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foci in its longest diameter is always the same, then draw an ellipse by the method above referred to : Let the line AB, No. 1 (Fig. 144), be its major or longest diameter or axis, and CD, drawn at the mid-length of and at right angles to AB, its minor or shortest diameter or axis. Anywhere on the sheet of drawing paper draw two lines, as ac, be, No. 2 making an angle with each other say 30- intersecting at c. On a c set off with the compasses from c, half the length of AB, No. 1, and describe the arc de, and on be with half CD as radius, and from the same centre, describe the arc fg, join df by a right line, and parallel to it draw lines through e arid y, cutting ac in point 1, and be in point 2. Then on the major axis AB, No. 1, set off from A and B the distance cl, MECHANICAL AND ENGINEERING DRAWING 97 No. 2, in the points II', and with it as a radius from II' as centres, draw arcs of about 30 through A and B. On the minor axis CD set off from C and D, the distance c2, No. 2, in the points mm', and with it as a radius and m m' as centres draw through C and D arcs similar to those drawn through A and B. For intermediate points take a straight-edged slip of paper SP and mark on its edge with a fine- pointed pencil in points 1 and 3, half the longer axis aA, or aB, No. 1, and in point 2 measured from point 1 half the shorter axis aC, or aD. If, then, this slip of paper be laid on the figure No. 1 and moved over it in such a way that point 2 in its edge is always on the major axis, while point 1 is on the minor, point 3 will describe, if carried through a whole revolution, a perfect ellipse. As, however, only a few points are required between the ends of the arcs already drawn, these can be marked off from the edge of the slip of paper by