352 HYDROGRAPHICAL SURVEYING [chap. xv. 



stated altitude, and the sextant Avill be set beforehand, the 

 observer using it giving the " Stop " to the theodohte 

 observer. 



In the others, the theodohte observer generally calls the 

 " Stop," and the sextant observer takes whatever altitude it 

 happens to be. 



At very low altitudes, it is difficult to observe the sun in the 

 artificial horizon, and in such cases the altitude is read off on 

 the vertical arc of the theodolite. 

 Eiimina- It is desirable to make the observations at a low altitude, in 

 Errors of ^rder to avoid the effect of any error in the verticality of the 

 Verti- azimuthal axis arising from inaccurate levelling or instrumental 

 Azinmthai defect, which increases with the altitude. Errors arising from 

 Axis and this cause are eliminated by reversing the telescope in its Y's 

 tion! ™* jfo^ li^l^ ^h® ^^t^ o^ observations. Errors due to the adjustment 

 of collimation not being perfect may be eliminated by taking 

 half the sets of observations with the telescope turned half 

 round in the Y's with the bubble upwards, and the remainder 

 of the observations with the telescope in its ordinary 

 position. 

 Changing To arrive at a satisfactory result in either case, it is necessary 

 Degree of ^^ tQ,^Q several sets, with a different degree of the arc pointed 

 at the zero in each, so as to eliminate the errors of the horizontal 

 arc of the instrument. 

 Correct- As it is the bearing of the sun's centre which we obtain 

 Sim's° by working out the azimuth, the aim of the theodolite observa- 

 Centre. tions is to get the horizontal angle between that centre and 

 our zero ; but it is manifest that we cannot trust our eye to 

 place the cross-wires of the telescope exactly on the centre of 

 the sun, nor can we place the wires truly vertical and hori- 

 zontal. 



It we could do the latter, we could arrive at the angles to 



the centre by merely observing the smi in one quadrant, and 



applying the semi-diameter x sec. alt. ; but we must not trust 



this, if we want fair accuracy. 



Method of In equal altitude observations, the method is to fix on an 



Bearing altitude for both sextant and theodohte, and set the vertical 



by Equal 



Altitudes, arc of the latter at it. In the forenoon, bring the sun so that 



it is in the lower half of the field, and approaching the vertical 



wire. The theodolite observer then keeps the limb of the sun 



