"Washington, George. Autograph July 1782 to Lt. Col. Tench Tilghman . . . A most unusual and fine letter in which Washington jokes about George III . . ." Expected price: $10,000-$15,000.

ON TUESDAY a sensational sale of autographs, books and manuscripts will launch Sotheby Parke Bernet's unique new enterprise, the 84th Street galleries.

For the first time in the long history of auction houses, all books, stamps and coins will be handled in the same building, a great change from the days when Sotheby's had its coins on one street, its books at the main Madison Avenue gallery and its stamps in Danbury, Conn.

The new gallery, a converted garage, actually opens today with a mock auction for children, complete with bidding paddles and prizes for the best value estimates. But the Joseph Roebling Collection auction Tuesday will attract Americana manuscript specialists from all over the world.

A few samples: 11 George Washington autographs, ranging from a survey map he made at 18 to a letter he wrote as president; autographs and other documents by the likes of John Adams, Benedict Arnold, Daniel Boone, Edmund Burke, Benjamin Franklin, George III, Alexander Hamilton (15 items), Jefferson, Jackson, Lincoln and Captain Kidd.

The tidy notion of combining these three compact and popular objects appeals to Jerry E. Patterson, the senior vice president who heads the new division. "There's a lot of overlapping in these areas," he commented during a recent tour of the nearly completed renovation. "Books and stamps are both concerned with engraving and printing. You often find a great stamp collector, like Arthur Hind [who once owned the world's most expensive stamp], is also a print collector of note. We have a coin expert who happens to be also an expert in illuminated manuscripts."

Sotheby's New York has been auctioning stamps for only four years, he added. They had a coin department, but it had lapsed for lack of an expert appraiser. This is a familiar problem in the world of antiquities. Sotheby's in London had a stamp department from 1874 until 1917, but then suddenly abandoned stamps altogether. Most likely, Patterson speculated, their stamp expert was killed in World War I.

"We don't do jewelry here because, for one thing, that calls for really heavy security, with $30 million in sales every year. Also, it's a different crowd." The people who buy jewelry also buy paintings and sculpture, it seems, so all those things are lumped together in the main building. Still, books, coins and stamps bring in some $20 million, which is about 10 percent of Sotheby's total turnover.

With 30 million stamp collectors in this country alone, plus a vast international clientele, stamp sales are big. The last one took four days and ran through 3,860 lots at a rate of 120 an hour. Thirty seconds to auction each lot. Books rate more frequent sales, but this is a quieter world, since few books bring more than $5,000, while stamps can go as high as the $850,000 paid last year for the famed British Guiana penny black.

"This new place is a dream come true for a lot of our specialists," said Patterson. There are clever touches: viewing rooms for stamps and coins, broad tables where maps can be spread, a wide counter for looking at books, shelves for thousands of rare books and the reference volumes so vital to the expert, who needs a full bibliography of every conceivable author. Bookshelves are far enough apart so a typewriter on a dolly can be run between them. Huge latches set apart the high-security rooms. The stamp showroom has plenty of natural light, necessary for the subtle grading of old coins by color.

Patterson was especially proud of some bookshelves on wheels. Rare books, which shouldn't be handled much anyway, can be auctioned right on the shelves and then wheeled into the shipping room. The eliminates one handling stage, a vital economy in the auction business, where items may be moved by hand 10 times or more from arrival to disposal.

He is partial to books himself, having headed Sotheby's book department.

"The reason stamps go so fast is that they are all keyed to the Scott catalogue; everyone knows in advance what they look like and pretty much what they'll bring. With books -- who can say what a Lincoln letter will bring? There's something emotional about autographs."

Some famous people wrote wonderful letters, he said. "George Bernard Shaw could write a note to the gas company and it would be a treasure." Others could be dull and routine. Eleanor Roosevelt and Robert E. Lee, colorful enough individuals, wrote terrible letters, he said. Today, the market for letters by famous scientists -- Einstein or Freud, for instance -- is very hot.

And the stories: They are endless. In 1926 T. E. Lawrence checked the manuscript of his masterpiece, "The Seven Pillars of Wisdom," into the Victoria Station checkroom in London. He came back the same day. The package was gone.

"That thing may well be in some attic somewhere, just ticking away, waiting to make someone a fortune," Patterson observed. He also noted that the manuscript of "Moby Dick" is long lost. He himself has had some exciting afternoons in the book trade. Once he appraised the library of a deceased opera singer. He found himself surrounded by endless rows of junk: paperback mysteries, popular novels, a family Bible.

