122 HYDROGRAPHICAL SURVEYING [chap. v. 



C may be of course anywhere on the line A B, and sup- 

 posing ourselves to be plotting from an original base A B 

 will probably be much nearer to A than to F G, but by taking 

 such a long radius we get a straight line in the true direction 

 of the angle laid off, and when we want to measure another 

 angle on to another object, perhaps three times the distance 

 of from A, we have a long line we are certain of, to do 

 it from. 

 Always Here let it be impressed upon the surveyor that all lines 



Lin^^°°^ drawn for plotting the main points, and indeed all points 

 (except very minor ones, on which the position of nothing 

 else will depend), must be drawn as long as possible, and 

 with more or less long chords, if we desire correctness. If 

 we have a line drawn between two stations which lie say 

 6 inches apart on the paper, and it only projects a few inches 

 beyond each, and we hereafter require to lay off an angle 

 from one, having the other as zero, to a station which will be 

 say 2 feet or more distant, we cannot do it correctly, as this 

 longer line will have to be directed by a prick which cannot 

 be farther off than the length of the zero line ; but by drawing 

 long lines with long chords, we are ready for anything, and it 

 will not matter whether the station we take for zero be near or 

 far, as we use, not it, but the long line ruled through it. 

 Length of In no case should a line to a station be laid off with a 

 Radius, protractor or chord whose radius is less than the distance of 

 the station, excepting in a rough plan which we want to do 

 rapidly, or in most parts of a running survey, where, pretensions 

 to accuracy being thrown to the winds, we get points near 

 enough for our purpose down with a protractor. 

 Zero It is important to remember in the selection of zero lines 



^*°®^' that the one should be preferred which makes the smallest 

 angle with the line to be projected from it, provided the object 

 selected for zero is farther off than the point to be plotted. 

 Lengthen- It is difficult to extend correctly a short line once drawn, 

 ingaLine. ^y simply ruling on with the straight-edge. If a longer hne 

 is wanted, it is better to lay off the angle to it again from 

 some other long line, with a sufficient radius. 

 Ruling a To rule a true straight line which will pass exactly over 

 straight ^j-^p centre of the pricks is by no means an easy thing. The 

 ruling pencil, which should be of the hardest lead manu- 



