Bill Dunlop and his bathtub-sized sailboat were two months overdue, lost somewhere off the coast of Australia in a half-million square miles of Pacific Ocean.

His wife Pam, who had flown from Maine to greet the 43-year-old solo sailor in the middle of a 27,000-mile trip, begged authorities to begin a search. But both the odds against his survival and the cost of a thorough search were enormous. Condolences, but little else, were offered.

Then the note was found, inside an empty margarine jar that had washed up on an Australian beach.

"Shipwrecked on island, no food, little water, time running out. 10-16-84."

Though it was not signed, Pam Dunlop was certain it had been written by her husband.

"I can tell you absolutely, positively, it's his," she said. Jeff Weinstein, a family friend who was with her when she saw the note said, "I've never seen a woman shake so much."

During the three weeks since the discovery of that note, its authenticity has been publicly debated by handwriting experts on two continents. Though the Air Search and Surveillance Center, Australia's equivalent of the U.S. Coast Guard, has refused to mount a search, Weinstein and Pam Dunlop have launched their own.

Bill Dunlop, a former truck driver from Mechanic Falls, Maine, is still missing.

"The search continues," his wife said yesterday from their home in Maine. "But I have nothing new to report."

There are hundreds of adventurers who leave safe ports each year to sail open seas in self-styled challenges. Blind sailors cross the Atlantic alone. Rowers pull their way across the Pacific. Celestial navigators, windsurfers and even jet skiers attempt feats that are judged courageous or crazy, depending on their success.

Too often failure means death.

When Dunlop left Portland, Maine, on July 31, 1983, for a planned 27,000-mile sail, he and his nine-foot boat Wind's Will were already certified seaworthy. In 1982, he had set a record for the smallest boat crossing of the Atlantic, west to east, when he sailed Wind's Will from Portland to Falmouth, England, in 78 days. He had also survived lesser solo sails, including a trip through the infamous Bermuda Triangle.

On the current trip, Dunlop had navigated down the Atlantic coast, through the Panama Canal, then across the Pacific to the Cook Islands. He had run aground on a coral reef near the Bahamas during a storm and was beaten and robbed by four muggers during a stopover in Jamaica. Dunlop, who weighed 240 pounds, had proved himself resilient.

The last time Pam Dunlop heard from her husband was June 23, when he left Aitutaki in the Cook Islands for the 3,000-mile sail to Brisbane, Australia. He expected to arrive in mid-August. By mid-September, his wife and friends were worried but not panicked. There had been no reports of particularly bad weather. And an ocean sailing timetable, particularly in a nine-foot boat, is never predictable.

By early October, however, she was pressing Australian authorities for help. Officials did issue an alert to boats and inhabited islands on Dunlop's scheduled route, but refused to do much more. Then the note was discovered.

The initial reaction, even by Pam Dunlop, was to brand the discovery a hoax. That impression was reinforced when an Australian handwriting expert reported there was no evidence to link the message to other samples of Dunlop's writing. Even more damaging to hope were tests that showed the margarine tub the message was sealed in had been manufactured in Australia, after Dunlop had left the Cook Islands.

But then Pam Dunlop, who had returned to Maine before the message was found, was shown a copy. She claimed "100 percent" certainty that the note, partly written and partly printed, was her husband's. Another handwriting expert from Massachusetts, Charles Gersin, was hired by Dunlop to study the note. Last week he said the note "resembles the genuine writing" of Dunlop.

"The more I look into this, the more I'm convinced," Gersin said. To explain the inconsistency of the margarine tub, Weinstein said the container could have been washed onto the same island as Dunlop.

"Just why the note was not signed is a mystery to all of us, though he could be delirious in a Robinson Crusoe-type situation," said Weinstein, who has undertaken the privately financed search along Australia's eastern coast.

If Dunlop was delirious when he wrote the note three weeks ago, there would seem to be little chance that he is still alive. But Pam Dunlop is not prepared to give up the search when there is any chance her husband might be on some deserted island, searching the sky for help.

"If there's a way to survive, he'll do it," she said, in a low, quiet voice. "He'll give it his best shot. That's what I'm counting on."