CHAP, v.] IRREGULAR METHODS OF PLOTTING 



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do not bear from such object S. 47° 20' E., but so much less 

 or more by the convergency ; and that in all cases of fixing 

 ourselves by means of true bearings observed from our own 

 position, the amount of convergency, due to the bearing and 

 distance of the object, must be calculated and apphed to our 

 bearing, before we can use it as a bearing from the object. 



Here, Fig. 23, let B A be the original meridian drawn at 

 the commencement of plotting through any station A. M is 

 the distant mountain. At X our main points are falling short 

 from some reason or another, and we are obliged to have 

 recourse to a true bearing of M, which we accordingly obtain. 

 Requiied to draw this true bearing from the fixed point M. 



Fig. 23. 



If we have the sheet graduated, it will not much simplify 

 matters, as it is a great chance if a meridian passes close 

 enough to M to use it without further correction ; but let us 

 suppose that we have no other meridian on the chart but 

 A B. We must lay off the true bearing from M, with A as 

 the zero, so we require the angle A M X. If M has been 

 observed from A, whence we had a true bearing by which 

 the meridian A B is directed, we have the bearing or angle 

 B A M. If not, we must measure it from the sheet by re- 

 versing the chord method ; drawing a line from A to M, and 

 measuring the chord to the line A B at a given radius with 

 beam compasses, and calculating the angle which corresponds 

 to it, or B A M. 



Now consider the figure again, M C, X D, being imaginary 

 meridians to assist conception. 



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