Practical engineering drawing and third angle projection, for students in scientific, technical and manual training schools and for ..draughtsmen .. . P and other points of the ellipse by using F and F^ ascentres, and for radii any two segments of A C. Q, for example, gives ^1 Q and C Q as arcs from F and F^, with radius equal to Q C, would intersect arcs from the same centres,radius Q A, in four points of the ellipse, one of which is P. 126. By the Boscovich definition, we are enabled to construct the parabola and hyperbola alsoby continuous motion along a string. For the parabo
Practical engineering drawing and third angle projection, for students in scientific, technical and manual training schools and for ..draughtsmen .. . P and other points of the ellipse by using F and F^ ascentres, and for radii any two segments of A C. Q, for example, gives ^1 Q and C Q as arcs from F and F^, with radius equal to Q C, would intersect arcs from the same centres,radius Q A, in four points of the ellipse, one of which is P. 126. By the Boscovich definition, we are enabled to construct the parabola and hyperbola alsoby continuous motion along a string. For the parabola place a triangle as in Fig. 82, with its altitude G X toward the focus. If astring of length G X be fastened at G, stretched tight from G to any point B, by putting a pencilat B, then the remainder B X swung around and the end fastened at F, it is then, evidently, asfar from B to F as it is from B to the directrix; and that relation will remain constant as the tri-angle is slid along the directrix, if the pencil point remains against the edge of the triangle so thatthe portion of the string fi-om G to the pencil is kept straight. ^ig-- © 127. For the hyperbola, (Fig. 82), the construction is identical with the i^receding, except thatthe string fastened at / runs down the hypothenuse and equals it in length. 128. Referring back to Fig. 35, it will be noticed that the focus and directrix of the jjarabolaare there omitted; but the former would be the point of intersection of a perpendicular from Aupon the line joining C with E. A line through A, parallel to O E, would be the directrix. 129. Like the ellipse, the hyperljola can be constructed by using two foci, but whereas in theellipse (Fig. 82) it was the sum of two focal radii that was constant, i. e., FP+FiP=FD + 42 THEORETICAL AND PRACTICAL GRAPHICS.
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Keywords: ., bookcentury1800, bookdecade1890, booksubjec, booksubjectlettering
