Two traditionally held beliefs were tested yesterday and found true:

The sun never sets on the British Empire.

And, only the British can give a proper garden party.

To these now may be added a new one: If you ask Defense Secretary Caspar Weinberger to your garden party, it won't rain.

"Secretary Weinberger guaranteed us until 7 p.m.," British Ambassador Sir Oliver Wright said yesterday, looking without fear at the clouds that only began to gather in time to speed the parting guests.

"The Air Force said we'd be safe until then, so we didn't worry," agreed Weinberger himself, who believed his military enough to bring out his vintage 20-year-old Leica camera for the event. "I'm not much of a photographer, but the camera is a good one," he said.

The secretary and his wife Jane were on the committee (in charge of keeping it from raining?) for the annual Folger Library Garden Party, a benefit for the theater and library. And 320 guests in furbelows and flounces, straw hats on both sexes, reticules and even striped trousers filled the empire-sized British Embassy garden. The Folger (courtesy of Estee Lauder) provided pink, yellow and blue parasols for the ladies -- since in these benighted times people can't be counted on to have the proper accouterments for garden parties.

Of course, Lady Marjory Wright was correctly attired in a long, white Laura Ashley Victorian dress. Wright, perhaps the greatest theater star that ever trod the boards at the British Embassy ambassador's residence, was chairman of the event.

Most of the entertainment was in admiring other people's costumes. Rose Fales had on a magnificent ostrich feather hat. "From Worth 1901, my dear, of course," she said. "It's Leila Wilson's mother's she's a Whitney hat. You freshen it by hanging it over the tub to steam, but this time it fell in."

Caroline Simmons, a Washington cave dweller, said the party seemed like old times. "We always had, if not garden parties, at least parties in the garden. Polly Logan once said that everyone who had at least a square foot of ground always had garden parties." Simmons said she went to the Junior League Thrift Shop in search of a proper Edwardian dress. "And then decided to wear this one -- it's at least 20 years old."

The women weren't the only ones dolled up for the occasion. Baltimore attorney Elroy Snouffer -- wearing an Ascot dress-gray afternoon suit, tall hat, what may even have been spats and a black cane with white tips -- said, "I bought my outfit from Moss Brothers in London when we were there in 1957. We were attending a garden party at Buckingham Palace and were told to wear 'Ascot' dress."

Added his wife Dorothy, "I was just going to shake Prince Philip's hand when two women pushed me out of the way. But he smiled at me."

Selwa Roosevelt, the U.S. protocol chief, relaxed in a chair at one of the white tables that encircled the green lawn. "I've really put in a week," she said. "We've been working so hard because of the Indians coming. You know the Gandhis are so young, and have so much energy, they want to do everything." Indian Prime Minister Rajiv Gandhi and his wife, Sonia, begin a state visit here Tuesday.

The evening ended with a spirited rendition of Gilbert and Sullivan from the Washington Savoyards company. The Rev. Christian Mendenhall, soon to leave Trinity College, wrote a new version of "When Britain Really Ruled the Waves" that began, "When Citicorp ruled in Reagan times, Folger went to ground," and ended with the Folger Theater being saved and "Shakespeare can be heard encore."

By that time, after Pimm's Cups, champagne, English trifle, salmon in baskets, no one cared if it rained.