130 HYDROORAPHTOAI. SURVEYING [cuw. v. 



The bearing of A from M = bearing of M from A + the 

 convergency, as M is nearer the pole than A, or 



(' M A=B A M+ convergency for difference of departure 

 of A and M. 



In like manner : 



C M X = M X D (the observed bearing from X)+ con- 

 vergency for difference of departure of M X. 



Adding, we have C M A+C M X = B A M+M X D+ con- 

 vergency for A X. 



Or A M X = bearing M from A + bearing M from X + con- 

 vergency for A X. 



To get convergency in this case, we must assume a position 

 for X, which we can roughly plot for the purpose, and measure 

 the distance A X and bearing B A X. 



We can then from this calculate the convergency required, 

 knowing roughly the latitude of A, for 



Convergency = distance x Sin mere, bearing x Tan Md. lat. 

 Drawing If ]M ig likely to be used much in this way, it wiU be worth 

 Meridian, while to lay a meridian off through M by plotting the bearing 

 A M C or B A ]M + the convergency for A M ; from which 

 meridian subsequent bearings can then be laid off, duly cor- 

 rected for convergency, for the distance between M and the 

 station from which the bearing is observed. 



Any hne passing through a station, the true bearing of which 

 can be calculated, will serve as a zero for laying down any 

 other true bearing from the same station. This is generally 

 preferable to actually drawing a meridian through the station. 

 Neglect of Of course, it will depend on the latitude how much error will 

 gency. ^^ introduced by neglecting the convergency ; but when it 

 is considered that in latitude 45° the convergency is equal to 

 the departure, it will be seen that a large error will result by 

 not applying it ; for in this latitude, supposing A and X are 

 30 miles apart, an error of half a degree would be made by 

 drawing a meridian parallel to A B, and laying off the bearing 

 observed at X from M. 



As a rule, therefore, it can never be safely neglected except 

 very near the Equator. 



If it is intended to lay off the true bearing of an object from 

 a station plotted on the chart, the convergency must likewise 

 be borne in mind, and the meridian to be ruled through X 



