SLOWER OLD FAITHFUL ISN'T SO RELIABLE NOWADAYS.

Byline: James Brooke The New York Times

A frost-covered bison placidly chewed its winter cud, but the tourists, standing in an expectant semicircle, were getting restless.

A chalk board at the visitors center announced that Old Faithful would erupt at 1:15. It was already 1:30.

"We only forecast; we don't schedule," Rick Hutchison, Yellowstone National Park's research geologist, said on a recent Wednesday afternoon.

Old Faithful, the national icon of dynamism and dependability, is getting a bit irregular.

"Old Faithful is slowing down," said Hutchison, who has been studying the park's leading tourist attraction since 1970. "There is good evidence that it is sensitive to earthquakes."

Oblivious to a multimillion-dollar tourism industry that relies on Old Faithful's being faithful, the scientist added casually: "One day, it might just quit permanently."

As he spoke, at 1:35 p.m., the prehistoric underground plumbing rumbled to life.

Flanked by billows of steam, a hissing jet of water rose 125 feet, then fell against a snow-bound backdrop of pine trees and grazing elk.

At the turn of the century, tourism promoters drew presidents and royalty by touting Old Faithful's eruptions as "every hour on the hour."

America's most famous geyser never erupted with the precision of a departing Union Pacific express train. But as recently as 1950, the average interval between eruptions was 62 minutes.

Over the last quarter-century, not even an instant in geologic time, there has been a marked change in the eruptions, noted by an unblinking infrared eye that is mounted on the visitors center and watches the geyser's silica cone 24 hours a day.

"In 1970, when I got here, the average interval was 66 minutes," Hutchison said. "Today it's 77 minutes.

"In the 1970s, it was more regular. Our forecasts had a margin of plus or minus 5 minutes. Now it is plus or minus 10 minutes."

Although defined by volcanic rock and water heated up to 255 degrees Fahrenheit, geysers are fragile.

In Nevada and New Zealand, geysers that once were considered permanent are now extinct because power plants tapped into underground steam and water systems. Geothermal plants have even reduced geysers in Iceland, home to Geysir, namesake of all geysers.

Yellowstone encloses the largest geothermal basin on Earth, with about 250 active geysers and numerous hot springs, bubbling mud pots and fumaroles or vents. In 1992, scientists became alarmed after a northern neighbor of the park, the Church Universal and Triumphant, drilled a geothermal well and filled a swimming pool. The only other wells that tapped into the underground veins of pressurized water were scientific research wells that were later capped.

The church later capped the well and agreed to give up geothermal water rights in return for 11 acres of national forest land and access to surface water from a Yellowstone hot spring. The deal is contingent on approval of the "Old Faithful Protection Act" by Congress.

The bill, which has been passed by the House of Representatives, would ban well drilling in a buffer zone around the 2.2-million-acre park.

"Of the 10 great geyser basins in the world, the only two that have not been destroyed by well pumping are Yellowstone and Kamchatka, Russia," said Bob Ekey, spokesman for the Greater Yellowstone Coalition, a regional environmental group.

While well drilling is not believed to have affected Old Faithful, the geyser remains vulnerable to another kind of human interference. About six yards below the cone's mouth, Old Faithful narrows to a 3-inch vent.

In the past 125 years, curious visitors have dropped every object imaginable into the geyser's cone - from buckets to tree logs to dirty laundry. In the 1880s, soldiers billeted here used to write to gullible relatives back home that their blue uniforms came back up pressed, folded and with a laundry mark.

"I have seAen everything come up out of there - underwear, pieces of furniture, beer cans, coins, rifle shells," Hutchison recalled of illicit experiments conducted by some of the 3 million tourists who visit annually.

"Eventually, if you throw enough stuff in, you can plug up a geyser."

While vandals have plugged a few of Yellowstone's geysers, the changes of recent years seem to be produced by seismic activity, most of it rarely felt on the Earth's surface.

"Earthquakes increased dramatically in Yellowstone last summer," said Robert Smith, geophysics professor for the University of Utah. Referring to small tremors detected by a network of sensors, he added: "We had as many as 100 a day."

Scientists theorize that earthquakes can have two effects on geysers, either speeding up or slowing down the rate of supply of water.

Quakes can shake loose particles and debris in rock channels that feed water to a geyser. In this case, the geyser would spew more water and steam.

Or, shaking the vast, underground pressure system, they can crack open new channels, redirecting water to other geysers or hot springs. With Old Faithful now erupting less frequently and less regularly, scientists speculate that its underground feeder system is literally losing steam.

For the moment, only seasoned geyser gazers notice the difference.

"Last week, they missed by 40 minutes," said David Leddon, a guide who drives in tourists daily in a van on caterpillar treads.

There were few complaints from the thin line of tourists who gathered by Old Faithful on a day when thermometers hovered at 20 degrees below zero.

"It was beautiful," Alice Manuell, a visitor from Hartland, Wis., said later, in the warmth of the visitors center. Referring to the unexpected delay, she said: "My toes started to feel like ice."