56 POPULAR LECTURES AND ADDRESSES. 



or specified by aid of two objects seen in line 

 or nearly so, or one over the other. The danger 

 lies altogether on one side of this line ; or, it may 

 be, a line so specified is a safe course between two 

 dangers. 



An outlying danger is completely circumscribed 

 by three lines, each specified according to the same 

 principle, and the navigator who knows the three 

 clearing lines, but nothing more for certain, takes 

 care to keep outside their triangle ; but with more 

 minute knowledge he may, when there is occasion, 

 cut off a corner of the triangle by guess or by 

 feeling his way by the lead. Generally, if the 

 danger be of large extent, four or five, or more, 

 clearing lines, forming a quadrilateral or polygon 

 circumscribing it complelely, are specified, still all 

 on the same principle. 



39. There are three serious limitations to the 

 complete usefulness and sufficiency of clearing 

 marks for pilotage : 



(i.) However well a pilot may know them, still 

 he must see two objects for each clearing line, 

 one of them generally at a considerable distance. 



