84 POPULAR LECTURES AND ADDRESSES. 



the celestial pole at the time of your observation. 

 From the observed altitude, then, of the stars or 

 sun, you can deduce the altitude of the pole 

 thus : 



1. If the star crosses the meridian under the 

 pole, add the polar distance to the observed 

 altitude. 



2. If the star crosses the meridian above the 

 pole, but north of your zenith, subtract its polar 

 distance from the observed altitude. 



3. If star or sun cross the meridian south of 

 your zenith, add its polar distance to the observed 

 altitude and subtract the sum from 180. 



So, in any one of the three cases the latitude 

 is calculated from your observation. 



In meridian observations for the latitude the 

 aid of the chronometer is not needed : the 

 observer keeps watching the altitude by aid of 

 a sextant till he finds it cease to diminish and 

 begin to increase (in case No. i), or till he finds 

 it cease to increase, and begin to diminish (in 

 case 2 or case 3). He thus finds, as nearly as 

 he can in each case, the least altitude or the 



