WHEN thick North Atlantic fog envelops the ship, it is time to huddle on a deck chair, snuggle under a thick steamer rug, and breathe the salty air. Perhaps the sun will burn through in a few hours.
Or perhaps it's time to visit the PaineWebber Art Gallery where those who have never made an Atlantic crossing can envision what it was like.
The gallery, in midtown Manhattan, opens a porthole to the era of the transatlantic ocean liner with ``Ships of State,'' the largest collection of ocean-liner memorabilia ever shown.
The show, which opened this month and runs through Dec. 9, includes artifacts from the heyday of ocean liners in the early 20th century, including shipboard menus, captain's logs, and postcards written by passengers.
Several remnants from famous sinkings are included, such as a coat worn by a survivor of the Titanic, and the passenger list from the final voyage of the Lusitania.
John Maxtone-Graham, chairman of the exhibition and an ocean-liner expert, is quick to point out the differences between transatlantic liners and present-day cruise ships. The word ``liner,'' he explains, implies a destination.
``Cruise ships just float around usually sun-drenched islands,'' says Mr. Maxtone-Graham. ``These ships traveled in all weather, and they were extremely durable. The ocean liner was not a pleasure ship, it was a ship to get somewhere.''
Indeed, prior to 1914 the great majority of transatlantic passengers were immigrants. Far from luxurious, their ``steerage'' class quarters were dirty and cramped, and their meals were sparse. Still, most immigrants endured the week's voyage uncomplainingly as the price of passage to a glittering new world.
``Can you imagine,'' asks historian Walter Lord, ``how the Statue of Liberty must have looked to an immigrant just coming to America?''
Like threads binding two continents, ocean liners traversed the Atlantic for more than a century, carrying millions of passengers. Although travelers sailed from nearly every port in Europe, they almost invariably landed in New York City.
Americans sailing to Europe arrived at Manhattan piers with crowds of other excited passengers, most of them accompanied by an entourage of friends and relatives who would come on board to wish them bon voyage. In keeping with the mood of festive excitement, the pier was sometimes covered with flowers. Ships that sailed at night were often adorned with strings of lights.
On board, the ships reflected the mood and characteristics of their country of ownership. The British lines served strong tea; the German lines had military bands and beer steins. As soon as one boarded the ocean liner, one entered the country that owned it.
What passengers did on their week-long journey was largely determined by the price they paid for their ticket.
Passengers were strictly divided according to class; each had its own dining room, purser's desk, library, even deck space. Although poorer passengers were forbidden to enter higher-class quarters, it was not uncommon for the first-class passengers to go ``slumming'' in the poorer ones, especially in the early days when the contrast between classes was sharpest.
IN-BOARD entertainment was a far cry from the activity-packed daily agendas of modern cruise ships. There were games like shuffleboard and Ping-Pong, and nearly every passenger walked around the deck for exercise. By far the most popular activity, however, was lounging on deck, reading or sleeping. The ocean liner was, in effect, a floating hotel.
Life aboard ship was not always enjoyable, however. As speed was of utmost importance for transatlantic liners, captains would try to adhere to their schedules even in the worst Atlantic storms.
But at week's end, passengers usually arrived at their destinations happy and relaxed. A week at sea in such close quarters produced many acquaintances, some friendships, and even a few romances.
The first transatlantic steam service was introduced in 1840 by Samuel Cunard, an Englishman, to transport mail and passengers across the Atlantic. In the early 1900s, demand for ocean liners increased dramatically with the huge surge of immigrants coming from Europe to the United States.
By 1920, the US government decided to cut off the immigrant flow, and transatlantic lines were forced to reverse direction, concentrating their energies on transporting American tourists to Europe. Until the mid-1970s, when airlines took over trans- atlantic travel, ocean liners arrived regularly in New York's harbor, profoundly shaping the city.
Although the ocean liner's primary purpose was transportation, Mr. Lord thinks that these ships were alluring for other reasons as well. ``There was a feeling of nostalgia and enormous romance in traveling by ocean liner,'' he says, ``yet there was a purpose in your voyage; it was more than just a vacation.''
Maxtone-Graham, who started crossing the Atlantic by ship when he was six months old, thinks people are missing out on something in the age of air travel. ``There is a wonderful sense of peace and pleasure at sea,'' he says. ``It is a kind of indolence that is rare in our lives, to be sure.''