Off South Carolina, the ocean suddenly changes color, from green to deep blue. You’re in the Gulf Stream now, in warm and salty water from the tropics, with swordfish, tuna and squid, in a current so strong that it lowers our sea level. Benjamin Franklin would learn about this current’s force. He was a Colonial postmaster before the American Revolution, and he’d noticed British mail ships were slow, much slower than other merchant ships. Why? He mentioned this to his cousin, Timothy Folger, a ship captain who’d hunted whales off New England. Ah, yes, that current off the East Coast, Folger told Franklin. Any fishermen worth their nets cut in and out to make better time — the whalers had even warned the mail ships to steer clear. But the Brits “were too wise to be counseled by American fishermen.” A map might help, and so they made a chart of this “Gulf Stream” from Florida toward Europe. It was one of the first maps to document its tremendous reach. Map or no map,