When John Muir, who fought ardently for the establishment in 1890 of Yosemite Valley and the surrounding Sierra Nevada as a national park, called this the ''gentle wilderness,'' he was speaking from personal experience. After all, he was a hardy Scotsman who had herded sheep in Yosemite's mountains and subalpine meadows for an entire summer, having been trained for such arduous activity by his youth on the Wisconsin frontier and his famous walk from Kentucky to Florida in 1867.

But to the citified and comparatively sedentary inhabitant of the 20th century, a visit to Yosemite in the height of the summer season can seem anything but gentle. As one of the National Park Service's most popular attractions, Yosemite accommodates more than 21/2 million visitors each year, of which 80 percent arrive in July, August, and September. A like proportion crowd themselves into the seven-mile-long central Yosemite Valley area, to the exclusion of Wawona, Tuolumne Meadows, White Wolf, Crane Flat, and the rest of the park, which is roughly the size of Rhode Island. The result can be traffic tie-ups, parking problems, and restaurant lines that would unhinge a native New Yorker.

Clearly, this is not what Muir had in mind when he urged us to ''climb the mountains to get their good tidings.'' By sticking close to civilization, Yosemite visitors run the risk of being inundated by fellow human beings and missing the wilderness experience altogether. One obvious strategy for visitors is to get into the back country, where solitude is as abundant as nature herself. Backpacking is not a solution for everyone, however, especially those who prefer comfortable accommodations or recognize their own ignorance of nature as a limitation.

For the reality remains: It is hard for most of us to commune with nature because we have lost the habit. We have neither common knowledge nor common sense about wilderness, because we are so far removed from sufficient experience of it by urban and suburban life. We do not understand that wild animals are wild. Even if most of us wanted to climb the mountains, we simply wouldn't know how or what to do once we got there.

Fortunately, the National Park Service understands that street smarts are not transferrable to the natural environment, and that there are as many ways of enjoying the park as there are visitors.

''Our role is to accept all park visitors and to help them experience Yosemite at their own level,'' according to Eileen Berrey, supervisor of the Park Information Office at the Valley Visitor Center, first stop on any tour of Yosemite.

''There is no right way to enjoy the park,'' Ms. Berrey stresses, because we each have different tastes and values. People often have an image of a park visit that includes a 20-mile hike but precludes taking the time to relax, absorb, and really drink in the surrounding beauty. Instead, Ms. Berrey recommends that visitors inquire at the Valley Visitor Center, then select a manageable number of activities and sights to cover within a flexible schedule.

''Coming to Yosemite is not like visiting Disneyland,'' she observes. ''Disneyland represents the triumph of artifice over nature. At Yosemite, the very opposite principle prevails: We are preserving natural processes rather than things. We let Half Dome, Bridal Veil Falls, and the other natural wonders stand as they are without labeling them or putting up signs. This sometimes distresses people,'' she acknowledges. ''They are afraid that they are missing something. But people understand when you explain it to them.''

Explaining the environment to people under the guidance of experienced naturalists and historians is the goal of dozens of interpretive programs for all age and ability groups offered free or at nominal cost by the National Park Service, the Yosemite Natural History Association, and the Sierra Club. Seminars and demonstrations on backpacking, survival techniques (reading maps, using a compass), bear behavior, and poisonous and edible plants provide an informative introduction to experiencing the wilderness without mishap.

Nature walks focusing on bird watching, geologic formations, diurnal and nocturnal animal life, insects, plants, stars, ecological systems, water, and other natural history subjects are offered from 6:30 a.m. to 8:30 p.m. every day through September.

Other seminars of varying lengths deal with Yosemite's human history, including the Indian Cultural Museum in the valley and the Pioneer Yosemite History Center at Wawona. Nature walks, discussion groups, and campfires are offered in Yosemite Valley itself and at three other activity centers in the park in Tuolumne Meadows, Crane Flat/White Wolf, and Wawona. All events are listed in the exhaustive Yosemite Guide, available at park entrance gates, ranger stations, and many stores in the area.

