606 HYDROGRAPHICAL SURVEYING [chap. xxi. 



sudden change of weather conditions imperilling their safety, 

 should be provided with specific instructions with regard to 

 acting upon such an emergency, including fog. This should 

 specify a rendezvous at some prearranged beacon, the duty of the 

 mechanically propelled boats being to first succour the mark 

 boats, who should be always prepared to sUp and buoy their 

 moorings rather than court any danger there may be in the 

 attempt to weigh them. 



SECTION XXIII. 



THE USE OF BEACONS WITH SHIP TRIAXGULATION 

 DURING IRRECtULAR OR DETAILED SURVEYS. 



The subject is sufficiently referred to in Chapters V. and VI., 

 where various examples are given of the employment of beacons 

 when carrying out an irregular triangulation of a coast by 

 means of the ship. The previous sections have more especial 

 reference to the use of beacons when conducting a survey in the 

 open sea or out of sight of land or other objects, etc. 



Floating beacons can often be resorted to in detailed surveys 

 on the larger scales, when, owing to the nature or trend of the 

 coast, the lack of drjdng banks, off-shore depths, etc., any other 

 method would present much difficulty or involve great delay. 

 This often occurs, where it may be desired to extend a triangu- 

 lation, or commence from some starting-point on shore, on 

 coasts that are generally straight, low, featureless, and densely 

 wooded, but may afford desirable anchorages. In such cases 

 the beacons should be moored Mith two or more bridles, and 

 with as small swinging area as may be practicable, for, in order 

 to obtain the necessary precision, we shall almost entirely 

 depend upon the simultaneous intersection of such beacons 

 by observers at shore stations. 



When conducting an extensive coast survey by beacons 

 where landing is difficult, such as on the West Coast of Africa, 

 it will greatly assist matters, and largely reduce the chances of 

 errors accumulating and remaining undetected as the triangu- 

 lation proceeds, if advantage is taken of at least one accessible 

 elevated station, commanding a view as occasion offers, where 

 an observer can be stationed whilst the ship is placed in the best 



