Emily Dickinson Lexicon

equilibrium, n. [L. æquss, equal + lībra, balance.]

Condition of equal balance between opposing forces or powers; balancing of the mind between motives or reasons: a state of indifference or of doubt, when the mind is suspended in indecidion, between different motives, or the different forces of evidence (NW) [this second definition may be possible because of the word “evidence”. ED may be using this as a word play because in the poem it seems that she may be referring to the fact that rats are evasive and not often seen, but the evidence that they were there is left in plain view.]