This document discusses Janssen's theory for calculating pressures in bunkers and silos used for storing materials like coal, cement, and grains. Janssen's theory assumes that a large portion of the weight of stored material is supported by friction between the material and walls, with only a small portion transferred to the bottom. The derivation shows that there is a constant ratio between horizontal and vertical pressures, and the vertical pressure can be calculated based on factors like the material unit weight, coefficient of friction, and silo geometry.