Per·i·tro·chi·um n.  Mech. The wheel which, together with the axle, forms the axis in peritrochio, which see under Axis.

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 Wheel n.
 1. A circular frame turning about an axis; a rotating disk, whether solid, or a frame composed of an outer rim, spokes or radii, and a central hub or nave, in which is inserted the axle, -- used for supporting and conveying vehicles, in machinery, and for various purposes; as, the wheel of a wagon, of a locomotive, of a mill, of a watch, etc.
 The gasping charioteer beneath the wheel
 Of his own car.   --Dryden.
 2. Any instrument having the form of, or chiefly consisting of, a wheel.  Specifically: --
 (a) A spinning wheel.  See under Spinning.
 (b) An instrument of torture formerly used.
    His examination is like that which is made by the rack and wheel.   --Addison.
 Note: ☞ This mode of torture is said to have been first employed in Germany, in the fourteenth century. The criminal was laid on a cart wheel with his legs and arms extended, and his limbs in that posture were fractured with an iron bar. In France, where its use was restricted to the most atrocious crimes, the criminal was first laid on a frame of wood in the form of a St. Andrew's cross, with grooves cut transversely in it above and below the knees and elbows, and the executioner struck eight blows with an iron bar, so as to break the limbs in those places, sometimes finishing by two or three blows on the chest or stomach, which usually put an end to the life of the criminal, and were hence called coups-de-grace -- blows of mercy. The criminal was then unbound, and laid on a small wheel, with his face upward, and his arms and legs doubled under him, there to expire, if he had survived the previous treatment.
 (c) Naut. A circular frame having handles on the periphery, and an axle which is so connected with the tiller as to form a means of controlling the rudder for the purpose of steering.
 (d) Pottery A potter's wheel.  See under Potter.
    Then I went down to the potter's house, and, behold, he wrought a work on the wheels.   --Jer. xviii. 3.
 Turn, turn, my wheel! This earthen jar
 A touch can make, a touch can mar.   --Longfellow.
 (e) Pyrotechny A firework which, while burning, is caused to revolve on an axis by the reaction of the escaping gases.
 (f) Poetry The burden or refrain of a song.
 Note: ☞ “This meaning has a low degree of authority, but is supposed from the context in the few cases where the word is found.”
 You must sing a-down a-down,
 An you call him a-down-a.
 O, how the wheel becomes it!   --Shak.
 3. A bicycle or a tricycle; a velocipede.
 4. A rolling or revolving body; anything of a circular form; a disk; an orb.
 5. A turn revolution; rotation; compass.
    According to the common vicissitude and wheel of things, the proud and the insolent, after long trampling upon others, come at length to be trampled upon themselves.   --South.
    [He] throws his steep flight in many an aery wheel.   --Milton.
 A wheel within a wheel, or Wheels within wheels, a complication of circumstances, motives, etc.
 Balance wheel. See in the Vocab.
 Bevel wheel, Brake wheel, Cam wheel, Fifth wheel, Overshot wheel, Spinning wheel, etc. See under Bevel, Brake, etc.
 Core wheel. Mach. (a) A mortise gear. (b) A wheel having a rim perforated to receive wooden cogs; the skeleton of a mortise gear.
 Measuring wheel, an odometer, or perambulator.
 Wheel and axle Mech., one of the elementary machines or mechanical powers, consisting of a wheel fixed to an axle, and used for raising great weights, by applying the power to the circumference of the wheel, and attaching the weight, by a rope or chain, to that of the axle. Called also axis in peritrochio, and perpetual lever, -- the principle of equilibrium involved being the same as in the lever, while its action is continuous.  See Mechanical powers, under Mechanical.
 Wheel animal, or Wheel animalcule Zool., any one of numerous species of rotifers having a ciliated disk at the anterior end.
 Wheel barometer. Physics See under Barometer.
 Wheel boat, a boat with wheels, to be used either on water or upon inclined planes or railways.
 Wheel bug Zool., a large North American hemipterous insect (Prionidus cristatus) which sucks the blood of other insects. So named from the curious shape of the prothorax.
 Wheel carriage, a carriage moving on wheels.
 Wheel chains, or Wheel ropes Naut., the chains or ropes connecting the wheel and rudder.
 Wheel cutter, a machine for shaping the cogs of gear wheels; a gear cutter.
 Wheel horse, one of the horses nearest to the wheels, as opposed to a leader, or forward horse; -- called also wheeler.
 Wheel lathe, a lathe for turning railway-car wheels.
 Wheel lock. (a) A letter lock.  See under Letter. (b) A kind of gunlock in which sparks were struck from a flint, or piece of iron pyrites, by a revolving wheel. (c) A kind of brake a carriage.
 Wheel ore Min., a variety of bournonite so named from the shape of its twin crystals.  See Bournonite.
 Wheel pit Steam Engine, a pit in the ground, in which the lower part of the fly wheel runs.
 Wheel plow, or Wheel plough, a plow having one or two wheels attached, to render it more steady, and to regulate the depth of the furrow.
 Wheel press, a press by which railway-car wheels are forced on, or off, their axles.
 Wheel race, the place in which a water wheel is set.
 Wheel rope Naut., a tiller rope.  See under Tiller.
 Wheel stitch Needlework, a stitch resembling a spider's web, worked into the material, and not over an open space. --Caulfeild & S. (Dict. of Needlework).
 Wheel tree Bot., a tree (Aspidosperma excelsum) of Guiana, which has a trunk so curiously fluted that a transverse section resembles the hub and spokes of a coarsely made wheel.  See Paddlewood.
 Wheel urchin Zool., any sea urchin of the genus Rotula having a round, flat shell.
 Wheel window Arch., a circular window having radiating mullions arranged like the spokes of a wheel.  Cf. Rose window, under Rose.


