Twenty-one climbers reached the summit of Mount Everest Thursday, including one American mountaineer who placed an instrument atop the world's highest mountain to measure its growth.

But a climber who had hoped to become the first amputee to conquer Everest was forced to turn back short of the peak.Storms had delayed the climbers until Tuesday, creating a backlog of teams eager for good weather in which to make their attempt on the 29,028-foot summit.

American Wally Berg, 43, a mountain guide from Boulder, Colo., climbed all night and reached the summit about 9 a.m. His base camp reported the weather was so clear that Berg spent two hours on the summit - an exceptionally long time - drilling a hole in its solid rock to insert the Global Positioning System receiver.

The system, along with another unit placed three-quarters of the way up the mountain, will measure the geological speed at which India is sliding under Tibet, raising the mountain roughly one inch every year.

Data from the Global Positioning System will also be used to measure weather patterns and help predict major earthquakes in the region.

The operation Berg carried out Thursday was developed by Bradford Washburn of Boston's Museum of Science, and the data will be used by scientists from the Massachusetts Institute of Technology, Yale University and the Walter Reed Army Medical Center.

Tom Whittaker, who lost his right foot in a car accident, was making his third attempt to climb Everest. The Nepalese Tourism Ministry said he had to turn back because of a pulmonary infection.

Whittaker, 49, of Prescott, Ariz., used a specially made artificial limb for the climb.