English: Two enormous gyroscopes being installed in the USS Henderson as a roll stabilizing system during its construction April 1917 at the Philadelphia Navy Yard in World War 1. The Henderson, a transport of 80 ton displacement, was the first large ship to be gyroscopically stabilized to prevent the ship from rolling from side to side with ocean swells. The gyros, built by Sperry Rand, consist of two 25 ton, 9 ft diameter flywheels which during operation are spun at 1100 RPM in opposite directions by 75 HP AC electric motors. Each gyro case is mounted on a vertical bearing which can be turned by a 75 HP servo motor. When a small sensor gyro on the ship's bridge sensed the ship roll, it ordered the servo motor to rotate the gyros about the vertical axis in a direction so the gyro's precession would oppose the ship's roll. During trials they were able to keep the ship roll down to 3 degrees in the roughest seas. This technology was replaced by roll stabilizer fins and is not used today.