CHAP. I.] TESTING STATION POINTER 33 



a station pointer on a published chart, or the adoption of this 

 instrument may have a disastrous result. 



In the first place, the chart may be from a rough survey, and 

 there may be absolute errors in the points on it ; and secondly, 

 the distortion caused in printing with damp paper always 

 changes the position of points, more or less, and with objects 

 in certain positions, this alone may make an error in a station 

 pointer fix. 



In navigating, therefore, with a published chart, of the 

 accuracy of which you are not certain, ahcays use a hearing, as 

 well as the sextant angles plotted by station pointers, or use 

 check angles to each fix. If the result is to show that the points 

 are not correct relatively to one another, use the compass only, 

 as it is less likely to get you into trouble with a defective chart, 

 for the reason that the non-intersection of three bearings will 

 at once indicate something wrong, and the navigator will choose 

 the points of danger in his course ahead to steer by, rejecting 

 the others whose positions with regard to him are of little 

 moment. 



BRASS SCALES. 



These must be examined by means of the beam compasses, 

 to see that their divisions are correct, more especially the 

 diagonal portion, as the makers are sometimes not careful 

 enough. If a scale is found to vary, it should be rejected. 



A brass scale should never be used for ruling, and never he 

 taken out of its hex. If it is, some day it will fall from the 

 table, get bent, and its correctness is gone. 



STEEL STRAIGHT-EDGE. 



This must be examined to see if its edge is exactly straight, 

 by ruling a very fine line, and reversing the straight-edge, 

 when, either ruling another line over the first, or examining 

 the coincidence of the edge with the line already ruled by 

 means of a reading-glass, will prove whether it is perfect. 



Placing steel straight-edges edge to edge is another method 

 when there are more than one ; but great care must be taken 

 with regard to the hght if this is done, as it is difficult to 



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