Screw \Screw\ (skr[udd]), n. [OE. scrue, OF. escroue, escroe,
   female screw, F. ['e]crou, L. scrobis a ditch, trench, in
   LL., the hole made by swine in rooting; cf. D. schroef a
   screw, G. schraube, Icel. skr[=u]fa.]
   1. A cylinder, or a cylindrical perforation, having a
      continuous rib, called the thread, winding round it
      spirally at a constant inclination, so as to leave a
      continuous spiral groove between one turn and the next, --
      used chiefly for producing, when revolved, motion or
      pressure in the direction of its axis, by the sliding of
      the threads of the cylinder in the grooves between the
      threads of the perforation adapted to it, the former being
      distinguished as the external, or male screw, or, more
      usually the screw; the latter as the internal, or female
      screw, or, more usually, the nut.
      [1913 Webster]

   Note: The screw, as a mechanical power, is a modification of
         the inclined plane, and may be regarded as a
         right-angled triangle wrapped round a cylinder, the
         hypotenuse of the marking the spiral thread of the
         screw, its base equaling the circumference of the
         cylinder, and its height the pitch of the thread.
         [1913 Webster]

   2. Specifically, a kind of nail with a spiral thread and a
      head with a nick to receive the end of the screw-driver.
      Screws are much used to hold together pieces of wood or to
      fasten something; -- called also wood screws, and screw
      nails. See also Screw bolt, below.
      [1913 Webster]

   3. Anything shaped or acting like a screw; esp., a form of
      wheel for propelling steam vessels. It is placed at the
      stern, and furnished with blades having helicoidal
      surfaces to act against the water in the manner of a
      screw. See Screw propeller, below.
      [1913 Webster]

   4. A steam vesel propelled by a screw instead of wheels; a
      screw steamer; a propeller.
      [1913 Webster]

   5. An extortioner; a sharp bargainer; a skinflint; a niggard.
      --Thackeray.
      [1913 Webster]

   6. An instructor who examines with great or unnecessary
      severity; also, a searching or strict examination of a
      student by an instructor. [Cant, American Colleges]
      [1913 Webster]

   7. A small packet of tobacco. [Slang] --Mayhew.
      [1913 Webster]

   8. An unsound or worn-out horse, useful as a hack, and
      commonly of good appearance. --Ld. Lytton.
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   9. (Math.) A straight line in space with which a definite
      linear magnitude termed the pitch is associated (cf. 5th
      Pitch, 10
      (b) ). It is used to express the displacement of a rigid
          body, which may always be made to consist of a
          rotation about an axis combined with a translation
          parallel to that axis.
          [1913 Webster]

   10. (Zool.) An amphipod crustacean; as, the skeleton screw
       (Caprella). See Sand screw, under Sand.
       [1913 Webster]

   Archimedes screw, Compound screw, Foot screw, etc. See
      under Archimedes, Compound, Foot, etc.

   A screw loose, something out of order, so that work is not
      done smoothly; as, there is a screw loose somewhere. --H.
      Martineau.

   Endless screw, or perpetual screw, a screw used to give
      motion to a toothed wheel by the action of its threads
      between the teeth of the wheel; -- called also a worm.
      

   Lag screw. See under Lag.

   Micrometer screw, a screw with fine threads, used for the
      measurement of very small spaces.

   Right and left screw, a screw having threads upon the
      opposite ends which wind in opposite directions.

   Screw alley. See Shaft alley, under Shaft.

   Screw bean. (Bot.)
       (a) The curious spirally coiled pod of a leguminous tree
           (Prosopis pubescens) growing from Texas to
           California. It is used for fodder, and ground into
           meal by the Indians.
       (b) The tree itself. Its heavy hard wood is used for
           fuel, for fencing, and for railroad ties.

   Screw bolt, a bolt having a screw thread on its shank, in
      distinction from a key bolt. See 1st Bolt, 3.

   Screw box, a device, resembling a die, for cutting the
      thread on a wooden screw.

   Screw dock. See under Dock.

   Screw engine, a marine engine for driving a screw
      propeller.

   Screw gear. See Spiral gear, under Spiral.

   Screw jack. Same as Jackscrew.

