Two British daredevils hovered yesterday within miles of reaching a goal that has eluded men since the first attempt was made in the middle of the 19th century - crossing the Atlantic by balloon.

Plagued by an eight-foot rip in the yellow helium balloon's inner seam and by extreme physical and mental exhaustion, Donald Cameron, 39, and Christopher Davey, 34, drifted to within 140 miles of Brittany on France's northwest coast, when the winds began to shift and push them north.

"We're on tenterhooks. The mission is hanging on knife edge," Davey radioed to the control center at Bracknell, England.

But the control center pleaded with the balloonists to keep going on their grueling odyssey that began Wednesday morning when they lifted off from St. John's Newfoundland.

"If they can get the right winds again, they could make Cornwall" in southwest England, "but we really don't know for sure where or when they'll land," said a spokesman at Bracknell. "They are disappointed at having to spend another night in the cramped conditions aboard the gondola."

Davey, a major in the British army, was reported to be airsick, and Cameron was stricken with the flu before the balloon lifted off. Both men were also sick with exhaustion, having slept less than three hours a night since Wednesday.

Crossing the Atlantic by balloon has been the dream of adventurers since the middle of the 19th century. At least six people have died since the first attempt was made in 1859.

The most recent attempt was made in October 1977 by Americans Dewey Reinhard and Steve Stephenson, who lasted 46 hours before being fished out of the ocean by the Canadian coast guard.

Other recent attempts include those of Ben Abruzzo and Maxie Anderson, who made it as far as the coast of Iceland in September 1977, and Karl Thomas, who was rescued by a Soviet trawler after two days aloft in June 1976.

Edward Yost of Tea, S.D., came closest of all, but was forced to ditch his craft 530 miles off the Portuguese coast in October 1976.

In February, 1974, Thomas L. [WORD ILLEGIBLE] 48, of Alexandria, took off from Horrisburg, Pa. in a gondola 6 feet diameter, suspended from 10 balloons each measuring 26 feet in diameter in an attempt to cross the Atlanta. He vanished and was presumed lost at sea.

Cameron and Davey's voyage has been far from uneventful. Midway through the flight, an eight-foot [WORD ILLEGIBLE] opened in the balloon's inner skin and helium began to leak. This forced the balloon to wallow about in bad weather for two days at lower altitudes because at higher levels there was a risk that the tear would worsen.

Early Saturday, however, when the tear showed no further signs or growing, the balloonists ascended to 13,000 feet to pick up the more favorable winds.

As they approached the Brittany coast last night, the weather was reported cold and blustery but not dangerous and the main problem because the balloonists flagging spirits.

As they considered abandoning their ship, their colleague at Bracknell center told them. "It seems you have more a morale problem than a meterological problem."

Davey's 73-year-old mother Edia has also been encouraging the pair.

"Stick out tonight and all my love and you'll make it," she said. "Good luck to you. Come in, I'm waiting at Bracknell."

Davey's reply could hardly be heard over the balloon's radio" . . . probably some time tonight or more likely early tomorrow . . ." he said.

It was still a question as to whether the balloon would land at night.

"It will be up to Donald to decide whether they will go for a landing in the dark," said the Bracknell center press spokesman, John Mansell.

"But making a night landing in balloon is very difficult, you can't see things like power lines and trees," he said.

"Then again, if it is a nice clear night with a bright moon, maybe they'll go for it. A lot of the decision will depend on their morale."

The monitors at the mission control said that the men's spirits had improved immeasurably since their decision to carry on.

"Everything is all right now," they quoted Cameron as saying. "I just wish the Atlantic were a bit smaller."