Lewis and Clark didn't have one. Neither did John Muir, Robert E. Peary or any of history's other famous trailblazers.

Yet today, "a lot of people carry cell phones into the back country, and that's all they carry," says Jonathan Dorn, equipment editor for Backpacker magazine. The false sense of security from a phone that may not work, plus the lack of safety essentials, can be a setup for disaster.

"If you're in a valley, you may not be able to get a [phone] signal," Dorn notes. "If you're on a mountain without a map or compass, you may not be able to tell people where you are." He cites a recent incident where a hiking party got lost, called search and rescue from a cell phone, but couldn't report their location because they hadn't brought a trail map.

"It took the park rangers more than 12 hours to find them," Dorn notes. "Turns out there was an old logging road about 100 yards west, which they could have easily found if they'd had a map."

Over-reliance on cell phones is one of several common safety mistakes vacationers make when going on day hikes or backpacking trips.

A recent study of 126 hikers climbing Keyhole Trail in Colorado concluded that "a significant proportion of respondents . . . were not adequately prepared for their hike," according to the International Journal of Wilderness.

More than half the hikers weren't carrying enough water, about a third weren't using sunscreen and 92 percent failed to properly acclimate themselves to high altitude before beginning their hike, reported physicians John Westfall of the University of Colorado Health Sciences Center in Denver and Bob Gramling of Dartmouth Family Practice in Waterville, Maine.

"It's not uncommon for people to fly into Denver, then go hike Keyhole the next day," says Gramling, who notes that the route climbs from 9,400 feet to 14,255 feet. Exercise at such an altitude can be extremely stressful and lead to illness unless people are properly acclimated. "The recommendation is that people spend at least two nights above 8,500 feet before participating in high-altitude exercise," he says.

Many hikers also fail to pace themselves properly. They start out too late in the day, push to reach a particular goal, then wind up exhausted and sweaty with a long hike back, just as the sun begins to go down and the air gets cold. And many don't realize that even if it's a beautiful 70-degree day when they hit the trail, it could be 35 degrees and sleeting at the top of the mountain.

"Hypothermia (abnormally low body temperature) is more common in the summer than in the winter," Gramling says. "More people are out hiking in bathing suits and don't bring proper clothing for warmth."

To keep a hike safe and enjoyable, preparation and good judgment are critical. "The biggest thing that will keep people from getting hurt or sick is humility," Gramling says. "It's important to recognize the awesome powers of nature that you have to try to be prepared for."

Many experienced hikers rely on a time-tested packing list, known as "The Ten Essentials," that was developed in the 1930s by a group of Seattle area climbing enthusiasts. Recently, Backpacker magazine's Dorn asked outdoor experts if this list was still relevant in an era of cell phones and computers. The answer was a resounding yes, says Dorn, who advises hikers to stow The Ten Essentials in their pack:

1. Map. Good trail maps are available at area hiking stores or from the National Park Service's Web site, www.nps.gov, or Backpacker magazine's site, www.backpacker.com.

2. Compass. Even if you blunder off a well-marked trail, a compass and map can help you pinpoint your location and find your way back.

3. Flashlight with extra batteries and bulb. A man who rescued a lost, cell phone-toting hiker told Dorn that "a cell phone's LED readout isn't bright enough to bring you down the mountain."

4. Extra food. If accidents or weather change your itinerary, it's important to have extra fuel on hand. Carry a surplus of at least one day's worth of ready-to-eat high energy snacks.

5. Extra clothing. Keep a set of clothes in reserve to protect yourself from hypothermia. Be sure the layer closest to your body is synthetic, to wick away moisture. The next layer should be a warm top in a non-cotton fabric such as fleece. In a mild climate, the outer layer might be a waterproof jacket. In a colder climate, you might need a backup down parka.

6. Sunglasses. Pick a pair that filters out UV rays.

7. First-aid kit. Stock adhesive bandages, alcohol wipes, antibiotic ointment, an elastic wrap (for sprains), a heat-retaining "space blanket" that folds to hankie size and medications to relieve common ailments such as headache, nausea and insect bites.

8. Pocketknife. Many potentially lifesaving tasks require a blade: Cutting clothing to assess injuries, shaving branches for kindling, punching holes to repair wind-wracked tents.

9. Matches in a waterproof container. Okay, today you could probably get away with a few cigarette lighters instead. But the important thing is to be able to start a fire.

10. Fire starter. A votive candle, dry tinder or some easy-lighting fuel can help jump-start a blaze for cooking food or warming cold bodies.

Since it's also essential that you drink enough water, Dartmouth's Gramling advises hikers to carry two liters of water, plus purifying tablets or a filter that will allow them to fill up at each water source.

One thing you can't pack, but shouldn't leave home without is "common sense," adds Dorn, who calls this quality "the Eleventh Essential. This means knowing when to turn back, when to whip out the fleece jacket . . . [and] leaving travel plans with your family. No piece of equipment can save someone who lacks experience, know-how and good judgment."