In heart... in the earth (or, better, in the land). — These in the text are in antithesis. The mischief conceived in the heart is weighed out, instead of justice, by these unjust magistrates. The balance of justice is thus turned into a means of wrong-doing. But, perhaps, we should rather arrange as follows:
Nay! with your heart ye work wickedness in the land,
With your hands you weigh out violence.