GENERAL NAVIGATION. 15 



Along most shores not much affected by bays, tidal rivers, etc., the 

 current usually turns soon after high water and low water. 



The swiftest current in straight portions of tidal rivers is usually 

 in the middle of the stream, but in curved portions the most rapid 

 current is toward the outer edge of the curve, and here the water will 

 be deepest. The pilot rule for best water is to follow the ebb tide 

 reaches. 



Countercurrents and eddies may occur near the shores of straits, 

 especially in bights and near points. A knowledge of them is useful 

 in order that they may be taken advantage of or avoided. 



A swift current often occurs in a narrow passage connecting two 

 large bodies of water, owing to their considerable difference of level 

 at the same instant. The several passages between Vineyard sound 

 and Buzzards hhj are cases in point. In the Woods Hole passage 

 the maximum strength of the tidal streams occurs near high and low 

 water. 



Tide rips are made by a rapid current setting over an irregular 

 bottom, as at the edges of banks where the change of depth is con- 

 siderable. 



Current arrows on charts show only the most usual or the mean 

 direction of a tidal stream or current; it must not be assumed that 

 the direction of a stream will not vary from that indicated by the 

 arrow. The rate, also, of a stream constantly varies with circum- 

 stances, and the rate given on the chart is merely the mean of those 

 found during the survey, possibly from very few observations. 



FIXING POSITION. 



Sextant method. — The most accurate method available to the 

 navigator of fixing a position relative to the shore is by plotting 

 with a protractor sextant angles between well-defined objects on the 

 chart ; this method, based on the " three-point problem " of geometry, 

 should be in general use. 



For its successful employment it is necessary: First, that the ob- 

 jects be well chosen; and, second, that the observer be skillful and 

 rapid in his use of the sextant. The latter is only a matter of 

 practice. 



Near objects should be used either for bearings or angles for 

 position in preference to distant ones, although the latter may be 

 more prominent, as a small error in the bearing or angle or in laying 

 it on the chart has a greater effect in misplacing the position the 

 longer the line to be drawn. 



On the other hand, distant objects should be used for direction 

 because less affected by a small error or change of position. 



