NAVIGATION. 55 



I. PILOTAGE OR NAVIGATION IN THE NEIGH- 

 BOURHOOD OF LAND. 



38. Sure and ready knowledge of the general 

 appearance of the places visible from the ship's 

 course is the first requisite in a pilot. It was 

 probably the only kind of navigational skill, except 

 taking soundings, possessed by the ancient Medi- 

 terranean navigators, or by European navigators 

 generally, until nine hundred years ago, when the 

 mariner's compass first became known in Europe. 



When there are outlying dangers (as shoals and 

 sunken rocks are technically called in navigation), 

 the pilot must know familiarly their positions, with 

 reference to visible objects on the shore, or on 

 islands and rocks standing out above the water. 

 Mere acquaintance with the general appearance of 

 the visible objects no longer suffices, and the pilot, 

 however unscholarly may have been his training, 

 becomes of necessity a practical mathematician. 

 The principle of clearing marks for dangers is of 

 the purest geometry. A certain line is described 