   Screw key, a wrench for turning a screw or nut; a spanner
      wrench.

   Screw machine.
       (a) One of a series of machines employed in the
           manufacture of wood screws.
       (b) A machine tool resembling a lathe, having a number of
           cutting tools that can be caused to act on the work
           successively, for making screws and other turned
           pieces from metal rods.

   Screw pine (Bot.), any plant of the endogenous genus
      Pandanus, of which there are about fifty species,
      natives of tropical lands from Africa to Polynesia; --
      named from the spiral arrangement of the pineapple-like
      leaves.

   Screw plate, a device for cutting threads on small screws,
      consisting of a thin steel plate having a series of
      perforations with internal screws forming dies.

   Screw press, a press in which pressure is exerted by means
      of a screw.

   Screw propeller, a screw or spiral bladed wheel, used in
      the propulsion of steam vessels; also, a steam vessel
      propelled by a screw.

   Screw shell (Zool.), a long, slender, spiral gastropod
      shell, especially of the genus Turritella and allied
      genera. See Turritella.

   Screw steamer, a steamship propelled by a screw.

   Screw thread, the spiral rib which forms a screw.

   Screw stone (Paleon.), the fossil stem of an encrinite.

   Screw tree (Bot.), any plant of the genus Helicteres,
      consisting of about thirty species of tropical shrubs,
      with simple leaves and spirally twisted, five-celled
      capsules; -- also called twisted-horn, and twisty.

   Screw valve, a stop valve which is opened or closed by a
      screw.

   Screw worm (Zool.), the larva of an American fly
      (Compsomyia macellaria), allied to the blowflies, which
      sometimes deposits its eggs in the nostrils, or about
      wounds, in man and other animals, with fatal results.

   Screw wrench.
       (a) A wrench for turning a screw.
       (b) A wrench with an adjustable jaw that is moved by a
           screw.

   To put the screws on or To put the screw on, to use
      pressure upon, as for the purpose of extortion; to coerce.
      

   To put under the screw or To put under the screws, to
      subject to pressure; to force.

   Wood screw, a metal screw with a sharp thread of coarse
      pitch, adapted to holding fast in wood. See Illust. of
      Wood screw, under Wood.
      [1913 Webster]

Micrometer \Mi*crom"e*ter\, n. [Micro- + -meter: cf. F.
   microm[`e]tre.]
   An instrument, used with a telescope or microscope, for
   measuring minute distances, or the apparent diameters of
   objects which subtend minute angles. The measurement given
   directly is that of the image of the object formed at the
   focus of the object glass.
   [1913 Webster]

   Circular micrometer, or Ring micrometer, a metallic ring
      fixed in the focus of the object glass of a telescope, and
      used to determine differences of right ascension and
      declination between stars by observations of the times at
      which the stars cross the inner or outer periphery of the
      ring.

   Double image micrometer, a micrometer in which two images
      of an object are formed in the field, usually by the two
      halves of a bisected lens which are movable along their
      line of section by a screw, and distances are determined
      by the number of screw revolutions necessary to bring the
      points to be measured into optical coincidence. When the
      two images are formed by a bisected object glass, it is
      called a divided-object-glass micrometer, and when the
      instrument is large and equatorially mounted, it is known
      as a heliometer.

   Double refraction micrometer, a species of double image
      micrometer, in which the two images are formed by the
      double refraction of rock crystal.

   Filar micrometer, or Bifilar micrometer. See under
      Bifilar.

   Micrometer caliper or Micrometer gauge (Mech.), a caliper
      or gauge with a micrometer screw, for measuring dimensions
      with great accuracy.

   Micrometer head, the head of a micrometer screw.

   Micrometer microscope, a compound microscope combined with
      a filar micrometer, used chiefly for reading and
      subdividing the divisions of large astronomical and
      geodetical instruments.

   Micrometer screw, a screw with a graduated head used in
      some forms of micrometers; turning the head one full
      revolution advances the position of the tip of the screw
      only by a little.

   Position micrometer. See under Position.

   Scale micrometer, or Linear micrometer, a minute and very
      delicately graduated scale of equal parts used in the
      field of a telescope or microscope, for measuring
      distances by direct comparison.
      [1913 Webster] Micrometric