The Collaborative International Dictionary of English v.0.48:

Wherethrough \Where*through"\, adv.
   Through which. [R.] "Wherethrough that I may know."
   --Chaucer.
   [1913 Webster]

         Windows . . . wherethrough the sun
         Delights to peep, to gaze therein on thee. --Shak.
   [1913 Webster]