NAVIGATION. 81 



ambiguity in the use of the word meridian, 

 according to which it is sometimes used for a line 

 on the earth's surface, and sometimes for the north 

 and south vertical plane denned above an am- 

 biguity not very inconvenient when we are on our 

 guard against any mistake which could arise from 

 it. It is exceedingly interesting, in respect to the 

 theory of gravitation and of the earth's figure, 

 though of no moment in respect to navigation, 

 to remark that, in reality, lines of equal longitude 

 are not precisely meridional lines, or true north 

 and south lines ; ?nd that lines of equal latitude 

 are not exactly circles, but slightly sinuous curves. 

 48. Just two kinds of observation are used in 

 astronomical navigation which are shortly desig- 

 nated as " altitudes " and " lunars" I shall say 

 nothing of lunars at present, except that they are 

 but rarely used in modern navigation, as their ob- 

 ject is to determine Greenwich time, and this object, 

 except in rare cases, is nowadays more correctly 

 attained by the use of chronometers than it can 

 be by the astronomical method. 



The astronomical observation, which is practised 

 VOL. in. G 



