The Collaborative International Dictionary of English v.0.48:

Equiponderant \E`qui*pon"der*ant\, a. [Cf. F.
   ['e]quipond['e]rant.]
   Being of the same weight.
   [1913 Webster]

         A column of air . . . equiponderant to a column of
         quicksilver.                             --Locke.
   [1913 Webster]