356 



HYDROGRAPHICAL SURVEYING [chap. xv. 



prime vertical, a small error in the altitude will but slightly 

 affect the azimuth. 



The azimuth of the sun having been obtained by the ordinary 

 rule of nautical astronomy, the true bearing of the object is 

 found by applying the mean of the theodolite angles of that set, 

 Methodby In finding a true bearing with sextant only, it will be more 

 accurate if two observers are employed — one to take the 

 altitude, the other to measure the angular distance at the same 

 instant. 



If only one observer is available, he must take altitude and 

 angular distance alternately, taking care to end with the same 



Sextant. 



Calcula- 

 tion of 

 Horizon- 

 tal Angle. 



observation as that with which he begins, so that the mean of 

 each kind will correspond as nearly as may be in time. Thus, 

 if he begins with altitude, he must also end with altitude. 



This method should not, however, ever be used when a 

 theodolite is available, and is only adopted for true bearings 

 from the ship, in an irregular survey. 



It is essential to observe both the nearer and farther limbs 

 of the sun and accept the mean ; observations of one limb are 

 liable to considerable error. 



In this instance we have to calculate the horizontal angle, 

 which A\dth the theodolite we obtained directly. 



The object should be so chosen that the line joining it 

 with the sun should not make a larger angle with the 

 horizon than 20°, and the less the better, as any inaccuracies 

 of observation will not then be much increased when the 

 horizontal angle is deduced. If Ave take an object 90° 



