The Collaborative International Dictionary of English v.0.48:

Librate \Li"brate\ (l[imac]"br[=a]t), v. i. [imp. & p. p.
   Librated (l[imac]"br[asl]*t[e^]d); p. pr. & vb. n.
   Librating.] [L. libratus, p. p. of librare to balance, to
   make even, fr. libra. Cf. Level, Deliberate,
   Equilibrium.]
   To vibrate as a balance does before resting in equilibrium;
   hence, to be poised.
   [1913 Webster]

         Their parts all librate on too nice a beam. --Clifton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Librate \Li"brate\, v. t.
   To poise; to balance.
   [1913 Webster]

WordNet (r) 3.0 (2006):

librate
    v 1: determine the weight of; "The butcher weighed the chicken"
         [syn: weigh, librate]
    2: vibrate before coming to a total rest; "the children's swing
       librated"