24 HYDROGRAPHICAL SURVEYING [chap. i. 



two circles must be our exact position X, as it is the only one 

 from which we could have obtained these tw^o angles at the 

 same time. See Fig. 2. 



The station pointer obtains us this position X without the 

 trouble of drawing the circles, as it is manifest that, if we have 

 the angle A X B on one leg of the station pointer and B X C 

 on the other, the only spot at which we can get the three legs 

 to coincide with the points A, B, and C, will be X. 



We place the station pointer, therefore, on the paper, bring- 

 ing the chamfered edges of the three legs of the instrument to 

 pass over the three points observed, and make a prick with a 

 needle in the nick in the centre, which will then mark the spot. 



A piece of tracing-paper on which the three angles are pro- 

 tracted will answer the same purpose, but, of course, this will 

 entail more time, and in the open air will give trouble, as liable 

 to be blown about by the wind. Nevertheless, this has often 

 to be used, as when points are close together on a small scale, 

 the central part of the station pointer will hide them, and 

 prevent the use of the instrument. 



A very useful instrument has been devised by Commander 



Oust for such occasions, and consists in a plate of transparent 



zylonite on which a graduated arc is engraved. The requisite 



angles are drawn on this with pencil, with the angles reversed, 



and the plate being turned over, so as to bring the pencil lines 



in contact with, the paper to obviate parallax, is used as a 



station pointer. 



Three This method of fixing is generally known as the " two circle " 



Circle method, but it is really the "three circle" method, for the 

 Method. "^ 



circle drawn through the two outer points and the observer's 



position is also involved. A comprehension of this is of value. 



Position of The chance of error in a fix varies greatly with the posi- 



" Points." ^JQj^ ^f ^Yic, three points with regard to one another and the 



observer. 



It is in general sufficient to realise that the more rectangular 



the intersection of the two circles, the less chance there is of 



any error in the resulting fix, but there are cases where the fix 



is admirable though these circles are almost tangential, because 



the third and larger circle produces a rectangular cut. 



With points and the observer's position placed as in Fig. 2, 



the two circles give a good intersection, and the fix is good. 



