TRAP.
One trap, adjustable to impale either mice or rats.
A wire cage carries a row of fixed, inwardly curved pointed hooks at its entrance, with a set of pointed spear-hangers suspended on an adjustable bar above them. Raising or lowering the bar widens or narrows the entry slot, setting the trap for rats or for mice. The animal pushes past the swinging hangers to reach bait suspended inside. On attempting to leave through the same slot, it meets the points of the hooks and hangers; if it presses against them, it is impaled.
“If, however, the animal should attempt to worry itself and force itself against the hook and hanger points, it would simply result in impaling itself upon the hooks and hangers.”— John Shepherd, from the specification
And the world did not beat a path to his door.