Whereby

from The Collaborative International Dictionary of English v.0.48

Whereby \Where*by"\, adv.
   [1913 Webster]
   1. By which; -- used relatively. "You take my life when you
      take the means whereby I life." --Shak.
      [1913 Webster]

   2. By what; how; -- used interrogatively.
      [1913 Webster]

            Whereby shall I know this?            --Luke i. 18.
      [1913 Webster]