spira

n.[ L. spiraculum, fr. spirare to breathe: cf. F. spiracule. See Spirit. ] 1. (Anat.) The nostril, or one of the nostrils, of whales, porpoises, and allied animals. [ 1913 Webster ]

2. (Zool.) (a) One of the external openings communicating with the air tubes or tracheae of insects, myriapods, and arachnids. They are variable in number, and are usually situated on the sides of the thorax and abdomen, a pair to a segment. These openings are usually elliptical, and capable of being closed. See Illust. under Coleoptera. (a) A tubular orifice communicating with the gill cavity of certain ganoid and all elasmobranch fishes. It is the modified first gill cleft. [ 1913 Webster ]

3. Any small aperture or vent for air or other fluid. [ 1913 Webster ]

a. Of or pertaining to a spiracle. [ 1913 Webster ]

n. [ L., fr. Gr. &unr_;&unr_;&unr_;, fr. &unr_;&unr_;&unr_; a coil. ] (Bot.) A genus of shrubs or perennial herbs including the meadowsweet and the hardhack. [ 1913 Webster ]

a. (Chem.) Of, pertaining to, or derived from, the meadowsweet (Spiraea); formerly, designating an acid which is now called salicylic acid. [ 1913 Webster ]

a. [ Cf. F. spiral. See Spire a winding line. ] 1. Winding or circling round a center or pole and gradually receding from it; as, the spiral curve of a watch spring. [ 1913 Webster ]

2. Winding round a cylinder or imaginary axis, and at the same time rising or advancing forward; winding like the thread of a screw; helical. [ 1913 Webster ]

3. (Geom.) Of or pertaining to a spiral; like a spiral. [ 1913 Webster ]

Spiral gear, or
Spiral wheel (Mach.), a gear resembling in general a spur gear, but having its teeth cut at an angle with its axis, or so that they form small portions of screws or spirals. --
Spiral gearing, a kind of gearing sometimes used in light machinery, in which spiral gears, instead of bevel gears, are used to transmit motion between shafts that are not parallel. --
Spiral operculum, an operculum whih has spiral lines of growth. --
Spiral shell, any shell in which the whorls form a spiral or helix. --
Spiral spring. See the Note under Spring, n., 4. [ 1913 Webster ]

n. [ Cf. F. spirale. See Spiral, a. ] 1. (Geom.) A plane curve, not reentrant, described by a point, called the generatrix, moving along a straight line according to a mathematical law, while the line is revolving about a fixed point called the pole. Cf. Helix. [ 1913 Webster ]

2. Anything which has a spiral form, as a spiral shell. [ 1913 Webster ]

Equiangular spiral, a plane curve which cuts all its generatrices at the same angle. Same as Logarithmic spiral, under Logarithmic. --
Spiral of Archimedes, a spiral the law of which is that the generatrix moves uniformly along the revolving line, which also moves uniformly. [ 1913 Webster ]

n. The quality or states of being spiral. [ 1913 Webster ]

adv. In a spiral form, manner, or direction. [ 1913 Webster ]