The second map in an occasional series depicts a turning point in transatlantic navigation.

(Library of Congress)

Thanks to the jet stream, westbound flights across the Atlantic take longer than eastbound ones. In the centuries before air travel, sailors dealt with a related time-sucking natural phenomenon, until a famous American intervened with “A Chart of the Gulf Stream.”

Who made this map?

Benjamin Franklin and his cousin, Timothy Folger, are credited with naming and mapping the Gulf Stream for the first time—the warm, strong ocean current that pushes northeast from the Gulf of Mexico, up the Atlantic coast, towards Europe. Though there were many editions, the map pictured above was printed by the American Philosophical Society in 1786, and now belongs to the Library of Congress.

What problem did it solve?

In 1768, Franklin was in London, working as deputy postmaster general for the American colonies. A visit by Folger, who captained a merchant ship, prompted Franklin to inquire about something peeving him. Why did it take British mail packet ships so much longer to reach America than it took regular merchant vessels?

It struck Folger that the British mail captains must not know about the Gulf Stream, with which he had become well-acquainted in his earlier years as a Nantucket whaler. Franklin later quoted his cousin’s explanation like this:

We are well acquainted with that stream, says he, because in our pursuit of whales, which keep near the sides of it, but are not to be met with in it, we run down along the sides, and frequently cross it to change our side: and in crossing it have sometimes met and spoke with those packets, who were in the middle of it, and stemming it. We have informed them that stemming a current, that was against them to the value of three miles an hour; and advised them to cross it and get out of it; but they were too wise to be counselled by simple American fishermen.

In other words, westbound British packet ships were losing precious time by sailing into and against the warm, strong current. Folger sketched out the rough location for Franklin, who soon made prints, along with his cousin’s directions for how to avoid what he dubbed the “Gulph Stream.”

Who used it?

Franklin passed out copies to those hapless British packet mariners, but again, they didn’t think much of the American’s sailing pointers, and apparently ignored them.

With the start of the American Revolution a few years later, Franklin’s allegiances shifted. He stopped distributing the Gulf Stream map to the British, and instead gave copies to the French, who used it to ship weapons and supplies to their American allies. After that, knowledge of the stream became “hugely important for transatlantic travel,” says Alex Clausen, a maps specialist at Swann Auction Galleries, where a copy of the map recently sold for about $8,000.

Is it accurate?

(NOAA)

Compare Franklin and Folger’s 18th-century chart to modern computer-generated models of the Gulf Stream, and they match up remarkably well.

While Franklin himself made observations of the stream on ocean voyages“I find that it is always warmer than the sea on each side of it, and that it does not sparkle in the night”—the accuracy of the chart is really due to Folger and his inherited whaling knowledge. Also, Spanish mariners had known about the Gulf Stream since the 1500s.

But Franklin was the one with the good instincts to map it, and that, combined with his general eminence, has landed him with most of the credit. More than two centuries after this chart was first published, Grumman Aerospace Corporation launched a landmark undersea expedition off the coast of Florida to study the depths of the Gulf Stream. The submersible’s name? What else: the Ben Franklin.

Check out the first installment of This Old Map here.

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