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round on c d until it is at right angles to the part B. When so swung we have virtually three "planes of projection," two of which are vertical and one horizontal, and each at right angles to the other. With planes in these positions it is evident that three different projections of an object may be obtained on them. Let the object be, say, a simple prismatic solid, as S, and let its position be such that two of its sides are respectively parallel to the two vertical planes A and B; then the view obtained on A, looking in the direction of the arrow s, will be a side elevation of S; and that on B, in the direction of /, a front elevation ; the plan P of the object in the HP being obtained as shown by the projectors to that plane. If, then, the part A of the VP, with its obtained side view of S, be swung back on c d into its original normal position with respect to the part B, we should have in the VP a front and side elevation of the original object S, and in the HP the plan P or view obtained when looking in the direction of the arrow t. On turning down the VP on the IL as a hinge until it and the HP become, as before explained, the one flat surface of the sheet of drawing-paper, the three true projections viz., the front and side elevations and the plan of S will appear as shown in Fig. 99. In that figure the assumed motion of the part A of the VP into the position of being at right angles to the part B, is shown by the dotted arcs and arrows, the winged arrow indicating the assumed motion of the plane A, and the barbed ones the transference of the projections. MECHANICAL AND ENGINEERING DRAWING 55 i<. 98