CHAP. XII.] LATITUDE BY STARS 277 



Single altitudes of a star near the prime vertical, either 

 before or after observations for latitude, will give the error of 

 the chronometer with sufficient accuracy for the purpose of 

 calculating the reduction to the meridian. 



Having thus made all preparations, and compared the 

 pocket watches with the standard chronometer, and another 

 as a check, we land to observe. We may here remark that 

 we must again compare on returning on board. 



Placing the artificial horizon north and south, we put on Observing 

 the roof, with the mark on it in the settled direction, according ^''^^^• 

 as our first star is north or south of the zenith. 



We then place the sextant on its stand, having first screwed 

 in the inverting tube with the weakest-powered eyepiece. This 

 should be adjusted to focus, as near as possible, before screwing 

 into the collar. 



Place the sextant and stand on the stool, so that one of the 

 three legs which support the stand is at right angles to the 

 meridian, and on the right of the observer. 



Set the vernier to a few minutes less than the estimated 

 double altitude of the star. 



Move the stool with the instrument on it, so that, looking 

 over the tube, we can see the reflection of our star in the 

 artificial horizon. 



Point the telescope at this, and set taut the screw that fixes 

 the handle of the sextant on to the bearing of the stand, then, 

 working the sextant right and left in the stand pivots, the other 

 image of the star will soon dash across the field. 



Unless the star is moving rapidly in altitude, there will be 

 no further need to move the sextant on its bearing, and care 

 should be taken that the screw is tight enough to prevent its 

 moving while turning the sextant up to read, or the telescope 

 will not point to the star when directed to the horizon again, 

 which it should do at once, and so save time in redirecting. 

 Beginners often neglect this, failing to see the necessity for 

 it ; and, losing time in looking again for the star after each 

 reading, miss one of the great advantages of a stand — viz., 

 that, once fixed, the stars will always be in the field without 

 any bother. 



With faint stars, when there are several of nearly the same 

 brightness close together, it is sometimes rather confusing. 



