The 15th earl of Westmoreland -- who also is known in England as Her Royal Highness' Master of the Horse -- came to Washington not long ago to open the new local office of Sotheby Parke Bernet on M Street and to attend a round of parties. He is the new chairman of Sotheby Parke Bernet Group Ltd., the international auction firm.

The first question Americans ask is: What does the Master of the Horse do, for heaven's sake? "I don't have any great responsibility," he said. "I am in charge of all the carriages and equipage in the Royal Mews. But actually there's a very good crown and quarry [that's a person's title] who does all the work. I suppose if he was kicked to death by a horse, her majesty would ask me to recommend a replacement, but that's about all I have to do.

"Oh, yes, I do ride on all occasions of the Trooping of the Color -- when the queen opens Parliament. I ride with her when there are occasions in which horses are involved in meeting foreign dignitaries; and when a royal child is married. No, I have no idea when Prince Charles might give me such an occasion to ride. But I wouldn't be surprised if Prince Andrew might be first. He's a great man for the ladies. Don't say I said that."

To be Master of the Horse is to be one of the three most important royal officers in England, Lord Westmoreland explained, answering a question. "The other two are the Lord Chamberlain, who takes care of the palaces and the houses, and the Lord Steward, who is traditionally in charge of royal tours. Originally, the Master of the Horse was in charge of raising calvary for the crown. I'm glad I don't have to do that."

Lord Westmoreland is a tall, distinguished looking white-haired man of 56. He carries himself like a man accustomed to ceremonial occasions. But he's something of a wit, and doubtless is good at relieving the tedium of royal events. He talks easily to everybody and makes a habit of passing on compliments. You could imagine the queen finds him agreeable to have around.

Lord Westmoreland earlier served the queen in another august position. He was one of two Lords-in-Waiting appointed by the queen. Five others are appointed by the cabinet to serve different departments. The term "in-waiting" was appropriate -- he waited at the plane and train station to officialy greet heads of state. As the queen's personal Lord-in-Waiting, he was sort of a super ambassador, her representatives to weddings, funerals and other state occasions. When heads of state visited Great Britain, Lord Westmoreland accompanied them on all their official rounds, and waved goodbye at the station.

"I suppose I have met almost everyone important from 1955 to 1978," he said. "The king of the Belgiums, the shah of Iran twice, the emperor of Japan, the head of Portugal.

"A great many of my collegues wondered how I could do all that and persue a business career, too. But it wasn't that difficult. State visits were predictable -- though they're less so now with the Arabs jetting about -- usually arranged way in advance. And they seldom took longer than the traditional three-day visit."

Former President Nixon recently impressed Lord Westmoreland by speaking without notes for an hour and a half, surveying the international situation, at a luncheon in London to promote his book. "I was tremendously impressed with his grasp of events. But I do suppose there was something rather superfical about it, as though his views lacked depth," he said.

Lord Westmoreland himself is very concerned about the world situation. "It's very dangerous. I'm afraid we are on a slope, it could be very easy to slide into war. This is all aimed at breaking up the Western alliance and isolating the United States. We must not let that happen."

As for the next decade, Lord Westmoreland says, he's still an optimist. "If we don't have another war -- in which case nothing is sure -- then I think we're in for a great growth period for Sotheby's. Depression or not, we are seeing an increasing number of people who think auctions are the best way to buy and sell. You could call it the socialization of works of art. Instead of the big estates, where a great deal of art is owned by one person, we are seeing many people owning fewer pieces. The major collections are disappearing."

Lord Westmoreland thinks the new Heritage Fund of $32 million just voted by the British parliament will help preserve British treasures. Decorative arts, historic houses and great gardens can receive help from the fund. New tax laws, as well, are being formulated to keep British collections together.

Such laws are too late for the Westmorelands. His family's stately home and its treasures were all sold in 1902 when the family hit a rough patch financially.

"Now and then when some family pieces come up for sale, other members of the family and I try to buy them," he said. Lord Westmoreland collects English sporting pictures himself (he's fond of hunting and was to present the Stratford-on-Avon Cup at Warrenton here) and English contemporary paintings. He has works by, among others, Graham Sutherland. His wife, a Finley from the family that owned The Scotsman newspaper, collects Early Victorian.

Now he and his wife have what he calls a "small and convenient house" on Chester Row off Eaton Square and another small country house in Gloucestershire, near Bath. Of the queen's castles, which he knows well, he prefers the smaller, more homey Balmoral in Scotland: "The most beautiful, charming place."

Lord Westmoreland suceeded Peter Wilson as head of Sotheby's. Wilson, who has headed the firm's immense growth in the past 21 years, was famous as an art scholar and historian. Lord Westmoreland came in as administration director in 1965 and initiated Sotheby's expansion program in the United Kingdom. His forte is the business side. "No one could compete with Peter Wilson as an art expert," he said. "It was time to look not at art but at the company. It's a time of readjustment. I think I can be more dispassionate than an art expert."

In New York, he inspected another new project: the 160,000-square-foot building Sotheby's is remodeling to house their decorative arts auctions. The new property, with a major French restaurant and connoisseur lectures, will open this fall on York Avenue. The old Madison Avenue Galleries will house paintings, prints, photography, books, stamps and coin auctions. The less expensive auctions, formerly held at Sotheby's uptown gallery known as PB 84, will now be divided between the Madison Avenue and York Avenue buildings.