The Collaborative International Dictionary of English v.0.48:

Trone \Trone\, Trones \Trones\, n. [LL. trona, fr. L. trutina a
   balance; cf. Gr. ?.]
   1. A steelyard. [Prov. Eng.]
      [1913 Webster]

   2. A form of weighing machine for heavy wares, consisting of
      two horizontal bars crossing each other, beaked at the
      extremities, and supported by a wooden pillar. It is now
      mostly disused. [Scot.] --Jamieson.
      [1913 Webster]

   Trone stone, a weight equivalent to nineteen and a half
      pounds. [Scot.]

   Trone weight, a weight formerly used in Scotland, in which
      a pound varied from 21 to 28 ounces avoirdupois.
      [1913 Webster]