The Collaborative International Dictionary of English v.0.48:

Rhythmic \Rhyth"mic\ (-m[i^]k), Rhythmical \Rhyth"mic*al\
   (-m[i^]*kal), a. [Gr. ????: cf. L. rhythmicus, F.
   rhythmique.]
   Pertaining to, or of the nature of, rhythm
   [1913 Webster]

         Day and night
         I worked my rhythmic thought.            --Mrs.
                                                  Browning.
   [1913 Webster]

   Rhythmical accent. (Mus.) See Accent, n., 6
   (c) .
       [1913 Webster]

WordNet (r) 3.0 (2006):

rhythmic
    adj 1: recurring with measured regularity; "the rhythmic chiming
           of church bells"- John Galsworthy; "rhythmical prose"
           [syn: rhythmical, rhythmic] [ant: unrhythmic,
           unrhythmical]