Parker replies: The two American machines are in the Washington, DC, area. The Harris machine (see photograph), used to make the tide predictions for the North African and Pacific campaigns, is on the National Oceanic and Atmospheric Administration’s campus in Silver Spring, Maryland. That machine is still operational, at least in that the pulleys and gears move; for an accurate tide prediction it would probably need recalibration. The Ferrel tide-predicting machine is in the Smithsonian’s National Museum of American History in downtown DC.

The Harris tide-predicting machine, constructed around 1910 and retired from service in 1965. (Courtesy of the national Oceanic and Atmospheric Administration.)

The Harris tide-predicting machine, constructed around 1910 and retired from service in 1965. (Courtesy of the national Oceanic and Atmospheric Administration.)

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The two British machines used by Arthur Thomas Doodson at the Liverpool Tidal Institute are in the Merseyside Maritime Museum in Liverpool.