The basic principle is to drag a wire attached to two vessels.
If the wire encounters an obstruction it will come taut and form a "V"
Image ID: theb2418, NOAA's Historic Coast & Geodetic Survey (C&GS) Collection
Photo Date: 1920?
• High Resolution Photo Available
of the U.S. Department of Commerce, National Oceanic & Atmospheric
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June 10, 2016