"Primitive societies noticed that the shadow of an upright post (or gnomon, from the Greek 'to know'), became shorter as the sun rose in the heavens, and lengthened again as the sun set."

-- From "The Discoverers" by Daniel J. Boorstin

Humans have measured their days in many ways.

Shadows, grains of sand, drops of water, springs, pendulums, dropping weights, descending stamps, burning candles, lighted wicks in oil and bells -- all mark the passage of life from here to there.

Daniel Boorstin, the librarian of Congress, tells of many marvelous clocks: The sundial that shot off a cannon, "installed by the Duke of Orleans in the garden of the Palais Royal in 1786, is said to have fired the shot that started the French Revolution." In 1335, a Milan campanile's clock divided the day into 24 by striking a bell.

Time is inexorable and thereby the most precious commodity. So it's understandable that over the eons, artists have ornamented clocks with jewels, embossed and inlaid precious metals, guarded them with figures of angels, eagles, doves, cupids and George Washington, and entwined them with vines and leaves.

And now Wendell Castle, perhaps this generation's best wood sculptor, has made 13 clocks that in their way tell of time past, present and future. Castle is artist in residence at the Rochester (N.Y.) Institute of Technology, where his only duty is to "make art." His up-to-the-minute exhibition, "Masterpieces of Time," ticks away at the Renwick Gallery through May 4.

Castle has used exotic woods, beautifully fashioned gold-plated brass and bronze, and exquisite woodworking -- time after time.

The "Octagonal" clock, Art Deco reinterpreted in Indian ebony "to express the transcendental nature of time," as the gallery notes say, carries a sterling message inset, "Time and tide wait for no man." Castle said (by phone), that before he made this clock, he'd never really thought of carving away time. His most recent works had been such trompe l'oeil marvels as leather coats on racks and tablecloths sliding off tables -- all sculpted from wood.

So when he made the "Octagonal" clock, at first it was just another piece of furniture. But time marched on, carrying Castle with it. He made 12 more, at first exhibited at Alexander E. Milliken Inc. in New York. (He shows at Fendrick Gallery in Washington.) "I think of time as magic," he said. "And 13 is a magical number."

All the baker's dozen, from 7- to 8-feet high, take some shape from the human body. The "Jester" clock, with a round facelike face and stubby armlike arms, a domino body and long legs, is the most obvious.

The "Sun God" clock is an obelisk with a setting sun for a middle and pyramidal legs, decorated with gilded bronze Horuses, or hawks. When you open the sun panel, inside is a desk with a dial with sterling drawer handles for eyes, a stem for a nose and a half-moon mouth.

"Magician's Birthday" has a pointy hat, like Merlin's, encircled by rings, and stands on legs with rings and numbers encircling the feet. "Rings have no beginning or end," explained Castle. "And it's all made of modules of 12."

"Arch," described by its name, opens to show shelves on one side. Gold-plated balls rotate around the glass clock face. At the bottom are tiers provided with gold balls to encode messages. (You provide your own code.)

"Trophy," made of ebony, tulipwood and gold-plated brass, "marks the ultimate prize: the gift of time," Castle said. Carved birds, top and bottom, on the "Time Flies" Australian lacewood clock threaten to fly it away at any moment. The "Dr. Caligari" clock, like the movie, is so abstract, Castle says, that nothing about it is "square, predictable, parallel, straight or even."

The "Desk" clock, literally that with 20 drawers, looks to Castle like a bow-legged cowboy. It's inset with mother-of-pearl and abalone shell.

And finally, for those who do not choose to know the time, Castle has made a timeless clock, "Ghost." A trompe l'oeil shroud shows bumps suggesting a grandfather clock that has, as the song says, "stopp'd short/ Never to run again/ When the old man died."