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This article originally appeared in Mount Vernon magazine, published three times a year by the Mount Vernon Ladies' Association.
Hidden away for more than 200 years, this French intelligence map helped American troops avoid British positions.
In the American War for Independence, 1781 is famous for the Franco-American victory at Yorktown, Virginia, but it could have been known for a disastrous defeat at New York City. A growing collection of documents within the Washington Library highlights what might have been and thankfully never was.
This previously unknown French intelligence map, acquired at auction by the Washington Library in 2021, helps tell this important story. Washington had not seen Manhattan since the fall of 1776, when the British ignominiously ran the Continental Army off the island. By 1781, the American forces had little idea of just how fortified the British positions along the Bronx River and Long Island Sound had become.
French naval officers—including one named Charles René Dominique Sochet, Chevalier Destouches—scouted the approaches to Long Island Sound from naval vessels. American spies, led by Benjamin Tallmadge, worked to locate crossing sites on Manhattan and Long Island. The objective of the massive reconnaissance was to get the British to unveil their artillery fortifications. Staff officers then collated all of this data into a 30-page battle plan.
This chart from the papers of Admiral Destouches highlights some of the reconnaissance efforts made. The French text over western Long Island details the treacherous currents around “Hell’s Gate” near what is now Rikers Island (marked “C”). The tidal waters of the Long Island Sound and East Rivers flowed into an unpredictable whirlpool of tides that both the Royal Navy and French Navy agreed should be avoided at all costs.
Before its acquisition by Mount Vernon, this map had been hidden away for more than 200 years in the private archives of the Destouches family. The document joins the Washington Library’s growing collection of historic maps, most of them part of the Richard H. Brown Revolutionary War Map Collection, which came to the Library in 2019.