The English scientist and clergyman Stephen Hales (1677-1761) pioneered the study of plant physiology, contributed the first major account of blood pressure, and invented a machine for ventilating buildings.

Stephen Hales was born in Bekesbourne, Kent, on September 17, 1677. He entered Corpus Christi College, Cambridge, in 1697, where he studied theology and took a degree in arts in 1703. He received his doctorate from Cambridge in 1733. At Cambridge, Hales was caught up in the backwash of Isaac Newton's great work at the university, and he acquired an interest in astronomy, physics, and chemistry as well as in biology. In 1708 Hales became perpetual curate of Teddington in Middlesex, and here he remained until his death. In 1719 he married Mary Newce, who died two years later without issue.

In 1711 Hales began his studies on blood pressure. True to his mechanistic views, he carefully measured the blood pressure of three horses and produced the first recorded estimates of blood pressure. Furthermore, he studied the pulse rates of various-sized animals and measured the heart's capacity to pump blood through the pulmonary veins. Hales also studied the effects of heat, cold, and various drugs on the blood vessels and experimented with animal reflexes.

Even though some research had been carried out by Jan van Helmont and Marcello Malpighi, Hales rightfully merits the title of father of plant physiology. Certainly in a century given over almost exclusively to the taxonomy of Carl Linnaeus, Hales's work was unique. In 1718, the year he became a member of the Royal Society, he read a paper entitled "Upon the Effect of ye Sun's warmth in raising ye Sap in trees." He then carefully measured sap pressure, velocity, and circulation in plants. Until this time sap circulation in plants was believed to parallel blood circulation in animals. Hales, however, clearly demonstrated that the transpiration of leaves draws the sap toward them at the same rate that the roots push sap upward. Furthermore, he found that plants draw some of their food from the gases in the air. He invented the pneumatic trough for collecting gases and developed gages and techniques to measure sap pressure.

Hales published his findings in Vegetable Staticks (1727), reissued in 1733 as volume 1 of his Statical Essays. Volume 2 was Haemastatics, primarily a summary of his earlier work on blood circulation. For his work he received the Copley Prize in 1739.

Hales thought his invention of a ventilator was his greatest contribution to the well-being of mankind. As early as 1741 Hales presented to the Royal Society a description of a ventilator to rid mines, prisons, hospitals, and shops of noxious airs. He published A Description of Ventilators (1743) and A Treatise of Ventilators (1758). Hales also sought ways to distill pure water from seawater, preserve meat and water for long ocean voyages, preserve foods in tropical climates, measure earthquakes, and prevent forest fires.

In 1751 Hales became clerk of the closet of the princess dowager, and subsequently he was made chaplain to her and to her son, the future George III. Though offered the canonry of Windsor by the royal family, Hales maintained an active ministry at Teddington until his death on January 4, 1761.

Further Reading

• The best work on Hales is Archibald E. Clark-Kennedy, Stephen Hales: An Eighteenth Century Biography (1929). General background studies include Charles Singer and E. Ashworth Underwood, A Short History of Medicine (1928; 2d ed. 1962), and Abraham Wolf, A History of Science, Technology, and Philosophy in the Eighteenth Century (1938; 2d rev. ed. 1952).