124 HYDROGRAPHICAL SURVEYING [chap. v. 



Com- Understanding then how to lay off angles by chords, and 



mearof having obtained by calculation as long a side as we can for a 

 Plotting, plotting base line, so as to plot as much as possible imvards, 

 or with decreasing distances, and not outwards to stations 

 farther distant than the original two, and having settled where- 

 abouts on the sheet this base line shall be placed, draw a 

 meridian line, parallel with the side of the paper, and passing 

 at one end of where the base is to be. Make a prick on this 

 line for one end of the base, using, as always for pricking, a 

 reading-glass, to ensure getting the prick exactly on the line. 

 Let us call this A. 



From A, lay off, with as long a chord as can be commanded, 

 the true bearing of the base, and having ruled this line of 

 bearing as long as possible, make another prick on it at the 

 required distance from A, for the other end of the base. 

 From the two base stations lay off angles to two other main 

 positions, and choose the one of these where the intersection 

 of the lines makes the nearest angle to 90° as the third station 

 to prick in, doing so with great care on the intersection of the 

 two lines. Then from this third station lay off an angle to 

 the fourth, and if this, when ruled, passes exactly over the 

 intersection of the two lines from the base stations, it can be 

 pricked in. All four stations are correct, and the groundwork 

 of the chart is laid ; but if there is any little triangle visible 

 with the reading-glass, all must be plotted over again, for 

 unless these first four stations are exactly right, nothing will 

 ever go right afterwards. 



These four stations settled, proceed in like manner with 

 other main stations ; but now we shall of course have three 

 intersecting lines for each station, and care must be taken 

 that these lines do truly intersect, and no station must be 

 pricked in that has not got three such converging lines 

 through it. 



The main stations down, smaller chords may be used for 

 secondary theodolite stations, and the protractors will come 

 in in plotting the marks and other minor points, the neces- 

 sary angles for which we may suppose some of the party are 

 getting, whilst the first main points are being carefully plotted. 

 As the chart fills, there will be many lines from which the 

 angle to a new point can be measured, and it is well to 



