CHAP, v.] USE OF THE SHIP FOR TRI ANGULATION 153 

 We can now test A G C D, having all its angles. 



In A G C D, given G D, C G, and all its angles, find 



C D. Compare the value of C D with that found 



previously. 

 In A B C D, given B C, C D, and Z B C D, find B D 



and Z^CBD, CDB. 

 In A B D, given A B, B D, and Z A B D, find A D 



and Z « B A D, B D A. 



We have thus found all the sides and angles in the quadri- 

 lateral A B C D, which can be plotted on any required scale. 

 Unless subsidiary marks have been shot in from the ship's 

 stations, there is no occasion to plot the latter. 



If necessary, the survey may be extended along the coast in 

 either direction on similar principles. 



The foregoing method was used with success on the East 

 Coast of Africa, and on that occasion no floating beacon was 

 used. In such a case it will be seen that, the first position of 

 the ship being at E, the second position should be somewhere 

 intermediate between E and F. It must be taken up with a 

 view to obtaining a sufficiently large receiving angle between 

 A and B, and at the same time to give a somewhat larger 

 receiving angle at C between the lines to it from the two posi- 

 tions of the ship. The receiving angle at C can always be 

 known roughly by the difference of its bearing from the two 

 positions of the ship. 



We are now necessarily dealing with receiving angles much 

 smaller than we should wish to use, and the case is merely 

 mentioned as an exercise of ingenuity when hard pressed. It 

 will, of course, be noted that the conditions of the problem 

 materially improve if the trend of the coast takes a more 

 favourable direction than that assumed in the example, enabling 

 the shore stations to see each other. 



Example XV. — Triangulation of a low ivooded coast destitute 

 of natural features, on which landing is 'practicable, using the 

 ship and floating beacons. 



If the stations on the coastline are visible from one another, 

 the triangulation is simple enough ; but if, as frequently 



