ORDINATES. Let 

 us consider any given point P at a distance r from the centre of the 

 Earth ; let u (Fig. 95) be the complement P'D of the latitude and / 



Fig. 95- 



the longitude CQ counted towards the east. We shall decompose 

 the magnetic force F at the point P into three rectangular forces, 

 one Z along the vertical and measured positively towards the zenith, 

 the other X in the meridian and directed towards the north, the 

 third directed towards the west. 

 Taking into account the ratios 



dx = rdU) dyr sin udl^ dz = dr, 

 the components of the force become 



(4) 



r sn u 



*.--*. 



Tr' 



