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]