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is from this line that all measurements of points in the plans of original objects have to be taken, when giving their elevations on the inclined plane OP. Then to find the required elevation of the line AB, set off from O on the line OP in b and a, the distances A and B6, that its ends A and B are from DL, and through them parallel to the IL draw projectors to cut those drawn from A and B in the plan in the points A'B' ; join these by a right line and it will be the elevation required. Then, as lines are the boundaries of " figures," the projection of a figure in a similar position with respect to the VP and HP is obtained in the same way. One problem in this connection will be sufficient to show the application of the principle. Let it be this Problem 58. Given tJie plan of a triangle, and the inclination of the plane OP ; to find tJie elevation of the trianyk. Let ABC (Fig. 160) be the plan of the triangle, and the inclination of the plane OP be 30 with the HP. Draw in the line OP at the given angle with the IL and the line DL as in the last problem. Find the position of the angular points ABC on the plane OP, and through MECHANICAL AND ENGINEERING DRAWING 123 124 FIRST PRINCIPLES OF them draw projectors parallel to the IL, cutting those drawn through the corresponding points in the plan. The intersections of these pro- jectors A'B'C' will be the elevations of the angular points of the triangle ; join these by right lines and the triangle ABC' will be the elevation required. If the plan of the original figure in its new position be required as well as its elevation, this is found in a similar way. Let the problem be