One of anatomy's great mysteries was solved when William Harvey explained the mechanics of blood circulation in 1628. Remaining to be investigated was blood pressure, the force of the blood pressing against the inner walls of the blood vessels. Blood pressure is higher during cardiac systole, when the heart's ventricles contract, pumping blood into the arteries. Blood pressure is lower during cardiac diastole, when the ventricles relax.

Harvey had noted that when an artery was severed, blood was ejected in spurts. Stephen Hales, an English clergyman with an avid interest in physiology, became intrigued: how great was the pressure the arterial blood appeared to be under? Hales began exploring the mysteries of blood pressure and its measurement while a student at Cambridge University in the early 1700s and continued around 1712-13 at his parish in Teddington, England. He conducted a series of experiments on domestic animals, beginning...