CHAP. XX.] 



PLOTTING 



459 



and so on for the remaining points, confining the calculation 

 to what is necessary to lay them down. 



The series of jDoints might, of course, be plotted simply on 

 their relative angular measurements and distances, but the 

 arbitrary plotting points afford means of cutting them in by 

 long chords, which give a higher degree of accuracy, besides 

 providing long lines m a transverse direction, which are con- 

 venient for carrying on the plotting of other points. 



Fig. no 



Writing in the Hydrographic Annual, Rear-Admiral F. C. 

 Learmonth, C.B., gives a further illustration of the use of an 

 arbitrary plotting point, X in Fig. Ill, which shows a tri- 

 angulation suddenly contracting in width to a long narrow 

 strait, in which sextant angles alone are possible. In such 

 a case errors are bound to creep in, and the only check lies in 

 carrying a true bearing observed at one end of the strait through 

 the sextant triangulation and comparing it with a true bearing 

 observed at the other end. If there is much discrepancy, 



