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CHAPTER VIII THE PROJECTION OF SOLIDS 21. ASSUMING that the student has followed the advice given in the last paragraph of Chapter VII., and thoroughly mastered the elementary principles of projection which we have expounded in it and Chapter VI., we can now proceed to apply those principles to the delineation of the simple geometrical solids of which engine and machine details are invariably made up. These solids are of two kinds viz., plane, and circular or curved. The first-named have all their surfaces plane figures, the projections of which the student already knows how to obtain, and the second includes all solids whose bounding surfaces are all curved, or plane and curved combined. What are known as the simple solids are the cube, the prism, the pyramid, the cylinder, the cone, and the sphere, the first three being plane solids, and the others circular or curved solids. A cube (Fig. 89) is a solid having six equal sides or faces, all of them squares. A j^'ism (Fig. 90) has two ends or bases parallel to each other, each being equal and similar figures ; its sides are rectangles. A pyramid (Fig. 91) has one base, its sides being triangles, with their vertices meeting in one point a, called the apex of the pyramid. (As it is advisable for the student to confine himself for the present to the study of the projection of plane solids, we defer any consideration of the circular ones until after the projection of curved lines in any position is understood by him.) 47 OF THK ' r ^\