196 HYDROGRAPHICAL SURVEYING [chap. viii. 



The soundings which will be joined together on the finished 

 chart by fathom lines — e.g., the three, five, ten fathoms, etc. — 

 should always be fixed, and in doing this it must be remembered 

 that it is the outer sounding of any of the same depth that will 

 be on the fathom line, and also the tide reduction must be taken 

 into consideration. This latter will, of course, be in many 

 cases only approximately known, so that exactly the right 

 sounding may not be fixed. 

 Direction The sounding-lines should be in ordinar}^ cases at right angles 

 of Lines, ^q h^q coast, and parallel to one another, as not only will a 

 better line be got for tracing the fathom lines, but the boat 

 will easier be kept in her right direction by observing two 

 objects which have been seen to be in transit, in the right 

 direction, at the commencement of the line. 



A narrow ridge should be examined by short parallel lines 



of soundings run on transits, if possible, and crossing the ridge 



at close intervals, in a direction at right angles to its length. 



Marks in In nice w ork on large scales it is generally necessary to place 



7^^D^^* two marks in line for this purpose ; but, for ordinary surveying, 



ing Lines, changing them from one line to the other will take far too 



much time for the purpose, and marks to answer all practical 



purposes may usually be found placed by Nature already. 



Wliile running out from the shore on a line of soundings it 

 is desirable to take off from the sounding-board tlie^ fixing 

 angles for the end of the line, in order to know when it has 

 been reached, without waiting to plot. 



When fixing near the end of a line, measure by station 

 pointer the angle between a point fixed on the sounding-board 

 and the point at which the next line of soundings cuts the coast- 

 line, remembering the fact that small angles between objects 

 at about the same distance are least affected by the movement 

 of the boat. Placing this angle on the sextant, endeavour to 

 pick up some object on the beach where the next fine cuts it, 

 to serve as the front transit mark for running in upon. The 

 back transit mark, if sufficiently distant, will remain nearly 

 unchanged, and should be brought in fine with the new front 

 mark. Fixing again at the end of the new line, shoot up the 

 proposed transit line, and amend it as necessary by taking the 

 required angle off by station pointer and setting it on the 

 sextant as before. 



