tral Forces; conſequently 'tis impoſſible there ſhould be any other Principle.
Give me Leave once more to introduce Sir Iſaac ſpeaking: Shall he not be allow'd to ſay, My Caſe and that of the Ancients is very different. Theſe ſaw, for Inſtance, Water aſcend in Pumps, and ſaid, the Water riſes becauſe it abhors a Vacuum. But with regard to my ſelf, I am in the Caſe of a Man who ſhould have firſt obſerv'd that Water aſcends in Pumps, but ſhould leave others to explain the Cauſe of this Effect. The Anatomiſt who firſt declar'd, that the Motion of the Arm is owing to the Contraction of the Muſcles, taught Mankind an indiſputable Truth; but are they leſe oblig'd to him becauſe he did not know the Reaſon why the Muſcles contract? The Cauſe of the Elaſticity of the Air is unknown, but he who firſt diſcover'd this Spring perform'd a very ſignal Service to natural Philoſophy. The Spring that I diſcover'd was more hidden and more univerſal, and for that very Reaſon Mankind ought to thank
