Descriptive geometry . races is an entirely different process from readingthe projections of a line or of a point. The traces of a planeoblique to H and V are not projections of the plane in theordinary sense. Let HQ and VQ, Fig. 59, be the horizontal and vertical tracesof a given plane Q. V / / / \/ V V /%> \ ^ / / N / /\/ / Fig. 59. Fig. 60. I Since the plane is indefinite in extent and cuts the groundline, it must pass into all four quadrants. Since the two traces are oblique to GL, it follows that theplane determined by them is oblique to both H and V. 49. The Slope of a Plane. The slop
Descriptive geometry . races is an entirely different process from readingthe projections of a line or of a point. The traces of a planeoblique to H and V are not projections of the plane in theordinary sense. Let HQ and VQ, Fig. 59, be the horizontal and vertical tracesof a given plane Q. V / / / \/ V V /%> \ ^ / / N / /\/ / Fig. 59. Fig. 60. I Since the plane is indefinite in extent and cuts the groundline, it must pass into all four quadrants. Since the two traces are oblique to GL, it follows that theplane determined by them is oblique to both H and V. 49. The Slope of a Plane. The slope of an oblique plane is de-scribed in the same terms as the slope of a line, and may perhapsbe best understood by reference to the planes of Figs. 59—63. V, § 49] REPRESENTATION OF THE PLANE 31 Plane Q in Fig. 59, shown pictorially in Fig. 60, would besaid to slope downward, forward, and to the left (or upward,backward, to the right). In Fig. 61 the slope is downward, backward, to the left (ab-breviated to ).. Fig. 61. In Fig. 62, the slope is downward, forward, to the right(). r /// p. \ t» V V f s Fig. (J2. In Fig. 63, plane Q, perpendicular to H, would be said toslope simply backward to the right (). VR J. LJ
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