The Collaborative International Dictionary of English v.0.48:

Tunnel \Tun"nel\ (t[u^]n"n[e^]l), n. [F. tonnelle a
   semicircular, wagon-headed vault, a tunnel net, an arbor, OF.
   also tonnel; dim. of tonne a tun; -- so named from its
   resemblance to a tun in shape. See Ton.]
   1. A vessel with a broad mouth at one end, and a pipe or tube
      at the other, for conveying liquor, fluids, etc., into
      casks, bottles, or other vessels; a funnel.
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   2. The opening of a chimney for the passage of smoke; a flue;
      a funnel.
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            And one great chimney, whose long tunnel thence
            The smoke forth threw.                --Spenser.
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   3. An artificial passage or archway for conducting canals,
      roads, or railroads under elevated ground, for the
      formation of roads under rivers or canals, and the
      construction of sewers, drains, and the like.
      [1913 Webster]

   4. (Mining) A level passage driven across the measures, or at
      right angles to veins which it is desired to reach; --
      distinguished from the drift, or gangway, which is led
      along the vein when reached by the tunnel.
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   Tunnel head (Metal.), the top of a smelting furnace where
      the materials are put in.

   Tunnel kiln, a limekiln in which coal is burned, as
      distinguished from a flame kiln, in which wood or peat
      is used.

   Tunnel net, a net with a wide mouth at one end and narrow
      at the other.

   Tunnel pit, Tunnel shaft, a pit or shaft sunk from the
      top of the ground to the level of a tunnel, for drawing up
      the earth and stones, for ventilation, lighting, and the
      like.
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