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make it, if possible, still more intelligible. The constructions to the left of the diagram are a repetition of the three first problems in Fig. 70, but show more fully, by means of the arrows, the direction of the projectors or visual rays with respect to the YP and HP. After the very full explanation given in Chapter VI. of the way of obtaining the projections of a straight line in any position, nothing more need be said in reference to it here than that the positions of the lines given in those problems are the positions of the edges which are all straight lines of the prismatic solids given in the diagram as the subjects for projection. The first solid, whose projections in the VP and HP of the diagram are figured 1, 1, is that of a right prism (of any material substance say wood) ; its bases or ends are rectangles, square with its sides, and its position with respect to the planes of projection is such, that its sides are perpendicular and its ends parallel to the VP, its upper and under sides are parallel to the HP, while its other two sides are perpendicular to it ; its ends being parallel to the VP are also perpendicular to the HP. As the sides and ends of the solid are plane rectangular surfaces, and in known position with respect to the VP and HP, their projections on those planes are obtained in the same way as those of any plane figures of the same form, and in the same position. 23. At this stage in our subject it will have become apparent to the would-be draughtsman that he must either be possessed of a perfect knowledge of the forms of the solids he is attempting to delineate, or have models of them to guide him in his delineations; in other words, he must have either a true conception or a possession