278 HYDROGRAPHICAL SURVEYING [chap. xii. 



and a beginner will find it very difficult to be sure of his star ; 

 but comparison with the star atlas, and consideration of how 

 the star wanted lies with regard to the others, will, after a 

 little experience, clear up the difficulty. 



If the star be a faint one, it is difficult to bring it down to 

 meet its image in the horizon by taking the sextant off its 

 stand ; and if it is a bright one, there is no need to do so, as 

 it cannot be mistaken if the estimated altitude is anywhere 

 near the truth. There is then, in regularly pre-arranged 

 observations, no necessity for doing this, and we shall trust 

 entirely to the vernier being set to the calculated altitude for 

 finding the star. 

 Advant- The levels on the index-bar of the sextant and on the arm 

 Levels on ^^ *^^® sextant stand having been accurately adjusted by means 

 Sextant of the sun as already described, and knowing the exact spot 

 Sextant on the roof of the artificial horizon to which the telescope 

 stand. should point on looking along its upper surface Avhen it is 

 truly directed to the centre of the artificial horizon, the calcu- 

 lated double altitude of the star is set on the sextant. The 

 bubble of the sextant level is brought into the centre of its 

 run by turning the sextant on its bearing in the stand in a 

 vertical plane. The slow motion given by the tangent screw 

 on the sextant stand, if so fitted, enables this to be done with 

 great nicety. The bubble on the arm of the stand is also 

 brought to the centre of its run by making the arm horizontal 

 and working the foot-screw. 



The sextant and stand are then moved bodily in the direc- 

 tion of the artificial horizon until the telescope points cor 

 rectly to the centre, the bubbles of both levels being kept 

 accurately in the centre of their runs. 



On looking through the telescope there can be no possibility 

 of mistaking the star to be observed. 



Having brought the two images into proximity by hand, 

 place the right hand on the screw at the end of the stand-leg 

 that has been arranged at right angles, and the left hand on 

 the tangent screw of the sextant, when, by working these two 

 screws, the images of the stars can be made to pass over one 

 another exactly, and the word " stop " given. 



At this signal the attendant bluejacket will hold the lantern 

 up for reading off. The light should be thrown on to the arc 



