52. Acts of third persons.

An obstruction which, if normally placed on a highway, would be lawful does not render the owner liable if it is tampered with and rendered dangerous by the unforeseeable act of a wrongdoer1, but a person who places an obstruction necessarily dangerous to users of the highway is not relieved from liability by the intervening act of a third person who moves it from one part of the highway to another2. Thus where a lorry was left on the highway, but in such a condition that it could not be set in motion except by four distinct operations, and