The cannons of the British warship H.M.S. Hussar were last fired in anger during the Revolutionary War. But when Central Park Conservancy workers arrived to clean one on Friday, the police said, they found it was still loaded with gunpowder and a cannonball, ready to fire.

The park workers found the ammunition when they removed a plug in the cannon’s barrel, said Paul J. Browne, the chief spokesman for the police. Then the workers dialed 911. When the bomb squad arrived, Mr. Browne said, “They tilted the barrel of the cannon and the cannonball rolled out.” Inside, too, was one pound and 12 ounces of gunpowder wrapped in wool. “In theory you could have fired that cannon,” Mr. Browne said, “because the powder was still working.”

The Police Department provided pictures of the pitted cannon angled upward as it might have been on deck; the cannonball, the size of a melon; and mounds of black gunpowder.

The powder was removed and taken to a gun range for safekeeping, Mr. Browne said, and the wool in which it was wrapped was returned to the conservancy as a historical artifact.