NAVIGATION. 105 



" If the distance between the moon and a star 

 " increased or decreased uniformly, the Greenwich 

 " times corresponding to a given distance, as 

 " found by the above rule, would be strictly 

 " correct ; but an inspection of the columns of 

 " the proportional logarithms in the Ephemeris 

 " will show that this is not the case ; a correction 

 " must therefore be applied to the time so found 

 " for the variation of the difference of the 

 " distances. This correction may be obtained 

 u by means of the table at page 490 of the 

 " present volume, in the following manner." 



[Here follow details of the method of inter- 

 polation to be used with examples of its 

 application.] 



III. DEAD RECKONING. 



59. I have now explained to you briefly, and 

 very imperfectly, navigation in clear weather. I 

 must next speak to you on a more sombre part 

 of our subject, navigation under clouds or through 

 fog. When no landmarks can be seen, and when 



