388 HYDROGRAPHICAL SURVEYING [chap. xvii. 



Gradua- A meridian having been drawn through one of the A^, a 

 Shfet^on ^^^^^^ perpendicidar is erected at each end. Knowing approxi- 

 Centrai mately the latitude of the spots at which the perpendiculars 

 en lan. ^^^ ruled, calculate the departure in each latitude corre- 

 sponding to the difference of longitude between the meridian 

 already drawn and a central meridian passing through the 

 nearest exact 5' or 10' of longitude. Ascertain very accurately 

 the scale of the chart from points as widely separated from 

 each other as possible ; the distance between them being 

 calculated in the triangulation or obtained from their astro- 

 nomical positions. 



Turn the departures, as found above, at the latitudes of the 

 two perpendiculars into inches according to scale of chart, 

 and set them off on these lines. Join the points thus found, 

 giving a central meridian passing through an exact 5' or 10' 

 of longitude. From the ^ through which the original meridian 

 is drawn, draw a long line perpendicular to that meridian ; 

 lay off from the perpendicular in direction of the pole half the 

 convergency due to the difference of longitude between the 

 original meridian and the central meridian. The point at 

 Avhich the line cuts the latter is in the same latitude as the A. 

 From this point on central meridian measure off N. and S. 

 the difference of latitude to the required margin on the scale 

 already found. Tlirough these extreme N. and S. points draw 

 lines perpendicular to the central meridian extending both 

 ways. 



Calculate departure in both marginal latitudes corresponding 

 to the difference of longitude between central meridian and 

 margin of chart E. and W., and also the convergency for 

 those departures in both latitudes. 



At the N. and S. marginal points on the central meridian lay 

 off from the perpendicular to the meridian half the respective 

 convergencies towards the pole, and on those lines measure 

 off the respective departures found above according to scale 

 of chart. The four corners of the chart are now fixed, and the 

 intermediate meridians are found by subdividing the per- 

 pendiculars between the central meridian and margin of chart 

 and joining the points so found at the N. and S. ends of the 

 chart. The parallels of latitude are found by subdividing 

 the half convergencies into as many parts as there are meridians, 



