482 HYDROGRAPHICAL SURVEYING [chap. xxi. 



in the ships, preferably round the standard or bridge compass, 

 above which should be disj^layed some suitably distinguishable 

 object to which to observe from the other vessels, all ot whom 

 observe the upper part of the beacon flag. 



Figure V., Case 1. 



The following essential data should be obtained simul- 

 taneously as far as possible : 



On board X (the controlhng ship, under way near Beacon 1). 



1. The four anglss between Y and Z and the Beacons 2 and 3, 

 whose sum equals 360°. 



2. The magnetic bearing of an object in the scheme easiest 

 to observe, and at a suitable distance. 



3. The direction of the ship's head. 



4. The connection of Beacon 1 with the scheme. 



5. G. M. T. or local mean time of the observation. (For 

 identification with Y and Z.) 



On hoard Y and Z. 



1. Angles between X and the two extreme beacons measured 

 through both X and the ship visible beyond X. 



2. Horizontal subtense angle of the length presented by X 

 between the two distinguishing marks near the extremes of 

 the vessel (a varying quantity). Measured with observing 

 sextant as accurately as possible. 



3. Magnetic bearing of an object in the scheme easiest to 

 observe, and at a suitable distance. 



4. The connection of Beacon 1 with the scheme. 



5. G. M. T. or local mean time of the observation. X should 

 endeavom' to take up a position so as not to mask the view 

 between Y and Z. 



The connection of Beacon 1 with the scheme can also now 

 be made on board each ship, or subsequently, when revisiting 

 the beacons. 



The signals should be rej^eated after sufficient time for re- 

 cording has elapsed to enable three independent sets of obser- 

 vations to be obtained, X remaining as steady as possible 

 throughout. 



