100 



HYDROGRAPHICAL SURVEYING [chap, iv 



earth. L is the position on ouf chart of K (see other figure). 

 R is the position of a similar point, invisible in first figure, 

 being on the other side of the earth. Meridians passing 

 through L and R are projected on the chart by the same 

 method as before, i.e., by drawing imaginary lines from the 

 centre of the earth through different points in the required 

 meridian ; they will be found to lie as T L, R 0, and their 

 extension will also pass through X, making an angle R X L, 



Fig. i6. 



which is the Convergency of the meridians ; and this will be 

 seen at once to be equal to the difference of the reverse bearings 

 of R and L, for — 



ZORL = OXL+RLX, 

 or OXL = ORL-RLX, 



i.e., Convergency = Bearing of L from R- Bearing of R 

 from L. 



A little consideration of this last figure will show that the 

 further towards the pole the central point J is, the greater 



