NH 57631 Sperry gyrostabilizer.

NH 57631 Sperry gyrostabilizer, installed in the engine room of USS Aramis, a 350-ton guardship and Admiral's barge in New York Harbor, circa 1920.

A gyrostabilizer is a gyroscopic device for stabilizing a ship, airplane, instrument mounting. Originally giant gyroscopes (up to 4 meters in diameter) were used to counteract roll in ships, but they were found to be too cumbersome. Now fins protruding from the ship's hull are moved hydraulically to oppose roll under the control of signals from small gyroscopes that sense roll and velocity.