NAVIGATION. 101 



great circle of the moon's orbital motion. Thus 

 supposing the ship to be navigating in tropical 

 seas, where a minute of longitude is equal to a 

 mile of distance, a careful navigator, with a good 

 sextant, whose errors he has carefully determined, 

 can, by one observation of the lunar distance, 

 find the ship's place within fifteen miles of east 

 and west distance. If he has extraordinary skill, 

 and has bestowed extraordinary care on the de- 

 termination of the errors of his instrument, he 

 may, by repeated observations, attain an accuracy 

 equivalent to the determination of a single lunar 

 distance within a quarter of a minute of angle, 

 and so may find the ship's place within seven 

 miles of east and west distance ; but, practically 

 we cannot expect that a ship's place will be found 

 within less than twenty miles, by the method of 

 lunars in tropical seas, or within ten miles in 

 latitude 60 ; and to be able to do even so much 

 as this is an accomplishment which not even a 

 good modern navigator, now that the habit of 

 taking lunars is so much lost by the use of 

 chronometers, can be expected to possess. 



