The Collaborative International Dictionary of English v.0.48:

Attemper \At*tem"per\, v. t. [imp. & p. p. Attempered; p. pr.
   & vb. n. Attempering.] [OF. atemprer, fr. L. attemperare;
   ad + temperare to soften, temper. See Temper, and cf.
   Attemperate.]
   1. To reduce, modify, or moderate, by mixture; to temper; to
      regulate, as temperature.
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            If sweet with bitter . . . were not attempered
            still.                                --Trench.
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   2. To soften, mollify, or moderate; to soothe; to temper; as,
      to attemper rigid justice with clemency.
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   3. To mix in just proportion; to regulate; as, a mind well
      attempered with kindness and justice.
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   4. To accommodate; to make suitable; to adapt.
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            Arts . . . attempered to the lyre.    --Pope.
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   Note: This word is now not much used, the verb temper taking
         its place.
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