When I found him by telephone, John Davenport of Northwest said he was sitting on the edge of one of the Washington Hospital Center's beds, waiting for a nurse to change his cast. Again.

"It's fractured in two places, and perhaps a third," said John, in that rueful tone that only four days flat on your back can produce. "But I'm okay. Really. Honest."

No one says "really" or "honest," much less both, if he's really and honestly okay. John Davenport's leg will mend, but his bank account and his spirit may not, at least not soon. John "got involved" the other day -- and unfortunately, he's paying off as a result.

John Davenport is a 39-year-old ex-Marine who's spending a year on temporary assignment as a librarian at the Nuclear Regulatory Commission. On the side, he's trying to peddle a screenplay to the Forces of Disney. The sale was looking good on Oct. 25 -- so good that there was a little skip in the Davenportian step as John walked south on 17th Street NW, past the headquarters of the National Geographic, at about 8 p.m.

Just then, a panhandler approached and asked John for 50 cents. "For my buddy across the street," the beggar said.

John took a look. The buddy, dirty and dressed in tattered clothing, was lurching around near the entrance to the Mayflower Hotel and holding his midsection as if it hurt.

John was reaching for two quarters when the scenario changed abruptly. A woman's screams erupted from in front of the Mayflower, and here came the buddy, dashing across 17th Street, a purse in his hand. His stomach ailment had cleared up with amazing swiftness.

An alley runs east-west just south of the Geographic's headquarters, and the purse snatcher seemed to be headed there. So John roared down the sidewalk to cut him off at the pass, as it were.

The two men reached the mouth of the alley at the same moment. John threw a flying body block at the thief, which knocked the man down and sent the purse flying. Apparently frightened, the thief picked himself up and ran away, emptyhanded.

But John proceeded to prove Newton's Eighteenth Law of Urban Dynamics ("A flying tackler in motion tends to remain in motion"). His shoes were not the greatest ("They were penny loafers with soles as clear as a cue ball"). John lost his balance, did a cartwheel on the sidewalk and came crashing down on his leg.

"You know how athletes are always saying, 'I could hear it pop?' I always thought that was nonsense. It isn't nonsense," said John. His leg broke once at the shin and once more at the thigh.

You're probably thinking that a broken leg is no big deal in 1984. Rescue squads arrive in minutes. Hospitals provide the best of care. And Uncle Sam's health insurance covers the bill.

Right on the first point. Right on the second. But for a federal temporary, as John is, wrong on the third. Only fulltime fedsies have coverage.

So there John lay, at Washington Hospital Center, running up a king-sized bill, knowing he'd have to pay every cent of it and wondering if it was all worth it.

To his everlasting credit, John Davenport says it was.

"Listen," he said, "a young woman (Cathy Novelli, an attorney from Arlington) has her purse back, and there's a guy running around who'll think twice before he tries anything like this again.

"Getting involved? I'm firmly for it. I'm really not worried about the hospital bill. I'll pay it, somehow. I'm sort of one of the eternal optimists. If you don't do something about these things, who the hell's going to?"