Report of the Board of Shell Fish Commissioners. 19 



running lines of soundings, are plotted by one of the engineers using 

 a three-arm protractor 1 , or position finder, for the purpose. 



The position of the launch at each of these intervals is deter- 

 mined by two angles, formed between the launch and three shore 

 signals, observed and measured simultaneously by the engineers 

 with setants 2 . 



By connecting consecutive positions of the launch as thus plotted 

 on the boat sheet, with straight lines, the course and position of the 

 lines of soundings on the oyster ground is shown. 



The three-point problem, which is involved in locating each of 

 the positions occupied during the survey, is illustrated by figure 7, 

 in which X represents the launch and A, B and C the signals on 

 shore. The angles AXB and BXC are those taken by the engineers 

 with sextants to locate the position of the boat at X. The right- 

 hand angle BXC (75 48') is transferred to the vernier between 

 the middle and right arms of the protractor, and the left-hand 

 angle AXB (58 16') is transferred to the vernier between the mid- 

 dle and left arms. When the protractor is thus set and laid on the 

 chart in such position that the left arm passes through the plotted 

 position of signal A, the middle arm through the plotted position of 

 signal B and the left arm through the plotted position of signal C 

 the point in which the three arms of the protractor meet, (the cen- 

 tre of the hub) is the point on the chart representing the position of 

 the boat when the sextant angles were taken. 



All angles taken during the survey are plotted on the boat sheets 

 by one of the engineers, while the other records the angles in an 

 angle record book. 



During the progress of the launch over an oyster ground the 

 leadsman, occupying a cage attached to the deck on the starboard 

 side, forward, throws his lead line at intervals of 20-30 seconds, 

 measuring the depth of water and testing the bottom at each cast. 

 Having made a sounding the leadsman reports the depth of water, 



1 A three-arm protractor (figure 1) is an instrument used for plotting obser- 

 vations made with sextants, of two angles to three known points for the location 

 of the point of the observer. The description of the theory of the sextant and 

 protractor and their use in hydrography requires the use of language too tech- 

 nical to be of general interest. 



2 A sextant (figured) is an instrument constructed for measuring the angle 

 between two objects (signals) from the position of the observer. 



