182 HYDROGRAPHICAL SURVEYING [chap. vi. 



A continuous series of true bearings is necessary, and it is 

 more convenient if they are taken at the beacons. 



Simultaneous angles must be observed at the same time to 

 correct the bearing to points as far as possible up and down 

 the coast. 



With a Thomson's compass in a favourable position — i.e., a 

 position where it is not liable to change — a compass bearing 

 will be admissible now and then. 



The first true bearing should be taken from the last beacon 

 from which the first breastmark is visible. No other bearing 

 is necessary till the points fixed from this position are passed, 

 when another is wanted to carry on and preserve the bearing. 



When the coast trends nearly north and south, latitudes 

 by twilight stars north and south are advisable every 30 or 

 40 miles to check the scale ; and similarly when coast is nearer 

 east and west, longitude observations east and west. 



Obtain shore observations when possible. 



To plot, begin when the first true bearing is attained. Draw 

 the meridian through the beacon from which it is observed, 

 and lay off the true bearing of the first breastmark. Let us 

 suppose we begin at Beacon III., and we are going to plot just 

 the Fleet III. to II., and then II. to I. 



Lay off the angles to II. and breastmark 2 B, drawing long 

 hnes. Calculate the patent log base III. -II. from the two 

 runs, and prick off Beacon II. 



Lay off all angles from III., first making sure that the whole 

 angle between first breastmark and the breastmark ahead is 

 the same both at dropping and picking up the beacon. 



Then lay off the angles from II., and prick in the two breast- 

 marks and primary points. 



Lay off on tracing-paper the angles from intermediate 

 Fix II., and all points should intersect and be pricked in. 

 Angles to points in Fleet II. to I. can be laid off from this 

 position. 



Plot intermediate Fix I. by the points already fixed, and 

 lay off angles. 



Lay off angles from II., using III. as zero. 



Prick in Beacon I. by distance, and test it by a station- 

 pointer fix, using II. and Breastmark 2, the centre being on 

 the Une drawn from II. to I. If tliis agrees, there should be 



