Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

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