Hales's measurement of blood pressure

One December, Stephen Hales had a mare tied down alive on her back. She was fourteen hands high, about fourteen years old, with a fistula on her withers. He opened the left crural artery about three inches from her belly, tied in a brass pipe and joined it to a glass tube nine feet long. When he untied the ligature the blood rushed half way up at once, then climbed with each beat until it stood 8 feet 3 inches above the level of the heart, rising and falling a few inches with every pulse.

Hales (1677 to 1761) was perpetual curate of Teddington in Middlesex from 1709 until his death and fitted his experiments around his parish duties. The horses he used were due to be killed as unfit for service. On a white mare he measured the pressure in the jugular vein and then in the carotid artery, joining the brass pipe to the tube with the windpipe of a goose so that the joint would give when the mare struggled. The blood rose to 9 feet 6 inches. He then let out blood 60 cubic inches at a time, measuring the height after each loss, until the mare died.

The Royal Society published his papers in 1733 as Statical Essays: containing Haemastaticks, 22 years after his first measurement of blood pressure. He estimated that blood entered the aorta of a mare during each contraction at 86.7 feet a minute, and he compared the elastic arteries to the air chamber of a fire engine, which turned the jerks of the pump into a more even spout. The analogy is now known by the German word Windkessel, meaning air chamber.

Hales also designed ventilators, which were fitted in the House of Commons and in ships, prisons and hospitals. He died on 4 January 1761 and was buried with his wife at St Mary's church, Teddington. In 1762 Princess Augusta, mother of George III, put up a monument to him by Joseph Wilton in the south transept of Westminster Abbey.