usbview download

Av a convenient distance from its base BC, in No. 1. Then to find the development of the surface required: in No. 3, Fig. 196, draw a line (indefinitely) at right angles to BC the base, and a projector from its apex parallel to that base, to cut it in A'. On the line through A' set off from A', the length of the longest side AB of the pyramid in a, and through a draw a line at right angles to A'a. From a', set off to 196 FIRST PRINCIPLES OF ---^ MECHANICAL AND ENGINEERING DRAWING 197 right and left in B and B', half the length of one edge of the base of the pyramid, then the line B'B will be the development of the edge B'B in No. 2. Join B and B' with A' by straight lines, then will the triangle A'B'B be the development of the longest face of the pyramid. For the adjacent faces BA'C, and B'A'C' : with BB' as radius and B,B' as centres, describe arcs to right and left, and from A as centre with the actual length of the edge AC in No. 1 as radius, cut the arcs last drawn in C and C'. Join AC' and A'C by right lines, and the triangles A'C'B' and ABC will be the development of the front and back faces of the pyramid. For the shortest face or that to the right in No. 1 of the pyramid in No. 3, with C as centre, and CB as radius, describe an arc, and from A' as centre, with AC as radius, cut that arc in C' ; join AC" with a right line, and the development of the four faces of the pyramid when laid out flat is complete. The actual length of A'C in No. 3 is the hyj)othenuse of a right- angled triangle, of which the line AC in No. 2 is the base, and the