alternately

from WordNet (r) 3.0 (2006)

alternately
    adv 1: in an alternating sequence or position; "They were
           deglycerolized by alternately centrifuging and mixing";
           "he planted fir and pine trees alternately"

from The Collaborative International Dictionary of English v.0.48

Alternately \Al*ter"nate*ly\, adv.
   1. In reciprocal succession; succeeding by turns; in
      alternate order.
      [1913 Webster]

   2. (Math.) By alternation; when, in a proportion, the
      antecedent term is compared with antecedent, and
      consequent.
      [1913 Webster]