1 Metaphors for unstable

He had, it is true, in his turn pledged himself to the King that he would no longer oppose the measures of the minister; but the pledges of Monsieur were known to be as unstable as water; and his chivalrous spirit was, moreover, aroused by the harsh treatment of his young and beautiful sister-in-law, with whom he passed a great portion of his time.

1 Metaphors for  unstable