"My heart sank," he said. "I figured I was in for a dreary afternoon. Then I spotted one old volume. It was a Kelmscott Press Chaucer, given to the singer by a fan. It brought $100,000, and that was years ago."

Another time, two truck drivers from Queens brought in a ragged book in a paper bag stained with egg salad. They had found it in a warehouse. It turned out to be a first edition of John Lyly's 16th-century romance "Euphues," the only copy to come on the market in modern times, the only copy Patterson had ever seen. It went for $8,500, and now would be worth about $60,000.

The 84th Street building is only the latest in a series of moves by the auction house. Last fall Sotheby's opened its block-long York Avenue galleries to handle decorative arts. It was there that the landmark auction of large IBM machines was held last winter, technically a failure (only one of the 14 items was sold, the rest not getting up to the seller's reserve, or buy-back, price) but in the long run opening a vast new field for auctions.

Another Sotheby's innovation is the electronic currency computer, which flashes the amount of each bid in dollars, pounds, francs, yen and whatever else you want, the very second the bid is made.

"Someday computers will handle the whole process," says President John L. Marion. "They'll manage collections, catalogue stuff, estimate values. We're using video already, for showing big items and cutting down on the handling. We even conduct auctions by satellite."

This technique is especially useful in the real estate business, a booming new venture that has brought in $80 million worth of property sales in North America and the Caribbean in four years. Charging a hefty 10 percent commission, Sotheby's real estate sales staff recently sold a 35-room Hollywood mansion with 10 acres to singer Kenny Rogers for $14.5 million. They had been asking $15 million, but what the heck.

"America has finally accepted the auction market as the best way to exchange items with no price tag on them," remarked Edward Lee Cave, head of the real estate division. "Today, you can move in and out of the art market even faster than you can move in real estate."

What it is all about is the preservation of wealth, not simply making money, auction people like to say. That may account for the tony atmosphere in the big auctions, and even as such "lawn sales" as the celebrated Garbisch auction at Pokety on Maryland's Eastern Shore last May, where close to $4 million in furniture and decorations were sold, from a $250,000 Chippendale desk to a $700 Dodge pickup.

Sotheby's began with a London bookseller, Samuel Baker, who had his first auction in 1744, a time when auction houses were common -- almost 60 of them existed in London alone. Baker thrived, selling the libraries of the great, including Napoleon's St. Helena collection. After Baker died in 1778, his nephew, John Sotheby, became a partner. In 1788 Sotheby's entered the coin world with a sale of duplicates from the British Museum.

Today Sotheby's has branches in 50 cities around the world, including 13 in North America. The continent accounted for $250 million in auction sales last year. The New York staff alone is 750, not counting real estate people. Sotheby's has been in New York since 1954, and later expanded by buying out Parke Bernet, then America's largest fine-arts auctioneers, who themselves dated to 1883.

The auction world is a world of hushed voices, sleekly pressed suits, manicured hands. It is a world of wild surmise. A man brought a six-dollar dish to one of Sotheby's Heirloom Discovery Days, held around the country in conjunction with local museum fund-raisers. The appraiser said it was Ming, worth $50,000. At auction it brought $76,000. Someone else found an old bronze statuette in a hatbox in Pittsburgh; today it resides in the Metropolitan Museum, a rare Italian Renaissance work. And there was the Chevy Chase family who wanted to do something about a carton of china that everybody had been stumbling over for years in their garage. It was T'ang. Sotheby's sold half of it for $500,000.

These days the rush is on to find lost treasure in the attic. The buyers are waiting eagerly -- this being a period when nobody trust the traditional ways of beating inflation. Just about everything, from Victorian furniture to pocket watches, is booming in value. There are exceptions, of course. Lalique glass, all the rage a few years ago, now is in oversupply, as are Tiffany lamps. Silver was down last summer, has been coming up again. Portraits do not sell well.

But look at photographs. In 1967 Patterson took a chance and auctioned some old photos that had been thrown in with the Will Weissberg book collection. They brought $17,865. Eyes popped. Last year the Sotheby's photo auction department cleared $2.3 million.

And old furniture. You're looking for an investment to keep your bucks alive for your children through 20 years of double-digit inflation? John Marion puts it this way:

Pick something with a limited supply. Say, 18th-century American furniture. There couldn't have been more than a half-dozen first-rate cabinet-makers in New England. In 1961 a fine early American bureau would have cost you about $16,000. That's a lot. But today you could sell it for, oh, say $300,000.

It beats Treasury bonds.Maybe it's even better than gold. You just have to be right.