AROUND THE WORLD IN A HUNDRED YEARS. In pictures and words, the National Geographic has taken armchair explorers on a century-long expedition from the Gal'apagos to the cosmos

EARLY in the life of the National Geographic magazine, photography was considered vulgar and was used gingerly. Elegant line engravings and illustrations adorned the magazine's pages. All that changed one day in 1904, when the first editor, Gilbert Hovey Grosvenor, suddenly had 11 pages to fill. He idly opened up a bulky envelope bearing exotic postage. Out spilled 50 first-ever photographs of the mysterious city of Lhasa in Tibet, taken by explorers for the Imperial Russian Geographical Society.

Filling pages of a serious magazine with a spread of nontechnical photos just wasn't done then; Grosvenor expected to get fired. Instead, people stopped him on the streets to thank him.

The rest, as they say, is history. The magazine that started out as a dry and technical journal has become over the course of a century a familiar fixture on coffee tables and in classrooms. The third-largest magazine in the country, and read in 167 out of 174 countries, National Geographic has been instrumental in shaping the way we view other cultures and creating a sense of awe at the beauty, order, and sometimes, cruelty of nature.

This year is its centennial. And the National Geographic Society is celebrating by looking both to the past and to the future.

The society has commissioned author C.D.B. Bryan to record the enormous changes it and the magazine have gone through in this last century. Mr. Bryan, a 1986 Guggenheim fellow and author of ``Friendly Fire,'' produced ``The National Geographic Society: 100 Years of Adventure and Discovery'' (Harry N. Abrams Inc., New York, 1987; 484 pp.; $45).

A massive book that will topple flimsy coffee tables, it's crammed with the magazine's now-famous, sumptuously colored photographs of explorers reaching summits, earthquakes and famines, quirky habits of animals, and the peoples of multitudinous cultures.

The magazine's illustrious history began with 33 explorers, biologists, military officers, and educators, who joined to form the National Geographic Society at a time when America was, as Bryan put it, having a ``love affair with science.'' Its second president was the inventor of the telephone, Alexander Graham Bell.

``There was a period of over 70 years in which the images people saw, the first impressions they had of foreign lands, came from pages of magazines like the National Geographic,'' Bryan says. ``It was the first to show the wildlife in Africa, the ancient towers and churches, photographs of Antarctica and the Arctic. It was the window on the world for the armchair explorer.''

The society drew a lot of armchair explorers; membership is now 10.5 million. One of the keys to its success, says Bryan, was Alexander Graham Bell's unique concept of membership: One didn't merely subscribe to a magazine; one was invited to become a member of the society, and received the magazine as its official journal. The circle of scientific elite invited their friends, and the circle widened until perplexed Midwest farmers were receiving the heavy, embossed invitations proclaiming, ``I have the honor of informing you that you are nominated for membership....''

What these farmers and others found over the last century was the universe brought to them on paper by a host of intrepid explorers. Comdr. Robert E. Peary, Charles Lindbergh, Amelia Earhart, Jacques Cousteau, Thor Heyerdahl, and Edmund Hillary are a few who charted the ocean's murky depths and oxygen-thin mountain summits.

Some were characters as well as trailblazers: There was the Viennese botanist Joseph F. Rock, who disappeared for years into the remote borderlands between Tibet and China. He bathed every night in an Abercrombie & Fitch folding bathtub and played Enrico Caruso records to mountain tribesmen.

One of Rock's tale's begins, ``I was quartered in the center of the village in a miserable old temple full of coffins. Darkness came on at midnight. The officers came in and announced that the brigands were outside and that the town could not be held against the impending attack. I never spent such a night in all my life.''

If the explorers were daring, the magazine often was not, as Bryan says. It was run by three successive generations of the Grosvenor family, and the ongoing theme was not to publish anything partisan. ``It lent a certain blandness to the reporting,'' says Bryan. The magazine got flak for not seeing the dangers of fascism early on, and for presenting a view of Harlem through rose-colored glasses.

But if Geographic was looking at the world through rose-colored glasses, says Bryan, so was America.

They're more courageous today, dipping into advocacy journalism in articles like ``Wild Cargo: the Business of Smuggling Animals,'' and ``Acid Rain - How Great A Menace?''

The current mission is to encourage readers and students to develop a critical sense of the role geography plays in defining their world.

``I feel that geography drives history,'' says president Gilbert M. Grosvenor. ``I suspect that had our leaders really understood the geohistory of Vietnam and Southeast Asia, I'm not sure we would have gone into that country, or at least the way we went in. We didn't understand the lowlanders, the real attraction of Ho Chi Minh, the relationship between the Viet Cong and the rest of Indochina, as it was then called.''

Appalled by reports of geographical illiteracy in the classroom, the society has launched a multifaceted Geography Education Program.

One project is a summer teachers' workshop started two years ago. Mr. Grosvenor says 30 percent of teachers teaching geography never took a geography course. Teachers from 22 states (so far) spend a month soaking up geography through lectures and workshops.

Other projects include a computer network that links up students around the country to work on science projects, and an interactive videodisc that permits teachers and students to program their own lessons.

While much of Earth's mass has been studied in the last century, the adventures aren't over, says Grosvenor. They've just taken on new forms: There are modern-day explorers retracing the Crusaders' path by horseback, the steps of Sinbad the Sailor, or St. Brendan's famous voyage across the North Atlantic in a leather boat. And the magazine's once all-male bastion has opened its ranks to women, who are now getting stories that men would not be able to get, like photographer Julie Cobb's story on life behind the veil of the women of Saudi Arabia.

``The same spirit that drove Livingstone and the great explorers in Africa the last 100 years or so,'' Grosvenor says, ``that same spirit will drive man to explore in the modern sense of the word, in the next century. ``We will discover ancient civilizations and learn more about our past, which I think we all admit is extremely important for looking ahead.''