20 GENEKAL NAVIGATION. 



of the ship. The moon is often visible during the day and in combi- 

 nation with the sun gives an excellent fix. 



The morning and evening twilight observations, besides their great 

 accurac}^, possess the additional advantage of greatly extending the 

 ship's reliable reckoning beyond the limits of the ordinary day navi- 

 gation and correspondingly restricting the dead-reckoning uncertain- 

 ties of the night. An early morning fix in particular is often of 

 great value. 



Observations of the stars at night require the use of the star tele- 

 scope in order to define the horizon. Though the same degree of 

 accuracy as at twilight can not be expected, night observations are 

 a very valuable dependence and should be assiduously practiced. 



Piloting. — The navigator, in making his plan for entering a 

 strange port, should give very careful previous study to the chart 

 and should carefully select w^hat appear to be the most suitable 

 marks for use, also providing himself with substitutes, to use in 

 case those selected as most suitable should prove unreliable in not 

 being recognized with absolute certainty. It must be remembered 

 that buoj^s seen at a distance in approaching a channel are often 

 difficult to place or identify, because all may appear equally distant, 

 though in reality far apart. Ranges should be noted, if possible, 

 and the lines drawn, both for leading through the best water in 

 channels and also for guarding against particular dangers; for 

 the latter purpose safety bearings should in all cases be laid down 

 where no suitable ranges appear to offer. The courses to be steered 

 in entering should also be laid down and distances marked thereon. 

 If intending to use the sextant and danger angle in passing dangers, 

 and especially in j^assing between dangers, the danger circles should 

 be plotted and regular courses planned, rather than to run haphazard 

 by the indications of the angle alone, with the possible trouble to be 

 api^rehended from wild steering at critical points. 



The alidade or dumb compass should invariably be mounted in 

 entering or leaving port and kept faithfully set to the magnetic 

 direction of the ship's head, changing promptly with everj^ change 

 of course, so that the observed bearings may be magnetic, and there- 

 fore ready for the chart without the necessity of waiting to apply 

 corrections. The chart should be on the bridge in readiness for 

 reference or use in plotting positions. 



The ship's position should not be allowed to be in doubt at any 

 time, even in entering ports considered safe and easy of access, and 

 should be constantly checked, continuing to use for this purpose 

 those marks concerning which there can be no doubt until others 

 gradually and unmistakably declare themselves. 



The ship should ordinarily steer exact courses and follow an exact 

 line, as planned from the chart, changing course at precise points, 



