Today is the 322nd anniversary of the birth of Stephen Hales, an English clergyman known for his careful biological research, particularly on the physiology and growth of plants. Hales conducted a number of experiments between 1719 and 1725 in which he measured the pressure exerted by water moving through a plant. He attached a glass tube or a simple manometer filled with mercury to a cut stem surface and measured the change in height of the sap, which indicated root pressure. Hales made similar measurements of blood pressure in domestic animals, and by injecting chemicals, estimated blood-flow rates in organs. Hales also discovered the dangers of breathing "spent" air in enclosed spaces and invented a ventilator, which he introduced on ships and in hospitals and prisons.
[Source: Roy Porter, Ed., The Biographical Dictionary of Scientists (Oxford University Press, ed. 2, 1994).]