Wheel \Wheel\ (hw[=e]l), n. [OE. wheel, hweol, AS. hwe['o]l,
   hweogul, hweowol; akin to D. wiel, Icel. hv[=e]l, Gr.
   ky`klos, Skr. cakra; cf. Icel. hj[=o]l, Dan. hiul, Sw. hjul.
   [root]218. Cf. Cycle, Cyclopedia.]
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   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.
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            The gasping charioteer beneath the wheel
            Of his own car.                       --Dryden.
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   2. Any instrument having the form of, or chiefly consisting
      of, a wheel. Specifically: 
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      (a) A spinning wheel. See under Spinning.
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      (b) An instrument of torture formerly used.
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                His examination is like that which is made by
                the rack and wheel.               --Addison.
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   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. --Brande.
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      (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.
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      (d) (Pottery) A potter's wheel. See under Potter.
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                Then I went down to the potter's house, and,
                behold, he wrought a work on the wheels. --Jer.
                                                  xviii. 3.
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                Turn, turn, my wheel! This earthen jar
                A touch can make, a touch can mar. --Longfellow.
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      (e) (Pyrotechny) A firework which, while burning, is
          caused to revolve on an axis by the reaction of the
          escaping gases.
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      (f) (Poetry) The burden or refrain of a song.
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   Note: "This meaning has a low degree of authority, but is
         supposed from the context in the few cases where the
         word is found." --Nares.
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               You must sing a-down a-down,
               An you call him a-down-a.
               O, how the wheel becomes it!       --Shak.
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   3. A bicycle or a tricycle; a velocipede.
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   4. A rolling or revolving body; anything of a circular form;
      a disk; an orb. --Milton.
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   5. A turn revolution; rotation; compass.
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            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.
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            [He] throws his steep flight in many an aery wheel.
                                                  --Milton.
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   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.
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Axis \Ax"is\, n.; pl. Axes. [L. axis axis, axle. See Axle.]
   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.
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   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.
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   3. (Bot.) The stem; the central part, or longitudinal
      support, on which organs or parts are arranged; the
      central line of any body. --Gray.
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   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.
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   5. (Crystallog.) One of several imaginary lines, assumed in
      describing the position of the planes by which a crystal
      is bounded.
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   6. (Fine Arts) The primary or secondary central line of any
      design.
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   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 co["o]rdinates 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 co["o]rdinates in space, the three straight lines
      in which the co["o]rdinate 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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