CHAP. XXI] OBSERVATIONS AT NIGHT 495 



When treating the angles, it is necessary to consider the 

 various corrections occasioned by change of position in each 

 ship, searchUghts situated at remote distances, angles observed 

 from different positions on board, all of which should be referred 

 to a definite point, say the standard or bridge compass of each 

 vessel, from which positions they can be then easily referred to 

 the beacon near by if necessary. Such corrections are quickly 

 dealt with in the manner previously described if the necessary 

 data are recorded at the time and not left to the memory. 

 A rough diagram illustrating the positions of the various ships, 

 their searchUghts, masthead flashing lamps, etc., and the 

 beacons near by, will much facihtate this operation should the 

 corrections appear somewhat involved in their appUcation. 



It is well to remember that the succeeding morning twihght 

 may often help to fill up any important gaps that there may have 

 been overnight, and thereby save the situation. We should also 

 be ready to take advantage of the clear atmosphere that often 

 occurs and precedes the haze that increases as the heat of the 

 day progresses during a period of fine weather. 



The vessels engaged should therefore, as a general rule, not 

 quit their overnight positions before a stated time in the morn- 

 ing, and display a large flag or other prominent mark from the 

 masthead whilst in position during daylight. 



The vessels will in the morning then be doubtless swung to the 

 opposite tidal stream, and the necessary corrections must be 

 applied before treating the angles then obtained with any former 

 that have been taken when swung in a different direction. 



SECTION XV. 



ADJUSTMENT OF THE TRIANGULATION. 



When the data thus obtained are received by the directing 

 vessel, the necessary calculations can be made for determining 

 the several ships' positions. W^hen station pointer fixes have 

 been obtained this is as described on p. 170. 



The bearings calculated from the fixes which are dependent 

 upon the original known data relating to the triangulated shore 

 objects will here afford a valuable check with those that we have 

 been able to observe. Should there be a disagreement in the 



