Inverson of circle: Into a circle, if the circle pass through inversioncenter o, it becomes a straight line.
圆的反演：反演成一圆。但当圆通过反演中心时变成一直线。
2
Based on the mass-period method, a measuring device named double spring inversion pendulum was designed, which could measure the center of mass and moment of inertia of an engine.