But visitor enjoyment and safety are only part of the National Park Service's extensive educational aims. Preservation of the park and its resources is a reciprocal responsibility. Tuning people in to the relationship between ''minimal impact'' camping that protects the environment and ''woods wise'' behavior that protects the camper promotes survival for all concerned.

Frequently visitors diminish or completely destroy their own and others' enjoyment of the park by seemingly innocent but dangerous actions that threaten both their own safety and the park's plant and animal life. Not only is it not nice to fool Mother Nature - it can be downright destructive.

''If all 21/2 million visitors picked a wildflower, there wouldn't be much left for the next person,'' notes Laurel Munson, supervising back-country ranger. ''This place is like a china shop. The wilderness is fragile, and it is our job to preserve the area in as natural a state as possible. Each of us, as individuals, must see this as our personal responsibility. We can't leave it for the other guy,'' she says.

''It's a fallacy that man is in danger from the animals,'' Ms. Munson asserts. ''It's the animals who are in danger from man. It may seem cute to feed the squirrels and deer, but we're really doing them an injustice. It's important that we let them continue their natural cycles if they are to remain wild,'' she says. ''Remember, we are the visitors. They live here.''

Ms. Munson's warning is especially relevant to bears. People often come to Yosemite fearing the bears are out to get them. In fact, bears are only after food, not confrontation with human beings. They have learned that it is more rewarding to snatch a salami than dig for grubs all day. To keep bears away, humans' food must be made unavailable.

''Physically encountering a human is a last-ditch effort for the bear,'' says bear management technician Michael Webb. ''Most injuries to humans by bears are the result of harassment or stupidity on the part of people. If you see a bear before it sees you, avoid it. Don't stop to take pictures. If it sees you before you see it, make a nuisance of yourself. Make loud noises, throw rocks at it. But if it gets your food, it's his. Don't try to get it back.

''It's been an especially hard winter and the bears are hungry. They're intelligent animals and will investigate all curious odors, including deodorants , shampoos, insect repellent, and other nonedible products. In addition to having incredible noses, bears recognize food containers, ice chests, and grocery bags by sight. Store food inside the trunk of your vehicle, covered with blankets and out of sight. Roll up your windows.

''In the back country, the only real way to keep bears from your food supply is to suspend it on cables 15 feet above the ground, 10 feet from the trunk, and 5 feet down from the nearest limb, using a counterbalancing system. Follow the directions outlined in the brochure 'Backcountry Use Regulations,' issued with your wilderness permit,'' Mr. Webb advises.Wilderness permits are required for all overnight use of the back country and are issued all year at Yosemite Valley , Wawona, Big Oak Flat, and, in summer, at Tuolumne Meadows.

Wilderness permits are issued free on a first-come, first-served basis, according to a daily quota established by the Park Service. Permits may be obtained the day before you start your hike, along with USGS topographical trail maps of the park and surrounding Forest Service property.

Wilderness permits enable the Park Service to limit the number of people using the back country at any given time, but they also protect campers. By recording who you are, where you're going, and how long you intend to stay, rangers can keep track of unintended fires and lost or injured campers, calling in search-and-rescue teams when needed. Always tell friends and relatives how long you will be gone. If you don't reappear on schedule, they should not wait until dark to report you missing.

Once you're out in the wilderness, though, safety and preservation of park resources depends very much on you. Even though Park Service staff patrol the back country, you can't count on seeing anyone when there are 700,000 acres of wilderness to cover. If you get lost, stay in one place out in the open. You will conserve energy and be easier to find.

For visitors coming to Yosemite from sea level, there is another caution to consider. It takes the average person five days to acclimatize physiologically to the change in elevation. Because you are breathing the same volume of air, but taking in less oxygen, even moderate exercise may be fatiguing.

Rangers advise visitors to slow down, take it easy, and leave the hectic pace of city life behind. John Muir would agree with that.

''Nature's peace will flow into you,'' he said, ''as sunshine flows into trees. The winds will blow their own freshness into you, and the storms their energy, while cares will drop off like autumn leaves.''

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