Pound Lock. The vessel has entered pound from lower level on left, the gate has been closed behind it and water is being allowed to flow through gate on right, gradually raising the water level so that the boat can float out. Pound locks are used on canals, or to transfer vessles from one waterway to another at a different level. From Nouvelle Invention de Lever l'eau Plus Hault (New Invention for Raising Water ...), Isaac de Caus, ( London, 1664). Engraving.