It resembles a skeleton from the late Devonian Period, when fish climbed into bushes. Darwin had little use for its artful angularity, being rich and desk-bound; Henry Ford no doubt considered it capital-degenerative, not likely to become obsolete, anti-consumptive.

Yes, it's a bicycle, wonderfully out of synch with post- industrial society, too slow for cars and too fast for joggers -- a perambulatory time machine. To function, it requires only your flesh on its bones, and a delicate balance between reality and irresponsibility.

More to the point, with a bicycle you have aerobics and enjoy the scenery. Just straddle the thing, and pedal. The stage sets on either side of you clank into motion. The faster you pedal, the faster the canvas unwinds. Sketched figures in sweaty tank tops and quilted shoes, and their disapproving dogs, become anomalous blips in the open-air frame.

To avoid cars, use the C&O Canal towpath. One gear -- the highest -- will do for endless miles of (preferably dry) conveyor belt, and surroundings that suggest provincial France, or the boyhood turf of Tarkington and Twain. For those wanting the requisite 20 minutes of doubled pulse rate, I suggest "crashing," so to speak, on the stretch between the Mile One marker under Key Bridge, and the Beltway. This is the domain of the beserk kayak and the tortured tendon, and your speeding will seem downright sociable.

Then slow down to a constant six miles an hour. This puts beneficial pressure on your quadraceps, hamstrings and calves, without jamming your joints; it opens the lungs and induces a feeling of well- being. You are doing something that is good for you, and enjoying it. The greatest menace is the Boy Scout wielding his own time machine, to the clash of pot and chow kit.

Horses pose in lush Maryland meadows; dirt roads dwindle into the wings. The future is a ragged hole in a russet, wooded tunnel, with birdsong piped in. String small binoculars around your neck, and keep the Peterson handy in the handlebar bag. Black-capped chickadees and tufted titmice appear when the Scouts disappear; if you're lucky, you'll see a piliated woodpecker.

The towpath suffers from a reputation for tree roots -- knobby, subliminal reasons not to make the trip -- but they're easily negotiable if you don't have to hurry.

Unlike the Sony Walkman, the time machine does not have a fast-reverse. You have to rewind the scenery yourself (or have someone pick you up in one of the cars you sought to avoid). If you turn around at Seneca, you can still be back in Washington in time for supper. Or you can go on to Harper's Ferry and overnight at one of the campgrounds decorated with pump handles, big oaks and maples, and a distant view of Potomac rocks.

Many people do use these spots, most of whom haven't the faintest interest in aerobics. They pass you two and three abreast, their panniers brimming with camera lenses and bottles of Bordeaux. On a recent trip, one paused up ahead to light a cigarette, and I pedaled through his cloud of cannabis.

Fifty miles is enough for one day. My shoulders ached, but not unpleasantly; I had a tenderness in that spot from which our prehensile ancestors grew tails -- the worst of my ailments.

I unpacked the Sierrawest high-tech collapsable boudoir from the rack and set it up within a few feet of fat carp nuzzling in the Potomac mud. I soaked my freeze-dried, foil- packed, dehydrated version of shrimp Creole (the real triumph of the space program) in water boiled on the Coleman. I dined on Forest Service gothic, with a view of boozing bass fishermen out among the rocks.

The next day I would dawdle the 10 miles into Harper's Ferry, listening to freight cars shunting on the Chessie tracks, more intimations of the 19th century, and watching a red-tailed hawk work the riverbank. Then an understanding spouse would pick me up at the confluence of the Shenandoah, less than one motorized hour from Washington.

I took the bicycle to bed with me, chained to the tent's aluminum support. Bicycles have been known to escape while owners sleep. The people who ride them away are so interested in aerobics that you can't catch them.

And all two-wheeled aerobists know that without your time machine, you're just part of the scenery.