CHAP. VIII.] SOUNDING 207 



by meridian distance with the farthest beacon at the other 

 extreme of the survey. "The other beacons are weighed and 

 placed in new positions as the survey progresses. 



Astronomical observations by twilight stars should be 

 obtained at beacons at intervals of about 20 miles, and the 

 intermediate triangulation, with the soundings dependent upon 

 it, is squared in between those positions, due consideration 

 being given, however, to the orientation derived from the chain 

 of compass bearings, which should not lightly be thrown over. 



Sectional lines off coral reefs are sometimes now required Reef 

 to show the exact slopes for scientific purposes, or for cables. ®° ^°^^" 



It is not an easy operation, and cannot be hurried. 



The soundings must be close, to show the exact slope when 

 it is, as in many cases, steep. 



The section must be run on a transit line, and there are 

 many ways of fixing the distance. 



A boat anchored on the edge of the reef for the outer transit 

 mark, with a long bamboo or other light spar stepped, will, 

 by means of vertical angles, afford a means of ascertaining 

 the distance up to perhaps half a mile, but beyond it will be 

 necessary to have another boat or mark on the reef at a fixed 

 distance from the transit-line, to which horizontal angles can 

 be taken, making, in fact, an exaggerated ten-foot pole. Other 

 methods will suggest themselves to the surveyor. 



The diagram should be drawn on a true scale — i.e., the 

 vertical and horizontal scales equal — an inch to 30 fathoms, 

 and the slope to the left, so as to facilitate comparisons with other 

 diagrams. 



In all sounding the lead-lines should be measured on return Measur- 

 on board, and a note made in the book, " Lead-line correct," j^elid- 

 or so much out. When the line has not been used for some Lines, 

 time, it should be measured before leaving in the morning 

 also ; but if it has been examined the evening before, this will 

 not be necessary. 



While on this subject, it may be noted that new lead-line 

 should never be used for boats' soundings. At the beginning 

 of the commission it may be necessary to do so, but afterwards 

 make lead-lines out of old well-stretched stuff that has been 

 used for deep lines for ships' sounding, and measure and mark 

 them when wet. 