 Ax·is n.; pl. Axes   A straight line, real or imaginary, passing through a body, on which it revolves, or may be supposed to revolve; a line passing through a body or system around which the parts are symmetrically arranged.
 2. Math. A straight line with respect to which the different parts of a magnitude are symmetrically arranged; as, the axis of a cylinder, i. e., the axis of a cone, that is, the straight line joining the vertex and the center of the base; the axis of a circle, any straight line passing through the center.
 3. Bot. The stem; the central part, or longitudinal support, on which organs or parts are arranged; the central line of any body.
 4. Anat. (a) The second vertebra of the neck, or vertebra dentata. (b) Also used of the body only of the vertebra, which is prolonged anteriorly within the foramen of the first vertebra or atlas, so as to form the odontoid process or peg which serves as a pivot for the atlas and head to turn upon.
 5. Crystallog. One of several imaginary lines, assumed in describing the position of the planes by which a crystal is bounded.
 6. Fine Arts The primary or secondary central line of any design.
 Anticlinal axis Geol., a line or ridge from which the strata slope downward on the two opposite sides.
 Synclinal axis, a line from which the strata slope upward in opposite directions, so as to form a valley.
 Axis cylinder Anat., the neuraxis or essential, central substance of a nerve fiber; -- called also axis band, axial fiber, and cylinder axis.
 Axis in peritrochio, the wheel and axle, one of the mechanical powers.
 Axis of a curve Geom., a straight line which bisects a system of parallel chords of a curve; called a principal axis, when cutting them at right angles, in which case it divides the curve into two symmetrical portions, as in the parabola, which has one such axis, the ellipse, which has two, or the circle, which has an infinite number. The two axes of the ellipse are the major axis and the minor axis, and the two axes of the hyperbola are the transverse axis and the conjugate axis.
 Axis of a lens, the straight line passing through its center and perpendicular to its surfaces.
 Axis of a microscope or Axis of a telescope, the straight line with which coincide the axes of the several lenses which compose it.
 Axes of coördinates in a plane, two straight lines intersecting each other, to which points are referred for the purpose of determining their relative position: they are either rectangular or oblique.
 Axes of coördinates in space, the three straight lines in which the coördinate planes intersect each other.
 Axis of a balance, that line about which it turns.
 Axis of oscillation, of a pendulum, a right line passing through the center about which it vibrates, and perpendicular to the plane of vibration.
 Axis of polarization, the central line around which the prismatic rings or curves are arranged. --Brewster.
 Axis of revolution Descriptive Geom., a straight line about which some line or plane is revolved, so that the several points of the line or plane shall describe circles with their centers in the fixed line, and their planes perpendicular to it, the line describing a surface of revolution, and the plane a solid of revolution.
 Axis of symmetry Geom., any line in a plane figure which divides the figure into two such parts that one part, when folded over along the axis, shall coincide with the other part.
 Axis of the equator, ecliptic, horizon (or other circle considered with reference to the sphere on which it lies), the diameter of the sphere which is perpendicular to the plane of the circle. --Hutton.
 Axis of the Ionic capital Arch., a line passing perpendicularly through the middle of the eye of the volute.
 Neutral axis Mech., the line of demarcation between the horizontal elastic forces of tension and compression, exerted by the fibers in any cross section of a girder.
 Optic axis of a crystal, the direction in which a ray of transmitted light suffers no double refraction. All crystals, not of the isometric system, are either uniaxial or biaxial.
 Optic axis, Visual axis Opt., the straight line passing through the center of the pupil, and perpendicular to the surface of the eye.
 Radical axis of two circles Geom., the straight line perpendicular to the line joining their centers and such that the tangents from any point of it to the two circles shall be equal to each other.
 Spiral axis Arch., the axis of a twisted column drawn spirally in order to trace the circumvolutions without.
 Axis of abscissas and Axis of ordinates. See Abscissa.

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