This paper describes a new method of finding the principal stress lines in a two-dimensional model by means of 5-8 rule as numerical integration by analytical theory of elasticity. The method is applied in detail to square and circular plates compressed by two diagonally opposite forces, and is found to be easy of operation and to yield results of good accuracy. In addtion, the determination of the special cases of 5-8 rule, and differential coefficient with arbitrary increaments is described in appendix.
