The Collaborative International Dictionary of English v.0.48:

Experimentally \Ex*per`i*men"tal*ly\, adv.
   By experiment; by experience or trial. --J. S. Mill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

experimentally
    adv 1: in an experimental fashion; "this can be experimentally
           determined" [syn: experimentally, by experimentation,
           through an experiment]