  Newton presents his first two laws as already “accepted by mathematicians and confirmed by experiments of many kinds” [P,  424].[24] For the third law, by contrast, he offers a variety of forms of support, including experiments on impact. One important element that becomes clear in his discussion of evidence for the third law — and also in Corollary 2 — is that Newton's impressed force is the same as static force that had been employed in the theory of equilibrium of devices like the level and balance for some time. Newton is not introducing a novel notion of force, but only extending a familiar notion of force. Indeed, Huygens too had employed this notion of static force in his Horologium Oscillatorium when he identified his centrifugal force with the tension in the string (or the pressure on a wall) retaining an object in circular motion, in explicit analogy with the tension exerted by a heavy body on a string from which it is dangling. Huygens's theory of centrifugal force was going beyond the standard treatment of static forces only in its inferring the magnitude of the force from the motion of the body in a circle. Newton's innovation beyond Huygens was first to focus not on the force on the string, but on the correlative force on the moving body, and second to abstract this force away from the mechanism by which it acts on the body. Three steps were thus involved in passing from the already familiar static forces to the more abstract Newtonian “dynamic” forces, one by Huygens and two by  Newton.[25]