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plane enters and leaves the solid parts of the object, and crosses the open parts, for these points determine what parts are in section and seen, and what are not. In Fig. 121 the upper part of the projection is shown as being solid right across, because the section plane cuts through a solid part of the cube's side. The same is seen in the lower part of Fig. 123 ; but in the opposite parts of each of these figures the section plane has cut through an opening in that side of the cube, and this is indicated by the gaps shown. The same reasoning will explain the projection obtained at Fig. 122, only in this instance the section plane has equally divided the four openings it passes through. With the foregoing explanation, the three sectional elevations of the original object should be obtained without difficulty, projectors being shown in the front . elevation of it (Fig. 1 20), partly drawn in, from all the important points cut through by the section planes. MECHANICAL AND ENGINEERING DRAWING I m 70 MECHANICAL AND ENGINEERING DRAWING As the side elevations of the prism and the pyramid are obtained in the same way as those of the cube so fully shown in Sheets 3, 4, and 5, a few problems in their projection are given in Sheet 6, for the student to solve without further aid than that afforded by the con- struction lines shown in the diagrams. Fig. 124 is the elevation of a square prism, assumed to be solid in the first instance, with its base resting on the HP, and two of its sides adjacent ones respectively parallel and perpendicular to the VP. It is required to give side elevations of the prism looking in the