British Prime Minister Margaret Thatcher came here to calm concerns about China's claims to this town of fast-shifting fortunes, and she was about as reassuring as a weatherman in the middle of a storm. She reported the inclemency, promised umbrellas, but failed to forecast sunny skies.

So, just as Thatcher, in a televised address today, was assuring this British colony of her "moral responsibility" for its well-being, a wealthy wholesale jeweler suddenly decided on a stronger hedge against the future: he dispatched an assistant to the Australian mission for an application to emigrate.

"There was nothing there at all," said the jeweler's assistant of Thatcher's remarks. "The businessmen running this place have a lot at stake. They must take care of their fortunes."

As a result, uncertainty burst into fear, which hits this gold-plated city where it hurts the hardest -- the financial solar plexus.

Before her 2 p.m. speech, the stock market already had tumbled 67 points, reflecting the unresolved nature of her visit last week to Peking. An hour later, it slipped another 17 points, closing about 8 percent lower for the day, which is one of the worst falls in recent years.

Hong Kong land values did even worse, dropping 10 percent for the day, while local currency came within 1 cent of suffering its lowest exchange rate ever.

"If you are not worried, you are either a liar or a fool," said Ronald Li, chairman of the Far East Stock Exchange here. "Nobody in his right mind will start a major project in Hong Kong until the dust settles."

The worry that broke into near financial panic today comes not so much from the nightmare of Chinese Communist takeover, but from the sober realization that the dream of unfettered capitalist control by Great Britain is coming to an end.

Last week, Chinese Premier Zhao Ziyang made public something Communist officials have been whispering for months -- China eventually will reassume sovereignty over the territory it lost to the British in the humiliating imperial wars of the 19th century.

No one has described how Peking will regain control, but it is generally believed nothing will change until London's 99-year lease on most of the region expires in 1997.

Thatcher has refused thus far to recognize Chinese claims of dominion and has sought to calm fears by committing her government to the "stability and prosperity" of the world's third largest banking center. She strengthened the commitment last week by getting Chinese leaders to endorse it and agree to open negotiations over Hong Kong's future.

She tried to apply the same balm on the first full business day after her China journey, reasserting in the speech and later news conference her "moral duty" to the colony's 5.2 million residents, many of whom had fled from the Communist mainland.

"We do have a moral responsibility," she told the news conference, "and, being British, we take it very, very seriously."

She played down the sudden financial reversals, saying, "Hong Kong is a sensitive place and in a sensitive place, you would expect the market to fluctuate. I do not think I would conclude too much on what happens in one day."

For Hong Kong residents, however, falling stock market prices are a sure danger sign. Even though change may be 15 years away, they view it as a flight of the investor confidence that turned a sleepy colonial outpost into a major international commercial center bristling with skyscrapers and Rolls-Royces.

Few other citizens of the world can see their future in front of them by studying the closed-circuit televisions in most bank windows quoting stock market prices as shares are bought and sold during the day.

"No good news," said Edward Wu, 50, a businessman, peering at closing prices through a downtown window. "The rich want to get their money out. People are afraid, that's all."

Albert Chan, 34, a government employe, gathered with dozens of others in a surrealistic setting to watch the Thatcher speech. Grouped before the television display rack at an electronics store, they stared at 19 sets of all sizes broadcasting her remarks at a business luncheon.

The only two televisions not tuned to Thatcher were listing stock market prices, falling as she talked.

"Everyone worries things will change," Chan said. "The system of government in China is so different than the one here."

Was he reassured by the prime minister's words, Chan was asked after the speech.

"We have no alternative but to rely on the British," he replied. "If I was a rich man, I could go to the United States or Canada. But, no matter what happens, I can only stay here."

Hong Kong's moneyed class has been sending capital abroad for years -- Li of the Far East Stock Exchange calls it "buying insurance" -- and many are planning to follow their investments, settling in other Commonwealth countries.

A local newspaper recently reported that travel agencies are fraudulently offering to relocate Hong Kong people in South American nations for as much as $12,000 for a family of four.

"The passion of Hong Kong," said Li, "is to be left alone and free from the plague of politics and political turmoil. We hope for peace, good laws and order so we can go to work and earn our pay in an honest way."