Screw

[Old Fr.] A round piece of wood or metal with a sloping ridge called a thread run¬ ning round it for fastening things together. As a mechanical power the screw is a modification of the inclined plane. The flipper of the seabear, the wing of the insect, of the bat, and of the bird are screws in principle, resembling the blade of the propeller, and they twist and un¬ twist during oscillation.