A thick fog was rolling in off the Atlantic when I turned right at the Bi-Lo and, half a mile later, pulled up in front of what my GPS told me was 714 Jungle Road. Jerry McNab was living in Edisto Beach, I’d been informed, and from late afternoon through last call he could usually be found at a bar called Ayel’s at this address.

I climbed out of my Jeep, zipped my windbreaker against the chill, and looked around. Beach houses on stilts, mostly, each of them with a yellow and green Edisto Realty “For Rent” sign strapped to its balcony. It was off-season, the week between Christmas and New Year’s, and the coastal island seemed sound asleep, the houses dark, no vehicles in their carports.

714 Jungle Road was a lonely strip mall, only half a dozen storefronts wide. There was an art gallery, a nail salon, a shuttered ice-cream parlor—and, yes, a bar with one sad string of twinkle lights blinking around its entrance and plate-glass window, but it was called the Invariable Length, not Ayel’s.

And then the penny dropped: Invariable Length, I-L, ergo Ayel’s.

I went up the eight weathered pine steps and in the door.

The bar was practically empty. Not a customer in sight, just a tall guy with a bushy red beard and a KEEP CALM AND DRINK LOCAL BEER t-shirt and a red-and-white striped apron tied around his waist behind the bar, drying a pint glass with a dishtowel that had seen better days.

I walked across the wooden floor and settled in on a stool, directly in front of him. “What’s your pleasure?” he said, his voice raspy but friendly, not looking up from the glass he was polishing.

“Something local?” I suggested, nodding at his shirt.

He grinned a lopsided grin. “That’d be Low Tide,” he said.

“That’s Charleston beer,” I objected. “Not exactly local.”

“It’s as local as we got,” he said. “There ain’t a brewery anywhere closer to Edisto Island than Low Tide, ’cepting Fat Pig, and their beers are a mite heavy for folks around here.”

“I’ll drink to that,” I said. “You got Shady Beach on tap?”

He dipped his chin and drew it into the glass he’d just finished shining, flipped a coaster onto the bar and set the beer before me in one smooth silent motion.

“The Invariable Length,” I said, just making conversation. “I expect there’s a story behind the name.”

He nodded at a framed newspaper article hanging on the paneled wall to my left. “Read all about it,” he said, and fished another glass out of the bar sink and went to work with his towel.

“I left my glasses out in the car,” I said. “You mind telling me?”

He sighed, set down the glass and towel, and finally made eye contact.

“In the 1840s,” he said, “the U.S. Coast Survey took on the job of creating an accurate map of the entire Eastern Seaboard. The problem was the measuring tools they had back then would expand and contract with changes in the air temperature, which made outdoor measurement unreliable.”

I took a sip of my beer. It’s a brown ale, robust but not too heavy, and it was fine, but nothing new. I have lived my whole life in Chucktown, and I generally keep a growler of Shady Beach in my fridge.

“Alexander Dallas Bache, who was the great grandson of Benjamin Franklin and Jefferson Davis’s roommate at West Point,” the bartender went on, sounding like a tour guide who’d told the same story so many times he’d lost any interest he might once have had in it, “was appointed the Survey’s superintendent in 1843, and he figgered out a way to solve the problem. He stuck an iron rod and a brass rod inside a tin tube and connected them at one end with some doohickey called a compensating lever, and that caused the different expansion rates of the iron and brass rods to cancel each other out and allow for accurate measurement, no matter what the temperature did.”

“Smart thinking,” I said, “but—”

He sniffed as if he could read my mind and cut me off. “He called his invention the Bar of Invariable Length, and it did exactly what it was supposed to do, allowing the Survey to map the coastline with amazing accuracy. They started right over there”—he jerked his bearded chin back the way I had come—“in what’s now the state park, with the Edisto baseline, just a mite under seven miles from end to end.”

“So you called this place the Bar of Invariable Length,” I said.

“Not me,” he said, rubbing the back of his rough hand across his lips and beard. “I just work here.”