Water tube

Tube \Tube\, n. [L. tubus; akin to tuba a trumpet: cf F. tube.]
   1. A hollow cylinder, of any material, used for the
      conveyance of fluids, and for various other purposes; a
      pipe.
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   2. A telescope. "Glazed optic tube." --Milton.
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   3. A vessel in animal bodies or plants, which conveys a fluid
      or other substance.
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   4. (Bot.) The narrow, hollow part of a gamopetalous corolla.
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   5. (Gun.) A priming tube, or friction primer. See under
      {Priming}, and {Friction}.
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   6. (Steam Boilers) A small pipe forming part of the boiler,
      containing water and surrounded by flame or hot gases, or
      else surrounded by water and forming a flue for the gases
      to pass through.
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   7. (Zool.)
      (a) A more or less cylindrical, and often spiral, case
          secreted or constructed by many annelids, crustaceans,
          insects, and other animals, for protection or
          concealment. See Illust. of {Tubeworm}.
      (b) One of the siphons of a bivalve mollusk.
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   8. (Elec. Railways) A tunnel for a tube railway; also
      (Colloq.), a tube railway; a subway. [Chiefly Eng.]

   Note: In the New York area, the subways running under the
         Hudson River are sometimes referred to as the tube.
         [Webster 1913 Suppl. +PJC]

   {Capillary tube}, a tube of very fine bore. See {Capillary}.
      

   {Fire tube} (Steam Boilers), a tube which forms a flue.

   {Tube coral}. (Zool.) Same as {Tubipore}.

   {Tube foot} (Zool.), one of the ambulacral suckers of an
      echinoderm.

   {Tube plate}, or {Tube sheet} (Steam Boilers), a flue plate.
      See under {Flue}.

   {Tube pouch} (Mil.), a pouch containing priming tubes.

   {Tube spinner} (Zool.), any one of various species of spiders
      that construct tubelike webs. They belong to {Tegenaria},
      {Agelena}, and allied genera.

   {Water tube} (Steam Boilers), a tube containing water and
      surrounded by flame or hot gases.
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Water tube \Wa"ter tube`\ (w[add]"t[~e]r t[=u]b`).
   1. (Zool.) One of a system of tubular excretory organs having
      external openings, found in many invertebrates. They are
      believed to be analogous in function to the kidneys of
      vertebrates. See Illust. under {Trematodea}, and
      {Sporocyst}.
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   2. Any tube for passing or holding water; specif., in some
      steam boilers, a tube in which water circulates and steam
      is generated.
      [Webster 1913 Suppl.]