The Collaborative International Dictionary of English v.0.48:

Barrel \Bar"rel\ (b[a^]r"r[e^]l), n.[OE. barel, F. baril, prob.
   fr. barre bar. Cf. Barricade.]
   1. A round vessel or cask, of greater length than breadth,
      and bulging in the middle, made of staves bound with
      hoops, and having flat ends or heads; as, a cracker
      barrel. Sometimes applied to a similar cylindrical
      container made of metal, usually called a drum.
      [1913 Webster +PJC]

   2. The quantity which constitutes a full barrel. This varies
      for different articles and also in different places for
      the same article, being regulated by custom or by law. A
      barrel of wine is 311/2 gallons; a barrel of flour is 196
      pounds.
      [1913 Webster]

   3. A solid drum, or a hollow cylinder or case; as, the barrel
      of a windlass; the barrel of a watch, within which the
      spring is coiled.
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   4. A metallic tube, as of a gun, from which a projectile is
      discharged. --Knight.
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   5. A jar. [Obs.] --1 Kings xvii. 12.
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   6. (Zool.) The hollow basal part of a feather.
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   Barrel bulk (Com.), a measure equal to five cubic feet,
      used in estimating capacity, as of a vessel for freight.


   Barrel drain (Arch.), a drain in the form of a cylindrical
      tube.

   Barrel of a boiler, the cylindrical part of a boiler,
      containing the flues.

   Barrel of the ear (Anat.), the tympanum, or tympanic
      cavity.

   Barrel organ, an instrument for producing music by the
      action of a revolving cylinder.

   Barrel vault. See under Vault.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Boiler \Boil"er\, n.
   1. One who boils.
      [1913 Webster]

   2. A vessel in which any thing is boiled.
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   Note: The word boiler is a generic term covering a great
         variety of kettles, saucepans, clothes boilers,
         evaporators, coppers, retorts, etc.
         [1913 Webster]

   3. (Mech.) A strong metallic vessel, usually of wrought iron
      plates riveted together, or a composite structure
      variously formed, in which steam is generated for driving
      engines, or for heating, cooking, or other purposes.
      [1913 Webster]

   Note: The earliest steam boilers were usually spheres or
         sections of spheres, heated wholly from the outside.
         Watt used the wagon boiler (shaped like the top of a
         covered wagon) which is still used with low pressures.
         Most of the boilers in present use may be classified as
         plain cylinder boilers, flue boilers, sectional and
         tubular boilers.
         [1913 Webster]

   Barrel of a boiler, the cylindrical part containing the
      flues.

   Boiler plate, Boiler iron, plate or rolled iron of about
      a quarter to a half inch in thickness, used for making
      boilers and tanks, for covering ships, etc.

   Cylinder boiler, one which consists of a single iron
      cylinder.

   Flue boilers are usually single shells containing a small
      number of large flues, through which the heat either
      passes from the fire or returns to the chimney, and
      sometimes containing a fire box inclosed by water.

   Locomotive boiler, a boiler which contains an inclosed fire
      box and a large number of small flues leading to the
      chimney.

   Multiflue boiler. Same as Tubular boiler, below.

   Sectional boiler, a boiler composed of a number of
      sections, which are usually of small capacity and similar
      to, and connected with, each other. By multiplication of
      the sections a boiler of any desired capacity can be built
      up.

   Tubular boiler, a boiler containing tubes which form flues,
      and are surrounded by the water contained in the boiler.
      See Illust. of Steam boiler, under Steam.

   Tubulous boiler. See under Tubulous. See Tube, n., 6,
      and 1st Flue.
      [1913 Webster]