464 HYDROGRAPHICAL SURVEYING [chap. xx. 



When these positions are plotted they give, not only the 

 points throiigli which tlie curve representing the parallel has 

 to be drawn, but also the actual position through which the 

 meridians pass. These meridians can thus be drawn directly, 

 because the bearing which each meridian makes with the line 

 joining its particular position to A can be obtained whilst 

 calculating the distance and bearing from A. 



In practice it will be necessary to draw, perhaj)s, only three 

 meridians in this manner, and the remainder inserted as follows : 

 Along each of the extreme meridians the distances of the paral- 

 lels of latitude are ticked off, measuring from the parallel 

 ah'eady completed. When the uppermost and corresponding 

 divisions on these extreme meridians are joined by a straight 

 line, this line wiU suffice just as well as a curved parallel for 

 marking along it the minutes of longitude into which the upper 

 border has to be divided. 



Meridians can then be drawn from the lower parallel to the 

 correspondmg longitudes marked on this upper hne. If these 

 meridians be divided off into the correct mmutes of latitude, 

 commencing at the lower parallel, as was done in the case of 

 the extreme meridians, points wiU be obtained enabUng the 

 curve for any or all the other parallels to be drawn. 



As a check, any particular intersection of a meridian and a 

 parallel can be constructed by means of its bearing and distance 

 from A. 



This method could be limited to plotting the four corners of 

 the chart from the initial position of A, and the remainder com- 

 pleted by the other methods, although what is considered its 

 chief advantage would be ignored — viz., plotting a curved 

 parallel ^Aithout using very small dimensions. In any case 

 it would provide a severe check in testing a gnomonic chart 

 produced in any other way. 



In connection with the subject of graduation of a gnomonic 

 chart, reference should be made to a very useful " Table for the 

 Graduation of Surveys and Charts on the Gnomonic Projection " 

 published by the Hydrographic Department, Admiralty, and 

 giving various alternative methods, with special tables for 

 their use. 



Mr. J. W. Atherton, of the Hydrographic Department, has 

 also compiled " Tables for determining Geodetic Positions, 



