Report of the Board of Shell Fish Commissioners. a 
Bureau of Fisheries in conducting surveys of oyster grounds 
in other States, and the Coast and Geodetic Survey in making 
a hydrographic survey. 
The approximate position of an oyster ground having been 
pointed out by the local assistant, the Chief Engineer, with 
the survey party on the launch ‘‘CANVASBACK,”’’ runs a 
zigzag or parallel series of sounding lines over it (see 
figure 6), the object of which is to ascertain the exact limits 
of the ground, the depth of water over it and the condition 
of the bottom. 
-_A copy of a polyconic projection, known as a boat sheet, 
showing the shore line of the region being surveyed, and the 
plotted positions of the signals erected on shore, is thumb- 
tacked to a chart board in front of the hydrographic engineers. 
By means of a three-arm protractor,! or position finder, the 
positions of the launch at the end of two- or three-minute 
intervals, while running the sounding lines, are plotted on 
this hoat sheet by the chief engineer, the locations of the 
launch being ascertained by means of sets of angles taken 
simultaneously by the engineers observing on three snore 
signals with sextants.* By connecting consecutive positions, 
as they are plotted by straight lines the course of the lines 
of soundings is shown on the boat sheet. 
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 their position 
at X. The right-hand angle BXC (75° 48’) is transferred to 
the vernier between the middle arm and right arms of the 
protractor and the left-hand angle AXB (58° 16’) is trans- 
ferred to the vernier between the middle and left arms. When 
the protractor is thus set and laid on the chart in such 
1A three-arm protractor (figure 5) is an instrument used for plotting observa- 
tions 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 pro- 
tractor and their use in hydrography requires the use of language too technical 
to be of general interest. 
*A sextant (figure 4) is an instrument constructed for measuring the angle 
between two objects (signals) from the position of the observer. 
