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81. As the development of the whole of the surface of a square prism would merely be a repetition of that given of the cube, in the ' Y^ OF THE UNIVERSIT MECHANICAL AND ENGINEERING DRAWING next problem only a part of a prism is taken, as the solid whose de- velopment is required. Problem 87 (Fig. 191). -Given the elevation of a truncated square prism, to find the development of its surface. Fig. 191 Let No. 1 (Fig. 191) be the side elevation of the prism, standing on its base in a vertical position. As its base is square, its vertical sides will all be of the same width ; therefore on the line c d produced inde- finitely, draw in first an elevation of the prism, looking at it in the direc- tion of the arrow. From c and c in It, set off in points 2, 2' a distance equal to cd in No. 1, and through them draw vertical lines, and cut them in points 3 and 3', by a projector from a. Join b' 3 and b 3'. For the base, and top surface of the prism, produce the lines c'b', c b, in both directions indefinitely. Then with c as centre, and c'2 as radius, cut b'c produced in point 1 ; and from 6', with b' 3 as radius, cut the same line in point 4, and complete the rectangle 4& (above) and the square c'l (below) as shown. From point 2 in cd produced set off a length 2'y, equal to c2', and through point y draw yx, parallel to 2' 3' ; join 3' and x, and the development is completed. If the resultant figure (the envelope of the prism) be cut out in paper, and treated in the same way as that of the cube, it will be found to fit the solid exactly.