CHAP. XVII.] GRADUATION 387 



To get other meridians, divide J C and D G as many times 

 as there are meridians required, and join them, as O S, P T, 

 Q V, etc. 



To get the parallels, which it will be remembered are curves, 

 divide the half convergency chord, already measured, into as 

 many parts as we have meridians. 



In our figure we want five meridians from D to G, therefore 

 we divide the chord into five parts, as 1, 2, 3, 4. Draw a small 

 portion of D 4, cutting R W in Z. Z will then be the position 

 on R W of the parallel of D G. By similarly drawing D 3 to 

 cut Q V in E, D 2 to cut P T in H, and D 1 to cut O S in F, we 

 obtain a series of points on the meridians, which, connected 

 together, will form the curve of the parallel D G required. In 

 high latitudes we want more meridians, to dra^^ the curve 

 exactly, than in low, and we must therefore be guided by 

 circumstances as to the number of them. 



Similarly, we obtain the curve of the parallel J C. 



To draw more parallels, divide each meridian between the 

 parallels obtained into as many parts as required, and. join 

 them. 



This process demands considerable care and accuracy in 

 drawing every line, and should be checked wherever practicable. 



The margin of the chart is marked by subdividing the dis- 

 tance between each parallel or degree to the unit required. 



There are other ways of drawing this graduation, all founded 

 on the same principle. As this is, in the wTiter's opinion, the 

 best of them, it is here given. 



All charts should be graduated, but in the case of plans of The 

 comparatively small extent, it will be sufficient to graduate ^i^^'^gf ^ 

 their margins in skeleton form, giving a scale of latitude and Plan, 

 distance, and also a scale of longitude. The graduation must 

 necessarily be on the gnomonic projection, except in the case 

 of a Mercator's chart, to which points have been transferred by 

 their latitudes and longitudes. It is needless to say that it is 

 impossible to graduate on Mercator's projection a sheet on 

 which points have been plotted by angles and distances. 



The skeleton graduation of plans is carried out by ruling the 

 true meridian through one of the stations forming the long 

 side of the plan, and gi-aduating on a central meridian on the 

 principles described on the following page. 



25—2 



