When the fog rolls in and the sea begins to spit, Harry O. Jones looks out his lighthouse window, 90 feet above the Atlantic, and worries about being crushed.

"Ships aim right for our radio beam," says Jones, one of two Coast Guard officers assigned to the Chesapeake Light Tower, a magnificently ugly, water bug of lighthouse set down where the Atlantic Ocean meets the Chesapeake Bay. "You're looking down where the Atlantic Ocean meets the Chesapeake Bay. "You're talking about a 90,000-ton vessel filled with cargo. If somebody falls asleep, it's our a--."

Called simply "Ches Light," the steel tower that is both warning light and elctronic beacon for shipping has never been scraped by a hull. That doesn't keep Jones and fellow Petty Officer Andrew M. Barnes, standing on a grey mesh catwalk, from following the lumbering course of supertankers massive enough to dwarf the 11-story light.

But Jones and Barnes won't have to keep wary eyes on the Atlantic much longer. The Ches Light, one of 20 Coast Guard-tended lighthouses that protect ships and the bay from each other, will soon by fully automated. Sometime later this month, Jones and Barnes will lock up and leave the tower to the care of computers and, in the best maritime tradition, a ghost.

The ghost is a mildly mischievous sort who opens and closes doors, changes oven settings and provides the spooky night noises essential to any self-respecting lighthouse. It is said to be the spirit of a man who was killed building the tower 16 years ago. Jones is worried that when the lighthouse is automated, the ghost will be left alone in the Atlantic with no one to haunt.

"You can't scare computers," says Jones, 29, who has served on the Ches Light since February.

When the Ches Light is fully automated, there will be only three manned lighthouses left on the bay (Cape Henry, Cove Point and Thomas Point off Annapolis). lThose are expected to lose their crews within a few years. Coast Guard officials say that every person taken off a light saves $18,000 in salaries and benefits.

What is lost, concede some officials, is a tradition with roots in both maritime heritage and popular myth. Those "intrepid tenders of the light," as one early American writer described them, "who live lonely lives above the pounding surf, safeguarding the great waterborne commerce of our Nation" will be reassigned.

"It is an end of an era . . . but I think most of (the lighthouse crews) are pretty happy about it," says Capt. Don Super of the 5th District Coast Guard headquarters in Portsmouth where the Ches signals will be monitored.

The Coast Guard crews who tend America's lighthouses resemble the craggy keepers of folklore very little. Since 1939, when the Coast Guard was granted dominion over this country's lighthouses, it began replacing the sometimes eccentric civilian keepers, known as "wickies," with seamen who didn't always find the solitude so pleasing.

"I don't know of anybody who volunteers for it," says James Klinefelter, a boatswain's mate on the Point Brown, the 82-foot Coast Guard cutter that serves as a lifeline to the Ches Light crew, delivering mail and supplies every two weeks.

Before automation, the Ches Light had a crew of six who served 28 straight days on the light followed by 14 days off. The routine was very routine.There were gauges to monitor, logs to keep and floors to mop. When the fog rolled in, the horn was switched on. And always there was boredom.

"I will not miss this duty at all," said Jones while conducting a tour of the tower last week. Jones and Barnes, a 19-year-old who signed up 14 months ago, have very little to do now that the computers have been plugged in. They have been left of their watery aerie as a second string.

While they wait, Barnes stays tuned to his soap operas. Jones picks up television stations in three states and both throw darts at their Ayatollah Khomeini dart board.

Their living quarters, 85 feet above the waves and reached by a rope sling from the cutter below, look very much like a college dormatory with a million-dollar view. The largest room contains a color television set, pool table, couch and a bookcase dominated by Reader's Digest condensed classics. Past a kitchen with microwave oven and down a military-gray hall, are four small rooms for sleeping and a larger one for computers.

The rooftop helicopter port is up one flight of steel stairs. The light tower, which shines a continuous beacon that can be seen for 24 miles, rises three more stories. On a clear day, you can see Virginia Beach from the tower. On a rainy one, you can barely see the sea.

"It's kind of hairy in real bad weather," said Jones, standing on the edge of the helicopter port, imitating a frightened man trying to wave away a 90,000 ton ship. "They've come as close as 500 or 600 yards before turning away."

Both Jones and Barnes say lighthouse duty has its advantages. The fishing is very good. The discipline is easy. And in the summer, the sea around the tower teems with bikinis.

But neither man feels that his leaving the light represents any loss to America's maritime tradition. Like many Coast Guard crews, Jones and Barnes regard lighthouse duty as something to endure. They know little lighthouse lore, and can sing no sea chanteys. What they are proud of is the self-discipline they summoned to cope with the isolation.

There are a few in the Coast Guard, however, who regard automation as an enemy of a tradition that is both romantic and heroic.

"I consider it a loss," said Mike Waldrop, a Coast Guard mechanic on the Point Brown, who spent his honeymoon on a lighthouse in Washington State's Puget Sound.

"I think the happiest times I ever spent in the Coast Guard were on that lighthouse," said Waldrop as the Point Brown returned to port, leaving behind the ghost and a tradition that soon will be, too.