Descriptive geometry for students in engineering science and architecture; a carefully graded course of instruction . ; and the traces on the 13 14 DESCRIPTIVE GEOMETRY are called Horizontal Traces, and these, of course, are always horizontallines. In mentioning these traces the initial letters only are used as , instead of the whole words. Planes having other attitudes than the two so far discussed, sometunes calledoblique planes, are shown at OPQ and VWZ. The student must reaHze that we are not now deaHng with plans and eleva-tions, but with intersections of the projection p
Descriptive geometry for students in engineering science and architecture; a carefully graded course of instruction . ; and the traces on the 13 14 DESCRIPTIVE GEOMETRY are called Horizontal Traces, and these, of course, are always horizontallines. In mentioning these traces the initial letters only are used as , instead of the whole words. Planes having other attitudes than the two so far discussed, sometunes calledoblique planes, are shown at OPQ and VWZ. The student must reaHze that we are not now deaHng with plans and eleva-tions, but with intersections of the projection planes, made by planes of which therecan be no plans and elevations. These imagined planes are indicated by theirtraces. Reproduce on a paper with XY marked, these planes as in Fig. 9. Fold thepaper along the line XY so that the is vertical while the remains hori-zontal, just as the projection planes are always supposed to be arranged in relationto one another, and it will be realized that RS is really at right angles to ST, andthat LM is really at right angles to MN. In the other cases, also, the angle. Fig. 9. which OP makes with PQ is much less than what it appears to be when the paperis not so folded, and so also with VW and WZ. In Fig. 9, if the Vertical Traces RS, LM, etc., be produced downward belowXY, the productions will show where the planes cut the below the level ofthe , and likewise, if the Horizontal Traces ST, MN, etc., be produced upwardbeyond the XY line, it will be seen where the planes RST, LMN, etc., cut in that part of it beyond the XY where the passes through it. Forgeneral purposes, however, it is sufficient to. represent the planes by two Huesmeeting in XY as in Fig. 9. The student should now consider Fig. 10 and reaUze that whereas the planeRST is perpendicular to the , as seen by the fact that its is perpendicularto XY, and shows its angle to the as a°, the line AB is parallel to the ,its plan being par
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