New Fathometer, 1930s Poster

Description from LOC: "New fathometer used at full speed. Washington, D.C., May 7. Dr. Herbert Grove Dorsey of the U.S. Coast and Geodetic Survey has invented an instrument to secure the precise measurement. The machine sends a sound, pitched so high that it is above audibility, travelling thru the water a known speed, the sound hits the bottom and returns as an echo at 20 times per second. The fathometer is so accurate that a change in depth of three inches can be detected. the indications are made by a flash from a neon tube, viewed thru a rotating disc, the flash of light appearing to stand still and the depth is read as easily as a clock. The instrument indicator is installed in the pilot house (at the top of the machine) and the transceiver (the disc at the right of the machine [?]) is located in the hull of the ship, 5/7/1937."