GENERAL NAVIGATION. 17 



officers of men-of-war should become expert in this method of fixing 

 a ship's position. 



In man}' narrow waters, also, where the objects may yet be at 

 some distance, as in coral harbors or narrow passages among mud 

 banks, navigation by sextant and protractor is invaluable, as a true 

 position can in general be obtained only by its means. Positions by 

 bearings are too rough to depend upon, and a small error in either 

 taking or plotting a bearing might under such circumstances put the 

 ship ashore. 



In all cases where great accuracy of position is desired, such as the 

 fixing of a rock or shoal or of fresh soundings or new buildings as 

 additions to the chart, the sextant should invariably be used. In all 

 such cases angles should be taken to several objects, the more the 

 better; but five objects is a good number, as the four angles thus 

 obtained not only prevent any errors, but they at once furnish a 

 means of checking the accuracy of the chart itself. If a round of 

 angles can be taken, the observer's accuracy is also checked. In the 

 case of ordinary soundings a third angle need be taken only occasion- 

 ally ; first, to check the general accuracy of the chart, as above stated ; 

 second, to make certain that the more important soundings, as at the 

 end of a line, are correctly placed. 



If communication can be had with the shore, positions may be 

 fixed with great accuracy by occupying with theodolite or sextant 

 two known points of the chart. The third angle of the triangle, that 

 between the two points at the position sought, should be measured as 

 a check. 



The compass. — It is not intended that the use of the compass to 

 fix the ship should be given up; in ordinary piloting the compass, 

 with its companion, the alidade, may be more usefully employed foi 

 this purpose, although less accurate than the sextant. 



If the accuracy of the chart be doubtful, the compass should be 

 used in preference to the sextant. 



In fixing by the compass it should always be remembered that the 

 position by two bearings only, like that by two angles only, is liable 

 to error. An error may be made in taking a bearing, or in applying 

 to it the deviation, or in laying it on the chart. A third or check 

 bearing should, therefore, be taken of some other object, especially 

 when near the shore or dangers. A common intersection for the 

 three lines assures accuracy. 



Compass bearing and sextant angle. — When only two objects 

 are visible, a compass bearing and a sextant angle may be used, and 

 a better fix obtained than by two bearings. 



Doubling the angle on the bow. — The method of fixing by 

 doubling the angle on the bow is invaluable. The ordinary form of 



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