10 GENERAL NAVIGATION. 



obtained. When the soundings are sparse or unevenly distributed 

 it may be taken for granted that the survey was not in great detail. 



Large or irregular blank spaces among soundings mean that no 

 soundings were obtained in these spots. ^Vhen the surrounding 

 soundings are deep it may fairly be assumed that in the blanks the 

 water is also deep ; but when they are shallow, or it can be seen from 

 the rest of the chart that reefs or banks are present, such blanks 

 should be regarded with suspicion. This is especially the case in 

 coral regions and off rocky coasts, and it should be remembered that 

 in waters where rocks abound it is always possible that a survey, 

 however complete and detailed, may have failed to find every small 

 patch or pinnacle rock. 



A wide berth should therefore be given to every rocky shore or 

 patch, and instead of considering a coast to be clear, the contrary 

 should be assumed. 



Fathom curves a caution. — Except in plans of harbors that 

 have been surveyed in detail, the 5-fathom curve on most charts may 

 be considered as a danger line, or caution against unnecessarily 

 approaching the shore or bank within that line on account of the 

 possible existence of undiscovered inequalities of the bottom, which 

 only an elaborate detailed survey could reveal. In general surveys 

 of coasts, or of little-frequented anchorages, the necessities of navi- 

 gation do not demand the great expenditure of time required for so 

 detailed a survey. It is not contemplated that ships will approach 

 the shores in such localities without taking special precautions. 



The 10-fathom curves on rock shores is another warning, espe- 

 cially for ships of heavy draft. 



A useful danger line will be obtained by tracing out with a colored 

 pencil or ink the line of depth next greater than the draft of the ship 

 using the chart. For vessels drawing less than 18 feet the edge of 

 the sanding serves as a well-marked danger line. 



Charts on which no fathom curves are marked must especially be 

 regarded with caution, as indicating that soundings were too scanty 

 and the bottom too uneven to enable the lines to be drawn with 

 accuracy. 



Isolated soundings, shoaler than surrounding depths, should 

 always be avoided, especially if ringed around, as it is doubtful 

 how closely the s^^ot may have been examined and whether the 

 least depth has been found. 



The chart on largest scale sliould always be used on account of 

 its greater detail and the greater accuracy with which positions may 

 be plotted on it. 



Caution in using* small-scale charts. — In approaching the 

 land or dangerous banks regard nuist always be had to the scale of 

 the chart used. A small error in laying down a position means only 



