At dawn, when no one was looking, he would take in the capital from the back of a horse. He liked the solitude of the Washington Monument, the curious shadows in the Hirshhorn sculpture garden, the pink glow of the Commerce Department building, the silhouette of the Capitol.

These rides were among the secrets of William P. Clark, the longtime intimate of President Reagan who left Washington quietly last week after four years as deputy secretary of state, national security adviser and, until last Thursday, interior secretary.

He was an enigma to many, a private man who hugged the shadows, revealing nothing of himself or his influence.

But to the U.S. Park Police officers who rode with Clark at 6:30 every morning for the past 15 months, he was simply a fifth-generation California rancher who came into his own in the saddle of the president's royal white Lippizaner stallion named Amadeus, cantering confidently through Rock Creek Park. You can learn much about a man from the way he handles his horse, they said.

"If you can control 1,100 to 1,200 pounds of a horse who has a mind of his own, and get him to work for you, almost to perform for you like a ballet artist, well, that says a lot about a person," said Park Police Sgt. Maj. Denis Ayres, Clark's most trusted riding companion. "He has tremendous patience, tremendous strategy. He surprised us, being such an exceptional rider."

Clark on horseback was a contrast with Clark as Cabinet secretary. The public Clark spoke tersely, measuring the impact of each word, displaying no emotion. But on Amadeus, he was expansive, even lyrical. Asked which part of his ride he liked best, he responded not with a place, but with a feeling: "I love the solitude, the loneliness. It is the most ideal of circumstances. No phones, no interruptions."

Last Thursday, Clark set out with Ayres from the National Park Service stables under the Calvert Street bridge in Rock Creek Park. It was his last day as interior secretary. "It is not easy, but the time has come," he said. "I've had 18 years of government service. I've loved it."

With Clark atop Amadeus and Ayres on Chili, his favorite thoroughbred, the two moved down jogging paths in the brisk morning air, coaxing their horses past patches of ice, up leaf-carpeted hills and west into Montrose Park in Georgetown. There, the high-spirited Amadeus broke into a canter on the green, and Clark rode him as if rider and horse were one.

Then, down 30th Street, where a young woman dressed in a stylish knit jacket was walking her shaggy Afghan. The dog began to howl. "Simon, hush!" the woman scolded. She looked up apologetically at Clark: "He's never seen a horse before."

Each time Amadeus passed a Georgetown boutique, he gazed at himself in the window and let out what Clark called "his wild whinny" -- a spectacle few Washingtonians have witnessed, since Clark usually was well beyond Georgetown by the time the city awoke.

"He sees a good-looking horse," Clark said fondly as Amadeus whinnied at his image in the window of Haviland Cleaners & Dyers on P Street. "He has a big ego, maybe the biggest in Washington. He's royalty, from the famous Spanish Riding School in Vienna. You can tell he knows it by his demeanor. When they feed him, he noses some hay out of his manger into a pile and he lies down to eat. My wife, when she heard about that, asked: 'Does he read a paper and watch TV, too?' "

The relationship began late in 1983, when Clark left the White House to become interior secretary. Rock Creek was one of more than 300 parks under his stewardship. The president's stallion needed exercising. And Clark missed his California regime of horseback riding, dating back to his childhood when he rode his horse to the school bus. Why not ride Amadeus himself?

The foliage reminded him of California in places, and the rides gave him a unique vantage point for viewing what he called "the most beautiful city in the world." The Royal Lippizaner, loaned to Reagan by the chamber of commerce of Austria in a gesture of friendship, became his horse of choice and Ayres his favored companion. ("He's as fine a rider as I've known, except for my father," Clark said.) Except when he was out of town or had the flu, Clark didn't miss a ride each day for 15 months.

Setting out by 6:30 each morning, he rode Ayres' beat with him, through Rock Creek, Montrose and Glover Archbold parks, past the C & O Canal and beyond. Last Thursday, the slim, 53-year-old Clark wore full horseman's dress -- buckskin jodhpurs made by a tailor from the Spanish Riding School, tall black leather boots that rose three inches above his knee, silver spurs, suede jacket, a riding crop and a beige felt Stetson. But many days, he wore a mounted policeman's uniform presented him by the Park Police. On his daily rounds, he waved to joggers -- who sometimes recognized him -- gave directions to motorists and picked up trash. "He really worked a beat. He covered it well. He would have been a very good policeman," Ayres said. "He was always anticipating what was ahead."

Combining his old diplomatic beat with his Interior calling, Clark took many an ambassador on the early morning rides. Former French envoy Bernard Vernier-Palliez, a veteran of France's World War II horse cavalry, was the best rider of all, Clark said. On Vernier-Palliez's last day in office, Clark rode Amadeus to the door of the ambassador's Kalorama residence, leading a second stallion by the reins, and the two galloped off on a farewell ride in the early morning mist. The embassy staff cheered from the driveway.

