CHAP. XV.] 



TRUE BEARING 



355 



In " single " observations, each set will consist of four Method by 



contacts, in each of which the sun Avill be tangential to the ?^,^^,'®, 



° Altitudes 



vertical wire in a different quadrant of the field. 



The mean of these y\nll then be the angle to the sun's centre, 

 corresponding to the mean of the four altitudes. 



When the altitude is being taken by a sextant, it will only be 

 necessary for the theodolite observer to be very exact with the 

 contact of the side-limb of the sun ; but his upper or lower limb, 

 as the case may be, should be as nearly touching the horizontal 

 wire as possible, to ensure the elimination of the wire error. 



Fig. 69. 



It is quite immaterial in which quadrant the observer com- 

 mences ; but whatever plan he adopts, he should always observe 

 in the same manner, as it prevents confusion and mistakes. 

 The sun will appear as in the diagrams in Fig. 69. 



When taking the observation with the theodohte alone, it 

 will of course be necessary to see that both the horizontal and 

 vertical wires are truly tangential to the sun's limbs. 



Six sets should give a very good bearing ; but if the theo- 

 dolite is a very small one, or is known to be badly graduated, 

 more may be necessary. 



Half the altitudes in the artificial horizon may be taken 

 with upper limb and half with the lower ; but this is not impor- 

 tant, as if the observation be made when the sun is near the 



23—2 



