CHAP. IV.] THE MAIN TRIANGULATION 83 



each plummet is suspended, brought in line with the flag, will 

 indicate the direction of a line passing exactly underneath the 

 flag ; the intersection of those lines is the spot required. 



Although we are about to speak of triangulation from shore Triangu- 

 stations, as carried on by means of the theodoHte, as this s^extant?^ 

 instrument is always available in a surveying ship, it must be 

 understood that, ivith care, an excellent triangulation may be 

 obtained wiXh. that invaluable instrument, the sextant. 



When entering sextant angles in the " Main Angle Book," it Sextant 

 is convenient to refer them all to one zero, measuring round to a Station 

 the right as if taken with a theodolite. This facilitates the 

 correction of angles if taken from a false station. 



The point on which care is principally needed is that the Horizon- 

 angles measured should be horizontal angles. A practised ^th Sex-^ 

 surveyor will usually be able to note some small natural mark ta-nt. 

 directly above or below the object whose angle is required, 

 and at his own level, to which to measure his angle, and in most 

 cases of using the sextant this will give a sufficiently near result. 



If the angle is small, a direct measurement \vill introduce more 

 error than would result from reflecting to each object in succes- 

 sion a third object making a very large angle, and taking the 

 difference between the two angles observed. If forced to use 

 the sextant for triangulation, another means may be used. 



From the end of a longish pole (boat-hook staff will do), 

 planted at a slight angle from the perpendicular, let a plumb- 

 line fall, and getting the object transit one point in the line, the 

 angle can be taken to any other part of it. The plumb-line 

 must not be too close to the observer, or it will be difficult to 

 keep the transit on, and parallax will creep in. 



When possible, a sextant angle between two objects at different 

 levels may be corrected, by observing the angles of elevation 

 (corrected for distance of shore horizon) of the two objects, 

 and calculating the true horizontal angle between them by 

 spherical trigonometry. 



It is a question of circumstances as to whether the main 

 triangulation is to be carried on by itself first of all, or in 

 combination with the secondary stations and marks. This in 

 no way affects the principle of the work, but only the detail 

 of what is done when the angles at the main stations are 

 observed. 



6—2 



