sb. and a. [ad. [ad. Gr. κογχοειδής mussel-like, f. κόγχη mussel-shell + -ειδης -form: see -OID: in mod.F. conchoïde.]
A. sb. Geom. A plane curve of the fourth order invented by Nicomedes.
If from a fixed point (the pole) straight lines be drawn intersecting a fixed straight line (the asymptote), and on these lines points be taken at a constant distance from their intersections with the asymptote, this succession of points will form a conchoid of Nicomedes consisting of two branches, one on each side of the asymptote.
1798. Frere & Canning, Loves of Triangles, 12, in Anti-Jacobin, 16 April (1852), 106. Ye Conchoids extend.
1807. Hutton, Course Math., II. 320. To find the point of inflexion in the Conchoid of Nicomedes.
1821. Coleridge, in Blackw. Mag., X. 255. I never take a turn round the garden without thinking of his billow-lines and shell-lines, under the well-sounding names of Cumaïds and Conchoïds.
1879. G. Salmon, Higher Plane Curves, ii. 44. A curve, called the conchoid of Nicomedes, invented by that geometer for the solution of the problem of finding two mean proportionals.
b. Spherical conchoid: Herschel’s name for a similar curve, traced on the surface of a sphere.
1797. Encycl. Brit., II. 483/1.
B. adj. = CONCHOIDAL.
1802. Howard, in Phil. Trans., XCII. 207. Its fracture is usually conchoid.
