To View the Turning
The term "gyroscope" literally means "to view the turning." The mechanism itself consists of a spinning mass, such as a disk or a wheel, which is mounted on a base in a manner that enables its axis to turn in one or more directions. The axis is thus able to maintain its orientation regardless of any movement by the base.
In 1832, Professor W.R. Johnson devised such a mechanism in order to illustrate the dynamics of rotating bodies, calling his model a "rotascope." Twenty years later, in 1852, the French scientist Leon Foucault referred to the device as a "gyroscope" when he published an account of his experiments with the gyro. He used the gyro to show the rotation of the earth.
In 1851, he had used the pendulum to demonstrate this rotation, showing that the direction of a pendulum's swing will change as the earth rotates. When you set a pendulum swinging, it will continue to swing in the same direction unlessaccording to Newton's First Lawit is pushed or pulled in some other direction. As it swings, the earth will rotate once every twenty-four hours underneath the pendulum, causing its swing to appear to change direction. If you stood watching the pendulum, after about a quarter of an hour you would be likely to notice that the line of the pendulum's swing had changed to a different direction. This would be especially clear if you marked the position of the line of swing in the morning and had the pendulum knocking down pegs arranged in a ring around its trajectory.
Such an arrangement can be seen in the pendulum stairwell of The Franklin Institute. It is natural to conclude that the change in direction indicates movement on the part of the pendulum, and stability on the part of the floor. However, it is actually the floor (which is connected to the earth) that is moving as its own "base" (earth) rotates, while the line of swing of the pendulum only appears to rotate.