Another frequent companion was U.S. arms control negotiator Paul Nitze, a second cousin of Clark's wife Joan. Just before last month's breakthrough U.S.-Soviet meeting in Geneva, Clark and Nitze rode up and down the Mall for 45 minutes in the shadow of the Washington Monument and the Capitol, talking out arms control issues. Was this a remake of Nitze's famous "walk in the woods" with Soviet negotiator Yuli Kvitsinsky during the 1982 Geneva nuclear arms talks?

"Actually, I called it our walk on the Mall," Clark said.

Other riding companions were Austrian Ambassador Thomas Klestil, U.S. Ambassador to Austria Helene Von Damm, a delegation of Mexican diplomats, Defense Secretary Caspar Weinberger, Commerce Secretary Malcolm Baldrige and Agriculture Secretary John Block.

Clark's tenure at Interior was perhaps his happiest in Washington. His State Department stint began badly when he was caught at his confirmation hearing unable to name the prime minister of Zimbabwe, Robert Mugabe. He was lampooned for weeks -- Newsweek magazine wrote of him under the headline "A Truly Open Mind" -- but Clark appeared undaunted, secure in his role as Reagan's confidant and adviser.

At the White House, it was no secret that Clark, as national security adviser, bitterly disliked the elbowing among the staff and was relieved to move west to Interior, where his domain spanned the nation's half-billion acres of parks and public lands. Amadeus, and horses in general, came to symbolize the change. In his expansive office, Clark hung a 1967 black-and-white photograph of himself, then-governor Reagan and his father, William P. Clark Sr. -- all in a row on horseback. Joe Bullock, Clark's driver at the White House and Interior, said he could sense that the move to Interior and the rides on Amadeus made his boss "happier, more relaxed. It's the kind of thing you can't put your finger on, but a lot of things, you just feel."

Clark said he believes that men can learn much from horses like this one. "There really is something to this thing called horse sense," Clark said. "He anticipates things -- problems, dangers. The lesson is always to stay one step ahead . . . This horse, to use an Al Haig phrase, is 'in charge.' "

He said he talks to Amadeus constantly and that Amadeus "talks" back. But, when asked what he has told the horse, Clark resumed a tight-lipped persona.

"I'll never tell."

Does Amadeus know national security secrets?

"He's full of them."

Clark says he can tell when Amadeus is upset or hasn't slept well, "and he can tell the same of me. He can tell when I'm leaving town and he pouts. He won't take his sugar." Clark brings his horse large sugar cubes flown in by diplomatic pouch from Austria.

The Lippizaner was named Maestoso Blanca (Majestic White Lady) when he arrived in Washington, but the name proved unwieldy at the Park Police stables, and Joan Clark, an impassioned devotee along with her husband of the composer Mozart, suggested the name Amadeus. Clark said he called the Austrian Embassy "to be sure no one in Vienna would be offended," and the horse was renamed. Reagan, who has ridden Amadeus several times, calls him "our good white friend," Clark said.

Reagan has named Clark as Amadeus' trustee and has asked his homeward-bound friend to take the stallion to California and to work him out daily on Clark's 880-acre ranch near Reagan's Rancho del Cielo.

Clark at times turned nostalgic on his last ride. Gazing at the row houses of Georgetown, he said: "They're so beautiful, so different from back home." And as for Amadeus, Clark repeatedly singled out signs of his intelligence: "Notice his ears. They always are pointed forward. And his eyes. He's looking around all the time. He's sensitive, intelligent. He sees every leaf, every stone in this park."

Their oneness became apparent when they reached a large patch of ice and Amadeus stiffened, raising his head and looking as wary as a child stepping into a dark corridor. Clark coaxed Amadeus forward to test his confidence. "Come on. Come on," he said. It became obvious that the horse was too uneasy, and Clark stopped pushing, guiding him instead through a series of sidesteps up the ice patch and onto dry earth.

As the ride ended, Clark spoke of a "hollow feeling" at leaving Washington, but professed no sadness at leaving a seat of power. "It has been a tremendous privilege. It's not the power that made it that way, it's a matter of duty. Ronald Reagan wanted people around him who did not necessarily look upon government as a way of life. I've kept that in mind through the 18 years," he said.

Clark said he will return often to Washington on assignments for Reagan. He said he is helping to arrange for Reagan's papers to be archived at Stanford University and will be Reagan's representative at upcoming negotiations over a 500-year-old land dispute between the Navajo and Hopi Indians.

The ride finished, he dismounted and led Amadeus into his stall. The horse was coaxed into accepting some sugar cubes. Clark lovingly stroked Amadeus' head for several minutes, speaking under his breath in gentle tones. Then he patted the horse once more and walked outside.

"I just told him I'd see him a little later," Clark said. "He said he'd believe it when he saw it."