86 POPULAR LECTURES AND ADDRESSES. 



accurately found by observing the instant when 

 the sun or a star crosses the meridian. But on 

 board ship this method cannot be practised, and 

 instead an altitude, whether of sun or star, is 

 observed when the body is anywhere out of the 

 meridian. Now remember that a star (or, neg- 

 lecting a very slight error due to change of 

 declination, the sun) is at its greatest altitude 

 when it is crossing the meridian, and you will 

 understand that, when the observed altitude is 

 anything less than the greatest altitude, you can 

 calculate how long time before or after its 

 meridian passage, must have been the instant of 

 your observation. The calculation requires a 

 knowledge of the declination of the observed 

 body, and of the latitude of the ship's place at 

 the time of the observation ; but if you have 

 chosen a star in the prime vertical, or very 

 nearly in the prime vertical, a very rough ap- 

 proximation to your latitude suffices. The 

 method most commonly practised at sea is to 

 estimate the latitude as accurately as possible by 

 dead reckoning from previously determined 



