SELECTIVE DOUBLE JAW TRAP
Catches the animal, measures it, and releases the small ones.
A small animal steps on the platform, releasing the small jaws, which close and in turn trip a lever releasing the large jaws. If the animal is small enough that the large jaws close completely, an extension on the large jaws pushes a tension lever out of its notch, which disables the small-jaw spring and frees the animal. If the animal is large enough to keep the large jaws apart, the tension lever stays in its notch and the animal remains held by both sets of jaws.
“It will be seen, therefore, that the animal of large size will be held in effect by two traps whilst the animal of small size will be held for a moment by the smaller jaws but when, as it were, the selecting action of the larger jaw has been effective the smaller jaws will be rendered ineffective by the complete release and loosening of spring 6 and the animal will be finally released.”— Joseph D. Zahm, from the specification
And the world did not beat a path to his door.