CHAP. I.] STATION POINTER 25 



Let us, however, take the same points with the observer's 

 position close to the centre object B, as in Fig. 3. We there 

 see that the two circles are nearly tangential, but the third 

 circle through the outer points and the observer, which the 

 station pointer also gives us, cuts at a right angle, and as the 

 position X cannot be off it, the fix is one of the best. 



Fig- 3- .' -. 



In such a case the whole angle between A and C should be 

 observed, if not too large (as in our figure), as the accuracy of 

 the fix depends entirely on this whole angle, and when so 

 near to B a little movement may make considerable difference 

 if B X A and B X C are separately measured. 



B Fig. 4. 



--' X 



Let us now take three points and the observer's position as 

 in Fig. 4, using the same letters. 



The angles we have observed give us X as the point of inter- 

 section. It is evident that it is difficult to localise this point 

 exactly, as all three circles so nearly coincide as make it im- 

 possible to say where the precise point is at which they inter- 

 sect, and, with the station pointer, we should find that we 



