Source : Webster's Revised Unabridged Dictionary (1913)

Whereby \Where*by"\, adv.
   1. By which; -- used relatively. ``You take my life when you
      take the means whereby I life.'' --Shak.

   2. By what; how; -- used interrogatively.

            Whereby shall I know this?            --Luke i. 18.

Source : WordNet®

whereby
     adv 1: as a result of which
     2: by or through which; "the means whereby we achieved our
        goal"