Source : Webster's Revised Unabridged Dictionary (1913)

Perpendicularly \Per`pen*dic"u*lar*ly\, adv.
   In a perpendicular manner; vertically.

Source : WordNet®

perpendicularly
     adv 1: straight up or down without a break [syn: {sheer}]
     2: in a perpendicular manner; "this red line runs
        perpendicularly to the green line"