NAVIGATION. 103 



(Inman, Norrie, and Raper) contain carefully 

 elaborated rules and sets of tables for the pur- 

 pose of making the practical problem of clearing 

 the distance as easy as possible. The conclusion 

 of the process of finding Greenwich time by a 

 lunar observation at sea I can best explain to you 

 by reading from the Nautical Almanac for 1876, 

 page 511, premising that six pages of the Nautical 

 Almanac are devoted to data for finding the longi- 

 tude at sea by the method of lunars. 



"Pages XIII. to XVIII. of each month, Lunar 

 " Distances, These pages contain, for every third 

 " hour of Greenwich mean time, the angular 

 " distances available for the determination of the 

 " longitude of the apparent centre of the moon 

 " from the sun, the larger planets, and certain 

 " stars, as they would appear from the centre of 

 " the earth. When a lunar distance has been 

 " observed on the surface of the earth, and re- 

 " duced to the centre by clearing it of the effects 

 " of parallax and re/raction, the numbers in these 

 " pages enable us to ascertain the exact Greenwich 

 " mean time at which the objects would have 



