"Congress made using the metric system legal in '65," Tom Thwaites said, his eyes scanning the steep climb ahead of us. "1865," he added.

"It struck me that it must have been Lincoln who was behind it." He strode determinedly upward.

"So we have John Wilkes Booth to blame for the United States not being a metric nation."

"Aha," I said, in a voice I hoped was loud enough to carry over the crunch of our feet through the carpet of dry leaves. I couldn't see whether Thwaites was smiling.

Five years retired after 30 years of teaching physics at Penn State, Thwaites is maybe better known for his exploits in Vibram soles. He is author of Fifty hikes in Central Pennsylvania and Fifty hikes in Western Pennsylvania, and founder, moving force, guiding spirit, and protector of the Mid State Trail, the second longest walking path in all of what he prefers to call Penn's Woods. He is also an outspoken champion of the international (or metric) system of units.

I had been reminded of these facts the previous Saturday, while flipping through the pocket-sized eighth edition of the guide to the Mid State Trail, in search of a suitable afternoon hike.

The guide, although it does not bear Thwaites's name, bears his reflection: it is authoritative, chock full of lore, and possessed of an unmistakeable puckish streak. Of a place called Crocodile Spring, it notes, "the name . . . arises from the absence of sharks. Presumably they were eaten by the crocodiles." Emanon Gap, it elsewhere helpfully points out, is "No name" spelled backwards.

Not until the heading "Metrication," however, does the guide hunker down to reveal a philosophy. "The MST was the first hiking trail in the United States to use metric measure," it states. And: "Metrication is a patriotic measure designed to help end our cultural isolation and ease our chronic balance of payments problems."

Back in eighth grade I was all ready for the metric system. It was coming, sure as tomorrow; sure as the conversions they made us do for homework. That was in 1972. I called Thwaites to find out what was the hold-up.

Which is how I found myself halfway up the shady side of Houselander Mountain, breathing hard, on a chilly morning in late October. We were on the Mid State Trail, whose initials, as Thwaites likes to point out, also stand for Metric System Trail.

In person, he is compact, bespectacled; his ruddy face perpetually verging on a chuckle. His untamed gray eyebrows look like caterpillars poised to attack. And he moves constantly, on the sturdy legs that have carried him umpteen-hundred kilometers, not a few since open-heart surgery in 1991. Except when we stopped at overlooks, once for lunch, he kept moving, and talked to the air in front of him. He did look back whenever it sounded like I had fallen.

"I can remember as a kid being in school, getting angry," he said, tracing the origins of his metric leanings. "There was this whole other set of marks on the rulers -- and nobody told us what they were for." The consternation has never left him.

And don't give him the standard lines. If the "colonial" system ("Don't blame the English," Thwaites says, "they converted 20 years ago") is easier, then why do we decimalize fractions of seconds? Creeping metrication, Thwaites calls it. The inconsistency galls him.

And if hewing to the old is just a matter of familiarity, well then. . . . Thwaites stooped to hurl a downed branch from the narrow footway. "What's the next shortest distance unit after the mile?" he asked. I made a noise like answering, but luckily he went on.

"The furlong," he said. "Then the chain -- the surveyor's chain, not the engineer's. Then the rod. We don't even have standard notation for these units!"

An anthropologist once told him that the American resistance to metric is an example of cultural inertia, Thwaites said a little later, "but I think it's more than that."

For years he asked his physics students a standard first question on exams: Name the countries in the world, besides the United States, that don't use the international system. The answer kept getting smaller. (It's now down to just Myanmar, which I still haven't found on a map.)

"The fact is, many, if not most, educated people in other countries have never even heard of our units. I think our resistance is an example of American arrogance."

He's felt the weight of that resistance, presumably in kilos, even from fellow scientists. "Some in my own department were opposed," he said sadly, as we sat down to lunch. "They saw no need for change."

Thwaites remains undaunted, publishing pro-metric letters to the editor of the local newspaper at a rate of about one a year.

"Oh, it'll happen, sooner or later," he said, pulling a quarter-head of cabbage from a paper bag. "But I think we've lost the opportunity to do it gradually."

Going metric would be all to our advantage, Thwaites argues. Economically, for one. If our manufacturing sizes matched up with everybody else's, he figures, we could sell a lot more of our finished goods overseas, instead of just raw materials. ("By not going metric, we're laughing at the unemployed.") But in other ways too. "When Australia went metric," he said, "they saved 40 percent of the time taken to teach kids math."

Thwaites wanted the MST, started in 1969, to be a learning tool, a way for young people, especially, to get to know the metric system first-hand. He himself has walked the entire 275.8 kilometers more than a few times pushing a custom-made two-meter measuring wheel, an orange-framed contraption he keeps folded behind the driver's seat in his old blue station wagon.

Once, he related, he rolled the wheel over a rattlesnake. It buzzed a little on being awakened, no big deal. "They put up with being stepped on a lot better than with being picked up," Thwaites said.