NA VI G A TION. 



greatest altitude, as the case may be. Practically, 

 he does help himself by finding by the aid of 

 his Nautical Almanac the time on his watch 

 within a few minutes of the precise instant when 

 the least or greatest altitude is to be observed ; 

 but then, though the altitude changes but very 

 little within five or ten minutes, before and after 

 this instant, the observer generally satisfies himself 

 that he has got the true minimum or the true 

 maximum by waiting till he finds the change 

 from sinking to rising, or rising to sinking. 



50. LONGITUDE. To determine the longitude 

 by astronomical observation, two things must be 

 done. The local time must be found from sun 

 or stars, and Greenwich time taken at the same 

 instant from your chronometer, or, failing the 

 chronometer, by lunars. The difference of the 

 times thus found reduced to angle at the rate 

 of 15 to the hour, 15' of angle to one minute 

 of time, 15" of angle to one second of time, is 

 your longitude east or west of Greenwich, ac- 

 cording as your local time is before or behind 

 Greenwich time. On shore local time is most 



