'Plumbline' in the Bible

Thus he shewed me: and, behold, the Lord stood upon a wall made by a plumbline, with a plumbline in his hand.

And the LORD said unto me, Amos, what seest thou? And I said, A plumbline. Then said the Lord, Behold, I will set a plumbline in the midst of my people Israel: I will not again pass by them any more: