ji'ro skope, an instrument for showing certain properties of rotation. It consists of a circular disk with a heavy rim, mounted upon an axis so that it will revolve with the least possible resistance. In the complete gyroscope, this axis is also mounted in a circular frame, which can be rotated in a direction at right angles to that of the disk, and the entire apparatus rests upon a circular standard, so that it can be turned in any direction. When revolving rapidly the disk is not easily moved out of its plane of rotation, and the axis retains its same direction, though the gyroscope be moved around a circle. For this reason the apparatus is sometimes used to explain the continual pointing of the earth's axis in the same direction. Small gyroscopes are used as toys.