The cases of branches and small branches are sometimes made very strong, with a view to resist rupture by the explosion of neighboring mines. For this purpose cases made of oak plank 4" thick are used, and the branch near its end is packed full of scantlings provided with rope-handles at their ends for withdrawing them after the mine is fired. This packing is, however, of doubtful utility, since a compression of the case sufficient to call the resistance of the packing into play is very apt to produce a permanent deformation of the cases, which will jam the scantlings and prevent their removal. For convenience in use the pieces of cases should be of uniform width. =24. Relative Advantages of Cases, and Frames and Sheeting.=--In favorable soil, cases, when they can be obtained, allow of more rapid progress and give a lining with a smooth interior. In very bad soil they cannot be used for the larger galleries. Frames and sheeting can be used in all soils which admit of mining operations, and can usually be improvised from materials found in the vicinity. =25. Sinking a Shaft by Frames and Sheeting.=--The size and position of a shaft are usually determined by the character and direction of the gallery which is to start out from it. It is evident (Pl. XI, Fig. 17) that the clear width of the shaft must be enough greater than the outside width of the gallery to allow the side sheeting of the gallery to be freely inserted outside the frames of the gallery and inside those of the shaft; also, that the shaft frames must be so spaced as to leave a clear space at the bottom for the gallery. This space must be equal to the clear height of the gallery, increased by the thickness of the cap-sill, the sheeting, and one or two inches for easy working. This and the thickness of one frame being deducted from the depth of the shaft, the remainder may be divided up into a number of equal or unequal parts called _shaft intervals_. In order that the sheeting may not yield under the pressure of the earth, these intervals seldom exceed 4 feet. The length of the shaft must be great enough to allow the miners to work freely, and to insert the sheeting for the first gallery interval. The sheeting for both shafts and galleries is cut in lengths about 1 foot greater than the interval between frame centres. =26.= The size and position of the shaft having been fixed, the top frame (Pl. XI, Fig. 15) is placed in position and secured by pegs, and the direction of its axis is accurately fixed by the score marks at the middle of the end pieces. The side and end pieces of this frame are respectively about 3 feet longer than those of the other frames, and are so halved together as to make of their ends four projecting _horns_, 1½' long, which keep the frame in place during the excavation of the first interval.