104 HYDROGRAPHICAL SURVEYING [chap. iv. 



the latitude, and difference of longitude from A, of F, which 

 is a station in a plan on a large scale we have made. 



For the purposes of this plan we have obtained the side 

 F C in the triangulation, which will serve as our base instead 

 of measuring another. 



Fig. 17. 



We shall commence by calculating the convergency for ten 

 miles of departure at the average latitude of the chart, as we 

 shall want it directly. 



Ill this case we fuid that — 



Convergency for 10' of departure = 11-92'. 

 Or for each mile of departure = 1-2'. 



We then find approximate latitude of B by the formula — 

 Diff . lat. = A B X Cos rough mercatorial bearing. 



We obtain the bearing, near enough for this purpose, by 

 finding the rough convergency and applying half of it to the 

 observed bearing of B from A, thus : 



Take departure from the traverse table, in this instance 9-5. 

 Multiply it by the convergency for a mile, just found to be 

 1-2, which gives us ir-4 as the rough convergency. Adding 

 half of this to the bearing of B from A, we get rough mer- 

 catorial bearing N. 69° 11' W., and working out the difference 

 of latitude, we find it to be 3' 38", which gives for the latitude 

 of B, 49° 34' 02", and for middle latitude 49° 32' 13". 



Then convergency = dist. x Sin mere, bearing x tan. mid. lat. 



Using the rough bearing just found, we get — 

 Convergency for A B= 11' 13-8". 



