Deviation of the compass

deviation \de`vi*a"tion\, n. [LL. deviatio: cf. F.
   d['e]viation.]
   1. The act of deviating; a wandering from the way; variation
      from the common way, from an established rule, etc.;
      departure, as from the right course or the path of duty.
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   2. The state or result of having deviated; a transgression;
      an act of sin; an error; an offense.
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   3. (Com.) The voluntary and unnecessary departure of a ship
      from, or delay in, the regular and usual course of the
      specific voyage insured, thus releasing the underwriters
      from their responsibility.
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   4. (Statistics, Physics) the difference between an expected
      value of an observation or measurement and the actual
      value.
      [PJC]

   {Deviation of a falling body} (Physics), that deviation from
      a strictly vertical line of descent which occurs in a body
      falling freely, in consequence of the rotation of the
      earth.

   {Deviation of the compass}, the angle which the needle of a
      ship's compass makes with the magnetic meridian by reason
      of the magnetism of the iron parts of the ship.

   {Deviation of the line of the vertical}, the difference
      between the actual direction of a plumb line and the
      direction it would have if the earth were a perfect
      ellipsoid and homogeneous, -- caused by the attraction of
      a mountain, or irregularities in the earth's density.
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