CHAP. IV.] CONVERGENCY 101 



will be the convergency of two meridians a fixed number of Conver- 

 degrees apart ; that when the pole P and J coincide, the ft^La-^ 

 meridians will radiate over the chart from that centre, and tor, and 

 the convergency will equal the distance between the ^g^ long. 

 meridians ; and that when J is on the Equator, the meridians ** Poles, 

 will be parallel, and convergency will be nothing. 



Parallels of latitude will appear on the chart as curves. Parallels, 

 concave towards the poles, and cutting each meridian at right 

 angles. 



The Equator being a great circle will be a straight line, and, 

 generally, the further from the Equator, i.e., the higher the 

 latitude, the greater will be the degree of curvature in the 

 parallels. 



More consideration will show that, the farther a part of the Distor- 

 flat surface is from the surface of the earth, the greater will ^^°^ 

 be the distortion of the positions resulting from this method 

 of delineating the globe ; or, in other words, that the distortion 

 increases from the centre of a gnomonic chart, and will 

 become very considerable towards the edges, if a large area 

 of the earth is attempted to be shown on a flat surface. 

 But in practice, a marine survey does not extend over a 

 sufficient area to make this distortion in any way apparent. 

 Our diagrams are of course much exaggerated in this 

 respect. 



It will be understood that the convergency is an actual 

 fact, and does not result merely from the method employed 

 in this projection. We have only considered it in con- 

 nection with the projection, as it is thought that by so 

 doing the nature of the convergency becomes more plainly 

 apparent. 



The mean of the two reverse bearings, or either one of Mercato- 

 them, plus or minus half the convergency, will give the Mer- g*'^. 

 catorial Bearing, so called from being the bearing which 

 each station will be from the other in a Mercator's chart, where, 

 the meridians being all parallel, the line joining the stations 

 will cut them at the same angle, this angle being also the 

 one at which the line on our gnomonic chart will cut a meridian 

 midway between the stations. 



The actual observed bearing of a distant object, if pro- 

 tracted on a Mercator's Chart, will not pass through its 



