The use of the regular mine for effecting a breach in the wall of a fortified place was well known, and often brought to bear. The miners began their work at some distance, and drove a shaft underground towards the part of the fortifications which seemed most assailable; they excavated beneath the foundations of the wall, supporting the substructure with wooden props until they had finished their work. Then they set fire to the props, and retired to see the unsupported weight of the wall bringing it down in a heap of ruins. The operation of mining was usually effected under the protection of a temporary pent-house, called a cat or sow.