CHAP. XV.] 



TRUE BEARING 



353 



in contact with the vertical wire, and below the horizontal one. 

 If the theodolite is truly levelled, he will not need to touch 

 his vertical tangent screw, but, if necessary, he must do so, to 

 keep the upper hmb of the sun as nearly touching the hori- 

 zontal wire as he can. When the upper limbs of the sun in 

 the artificial horizon are in contact, the observer calls " Stop," 

 and the motion of both tangent screAvs of the theodolite 

 ceases. The liorizontal arc is then read. 



Then, without moving the theodolite in altitude, the other 

 limb of the sun is brought on the other side of the vertical 

 Avire, and the reading made when the artificial horizon observer 

 gives " Stop," on the lower hmbs of the sun coming in 

 contact. 



The sun will thus have passed between opposite quadrants 

 of the cross-wires, as in the diagram Fig. 68. 



Fig. 68. 



Similar observations are made at the same altitude in the 

 afternoon, the lower limb coming first. Each set Avill thus 

 consist of two observations a.m. and two p.m. In this method 

 the time must be taken exactly, which is a drawback, as it 

 either requires tlu'ee persons or that one should take time 

 as well as his observation. There is, however, no necessity 

 to know the local time very exactly ; all we want is the true 

 elapsed time. 



To work out the equal altitude observation, the means of Calcu- 

 the times, and of the horizontal angles of a.m. and p.m. respec- Bearhie 

 tively, are taken. by Equal 



If the sun had no motion in declination, the mean of a.m. 

 and p.m. horizontal angle would be the angle on the horizontal 

 arc corresponding to the true meridian, or, in other words, the 

 bearing of the zero ; but as tliis is not so, we must work a correc- 

 tion similar to the equation of equal altitudes when obtaining 

 time, to be applied to this mean of the angles. 



23 



Altitudes. 



