168 HYDROGRAPHICAL SURVEYING [chap. v. 



marks, and the angle to this second zero observed from the 

 main station zero. 



This second zero is wanted to be in such a position with 

 regard to the marks that any shght movement in the ship 

 will make the least possible difference in the angles to be 

 observed between it and the marks. It must be, therefore, at 

 about the average distance of the marks. It A\'ill not do to 

 choose some object miles away behind the marks, as the least 

 swing of the ship will at once alter the whole of the angles. 

 Generally speaking, the central mark to be fixed will answer 

 the purpose best, but in many cases it will be found necessary 

 to change this zero for some marks, measuring from some other 

 object at an equal distance from the ship. 



If discretion be used in the selection of subsidiary zeros, 

 such zeros being connected simultaneously with the main 

 points used for fixing (of which one should usually be very 

 distant), any small movement of the ship will have but a small 

 effect on angles taken subsequently. 



The governing principle is that a small angle between the 

 objects at nearly the same distance is not sensitive to small 

 movements in the position of the observer. 



All angles that have a tendency to be sensitive should be 

 observed simultaneously with the fixing angles ; those that 

 are less sensitive afterwards. 



The objects selected for observation should be divided into 

 groups, each group having its own zero. 



The degree of accuracy obtainable in triangulation in which 

 the ship is used as a " point " is largely dependent upon a clear 

 perception of these considerations. 

 Repeating When the minor angles have been taken, repeat the main 

 Angles. angles to see if the ship has moved, giving another signal to 

 the shore station for another angle from it. All mark angles 

 should then be observed again to check errors. 



It need scarcely be said that the more rapidly these angles 

 are taken, the less the chance of any error arising from varia- 

 tion of ship's position, by change of direction of current, wind, 

 etc. An experienced hand should therefore be chosen for this 

 work. 

 Telescope A sextant with a telescope of high magnifying power is most 

 tant. useful. On this head, see p. 12. 



