NAVIGATION. 57 



It often happens that, through rain or haze, the 

 more distant of the two objects is invisible alto- 

 gether, although the nearer may be well seen, 

 and thus the clearing specification is absolutely 

 lost. 



(2.) A stranger, however well prepared by 

 reading his book of sailing directions, must have 

 superhuman quickness of perception to always, 

 when running at a high speed, recognise with 

 sufficient readiness the successive pairs of objects 

 constituting the clearing marks for dangers which 

 he must skirt along or pass between in his 

 course. 



(3.) Often while there are good single objects 

 to serve as near landmarks visible from the ship's 

 course, it may be impossible to find, beyond 

 them, any distinct marks, or any marks at all ; 

 as when there is too uniform a background of 

 hills, or when there is no background at all, the 

 land being flat, with no buildings or trees dis- 

 tinctly visible in the distance. For one or other, 

 or all, of these reasons, the azimuth compass is 

 continually in requisition for pilotage. Thus the 



