386 



HYDROGRAPHICAL SURVF.YING [chap. xvii. 



from these perpendiculars lay oflf, on the side of the pole, half 

 the convergency calculated for the difference of longitude in 

 the latitude of C and D respectively, as C 11, D 5, cutting the 

 opposite meridians respectively in J and G. Then J will be on 



Fig- 77. 



the same parallel as C, and G as D, and J D, C G, should be 

 equal.* 



To get the central meridian of the chart, bisect J C and 

 D G in A and B, and join them. 



Then J G joined should intersect C D in the central meridian. 

 This is a capital check for our correctness so far. 



* See Appendix B. 



