=54.= The galleries are constructed as follows (Pl. XII, Fig. 31): A double sap with a width of 8' is broken out from the side of the trench in the direction required and is driven forward in the usual manner, but with a continual increase in depth, at a slope not exceeding 1/4. The side slopes are as steep as the earth will allow. Two blindage-frames are set up vertically on the sides of the sap, 7' apart in clear, with their tops at the level of the tops of the trench gabions, their bottom horns resting in holes dug for them. These frames are prevented from falling inward by another frame placed crosswise upon them, with its horns resting on their cross-pieces. The side of this top frame toward the front may be held up by a stake or crutch, and the second pair of frames be placed at such an interval that their horns will interlock with those of the top frame. Successive frames may be placed in the same manner. The covering or “roof” is formed by three or four layers of fascines placed across the trench on top of the frames, and covered with earth thrown back upon them as the work proceeds. The sides of the gallery are held up by fascines, etc., laid along outside the frames. As soon as the bottom of the blinded gallery has reached the proper depth a mine-gallery may be started and carried forward. =55.= The blindage-frames described above give to the gallery a clear width and height of 7'. For smaller galleries the blinds may all be made shorter and of lighter scantling; or, if desired, those for the sides may be of a different length from those for the top. =56. Rate of Advance of Galleries.=--The following table gives an estimate of the men and tools required for shafts and galleries, with the probable rate of advance in good soil: