▪ I. cycloid, n. (a.)
(ˈsaɪklɔɪd, ˈsɪk-)
[See next.]
1. Math. a. The curve traced in space by a point in the circumference (or on a radius) of a circle as the circle rolls along a straight line.
The common cycloid is that traced by a point in the circumference of the circle, and has cusps where this point meets the straight line; that traced by a point within the circle is a prolate cycloid (with inflexions); by a point without the circle, a curtate cycloid (with loops).
| 1661 Boyle Spring of Air (1682) 101 Each point will by this compound motion describe on the plain..a perfect cycloid. 1727 Swift Gulliver, Voy. Laputa ii, A pudding [cut] into a cycloid. 1812–6 Playfair Nat. Phil. (1819) I. 135 The line in which a heavy body descends in the least time from one given point to another..is an arch of a cycloid..Hence the cycloid is called the line of swiftest descent. |
b. companion to the cycloid: the curve formed by successive positions of the point of intersection of a horizontal line drawn through a fixed point in the circumference of the rolling circle with a vertical line through its point of contact with the (horizontal) line on which it rolls.
| 1857 Whewell Hist. Induct. Sc. II. 244 The curve must be of the nature of that which is called the companion to the cycloid. |
2. Zool. A cycloid fish: see next.
| 1847 Ansted Anc. World x. 246 Two orders of Fishes..the Ctenoids and Cycloids. |
3. Psychiatry. A person characterized by a tendency to alternate between exhilaration and depression. Also attrib. or as adj.
| 1925 W. J. H. Sprott tr. Kretschmer's Physique & Char. xi. 127 In an adverse situation the cycloid is either sorrowful or hot-headed, but he is never in the very least nervous. 1927 Henderson & Gillespie Text-Bk. Psychiatry ix. 190 The ‘syntonic’ or ‘cycloid’ personality, in which, if mental illness develops, it tends to be of the manic-depressive kind. 1929 F. H. Garrison Hist. Med. (ed. 4) 679 Cycloids are cheerful, temperamental, well adjusted to the business world, but liable to sudden alternations of exhilaration and depression. 1943 H. Read Educ. through Art iv. 76 The ‘depressive cycloid’ temperament is certainly our old friend the melancholic humour. |
▪ II. ˈcycloid, a.
[ad. Gr. κυκλοειδής, κυκλώδης circular: see cycle and -oid.]
Resembling a circle; spec. in Zool. a. Of a somewhat circular form, with concentric striations; applied to the scales of certain fishes. b. Belonging to the Cycloidei, or order of fishes with cycloid scales.
| 1847 Ansted Anc. World iv. 62 The remaining two groups [of Fishes] are called respectively Ctenoid..and Cycloid..from the shape and structure of the scale. 1851 Richardson Geol. (1855) 283 Nearly all the cycloid genera..are extinct. 1872 Nicholson Palæont. 326 Scales cycloid or rhomboid. |