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planes of projection, is found in the same way as here shown, we can without further example proceed with the application of the principles involved, to the projection of solid objects similarly placed. Now, a solid object, such as the block of wood taken for explanatory purposes in Fig. 157, has not only length and breadth, but thickness also, and each of these dimensions has its representative lines in the projections that are obtained of it, when it is inclined to both the VP and HP. The actual thickness of an object or its measurement from surface to surface is the distance through it at right angles to the surface from which it is measured. If the thickness be represented by a line, then the line will be perpendicular to that surface, and its projections will depend upon its position with respect to the planes of projection. In Fig. 162, if the plan of a line be represented by the point a, then, when the plane to which the line is assumed to be perpendicular MECHANICAL AND ENGINEERING DRAWING 125 Fiy. 160 Fiy. 161 126 FIRST PRINCIPLES OF is inclined, say, 30 to the horizontal, its side elevation on that plane OP will be a'b. To find its front elevation, we draw a projector through a perpendicular to the IL, and from a and b the two ends of the line projectors parallel to the IL, to cut the one through a in a'b'; then a line joining these points is the front elevation of the original line a'b when inclined to both planes of projection. To apply this reasoning to the projection of such an object as the ; rectangular block of wood in