EATTLE, Jan. 7 — In a test critical to theories of cosmology, scientists have for the first time measured the speed at which the force of gravity moves. And, once again, it appears that Einstein has been proved right, scientists announced today at a meeting here of the American Astronomical Society.

"Newton thought that gravity's force was instantaneous," said Dr. Sergei Kopeikin, a physicist at the University of Missouri. "Einstein assumed that it moved at the speed of light, but until now, no one had measured it."

By observing a slight "bending" of radio waves when Jupiter passed nearly in front of a more distant cosmic object, scientists said they determined that gravity's propagation speed is equal to the speed of light. They said their finding was within an accuracy of 20 percent, which they considered good enough to conclude that gravity's velocity is probably indeed equal to the speed of light.

The result came as no surprise to other scientists.

The speed of gravity was one of the last unmeasured fundamental constants of physics.

The measurement was made on Sept. 8 using radio telescopes of the Very Long Baseline Array in the United States in conjunction with one in Germany. The simultaneous observations of Jupiter by such a broad system of telescopes were unusually sensitive to the slightest deflection of radio waves caused by the planet's gravity.