On the definition of "straight line". Mozi said that the three-point line is a straight line. As the definition of straight line, three-point * * * line is widely used to measure the height and distance of objects in later generations. Liu Hui, a mathematician in the Jin Dynasty, used three-point * * * lines to measure the height and distance in the monograph Calculation of Islands. After the Han Dynasty, the "Mountain Sight" on the crossbow machine was invented accordingly.