Which preposition to use with scalae

in Occurrences 1%

i. part 2, p. 7. 'Opposuit ei providentia Nisbetum: qui summâ doctrinâ consummatâque eloquentiâ causas agebat, ut justitiae scalae in aequilibrio essent; nimiâ tamen arte semper utens artem suam suspectam reddebat.

Which preposition to use with  scalae