SAN FRANCISCO, DEC. 11 -- Sensitive measurements of gravity made on and near a 2,000-foot television tower in North Carolina suggest the existence of a new fundamental force that adds a tiny boost to gravity's attraction, scientists said today.

The finding "is very exciting because it indicates the force we call gravity is much more complicated than we thought," said Paul Boynton, a physicist and astronomer at the University of Washington in Seattle.

The four fundamental forces of nature are gravity; electromagnetism; the strong force, which holds the centers of atoms together, and the weak force, which makes some atoms break down in radioactive decay.

Studies by Boynton and others previously found evidence for a fifth force, which slightly counteracts gravity's pull over 10- to 1,000-yard distances.

The new finding, reported at the fall meeting of the American Geophysical Union by Donald Eckhardt, an Air Force geophysicist, suggests the existence of a sixth force.

When Eckhardt and colleagues from the Air Force Geophysics Laboratory at Hanscom Air Force Base in Massachusetts carefully measured changes in gravity as they went up the television tower, "a clear pattern emerged indicating that not only is there a fifth force, but there is a sixth force as well," they said in a statement.

The possible existence of a gravity-enhancing sixth force and a gravity-counteracting fifth force interests the Air Force because the forces might alter the trajectory of missiles that depend on inertial guidance systems, Eckhardt said.

Other scientists praised the experiment for showing gravity is influenced by two poorly understood forces, but disagreed whether the new attractive force found by Eckhardt is a sixth fundamental force of nature or is part of the still-disputed fifth force -- or whether both are simply parts of gravity.

Eckhardt measured Earth's gravity on and 110 miles around a Garner, N.C., television tower.