82 POPULAR LECTURES AND ADDRESSES. 



regularly by day and frequently also at night in 

 practical navigation, consists simply in measuring 

 by the sextant the apparent altitude of the sun or 

 star above the horizon ( 5 above) and noting 

 accurately the hour, minute, and second by the 

 ship's chronometer, at which the observation is 

 taken. The immediate results of the observation 

 are corrected according to explanations I have 

 already given you in respect to the following 

 several particulars index-error, dip of the horizon, 

 and refraction ; also for the sun's semi-diameter, 

 when it is the sun, not a star, that is observed. 



49. LATITUDE. With these definitions and 

 explanations premised, we are prepared to under- 

 stand readily how latitude and longitude are 

 determined by actual observation of stars or sun. 

 If there were a bright enough star exactly at the 

 celestial pole of whichever hemisphere we are in, 

 we should only have to observe its altitude above 

 the horizon, and that would be the latitude. In 

 the northern hemisphere, Polaris, as I have told 

 you, is seen describing daily a small circle of i 21' 

 distance from the true north celestial pole ; and 



