JMS Pearce
Hull Royal Infirmary, United Kingdom

As a student I learned about Stephen Hales as the parish priest who first measured blood pressure — in a horse’s leg. The mists of time and waning memory have made his several astonishingly original works unknown to many. Samuel Johnson’s famous dictionary made few references to individuals, but one exception was Stephen Hales. Defining ‘Ventilator’, Johnson wrote ‘An instrument contrived by Dr Hale [sic] to supply close places with fresh air’.

Fig 1. Stephen Hales (1677–1761).
[Wikimedia Commons (Public Domain).

Stephen Hales, DD. FRS. (1677–1761) (Fig 1) was an eighteenth-century experimental physiologist.  Like Fr. Thomas Linacre (c. 1460–1524), Fr. Gabriele Falloppio (1523–62), Reverend Gilbert White FRS (1720-93), and many other priests, he was a well-educated amateur, probably of independent means, who although he devoted much of his life to religion and public service, pioneered experiments and observations in biology and physiology. Hales’s reputation, awe-inspiring in his time, was based on a fervent appetite for uncovering the fundamentals of animal and plant physiology.

He was born at Beakesbourne in Kent, the tenth child of Thomas Hales (1641–1692) and a descendant of Sir John Hales, Baron of the exchequer under Henry VIII. His mother, Mary Wood (1648–1687), hailed from Abbots Langley in Hertfordshire. Hales attended Reverend Richard Johnson's school in Orpington, Kent and was a pupil of the Reverend Mark Hildesley, of Murston, at a nearby parish (1694). Hildesley soon discovered his ‘too improveable genius, especially in the philosophical way, to be confined to a country parson's institutions’. He studied at Bene’t College (later named Corpus Christi College), Cambridge, receiving his BA., then MA. in 1703, and became a Fellow of the college. At this time there was a buzz of scientific activity in the university, which fired his imagination. Hales left Cambridge in 1709, but his fascination with science was firmly rooted.  He was appointed Perpetual Curate of the parish of Teddington in 1709 where he remained until his death. Hales married Mary Newce, daughter of the rector of Much Hadham ; she died probably in childbirth a year later.

Between parish duties, in his own laboratory he performed experiments relating to dissections, physiology, physics, chemistry, and botany. Hales began this work during his collaboration with William Stuckeley (MD., FRS.) at Cambridge. He constructed an orrery; Stukeley drew a picture of it and used the telescope which had been installed by Newton over Great Gate. The two men studied optics, astronomy, and microscopy. In a busy life, he published Vegetable staticks (1727) and Statical[1] essays (1733), founded principally on papers read before the Royal Society. As the local priest, amongst many activities he helped his parishioners to obtain a fresh water supply, and to improve the quality of the often ‘noxious air’ he invented artificial ventilators.

In later life, Hales became Clerk of the Closet to the Princess Dowager Augusta in 1751, and Chaplain to Prince George, afterwards George III.[i]  


Blood pressure

Hales began researches in a series of experiments on blood pressure at his Parsonage in Teddington, Middlesex, probably in December 1710[ii]:

I caused a mare to be tied down alive on her back; she was 14 hands high, and about 14 years of age,… having laid open the left crural artery about 3 inches from her belly, I inserted into it a brass pipe whose bore was 1/6 of an inch in diameter; and to that, by means of another brass pipe which was fitly adapted to it, I fixed a glass tube, of nearly the same diameter, which was 9 feet in length; then untying the ligature on the artery, the blood rose in the tube 8 feet 3 inches perpendicular above the level of the left ventricle of the left ventricle: … when it was at its full height, it would rise and fall at and after each pulse 2,3, or 4 inches.” [iii] (Fig 2.)


Fig 2. Stephen Hales. Statical Essays: containing Haemastaticks. London. Innys & Manby, 1733.


In different species he continued his studies on cardiac output, blood pressure, systolic velocity of the blood, and ‘time taken for the heart to expel a weight of blood equal to that of the animal’s body in minutes.’ With impressive originality he recognized two main contributors to arterial pressure: cardiac output and peripheral resistance.

Subsequent investigations into the arterial systems of animals appeared in 1733 under the title Haemastaticks. He expanded his researches, experimenting on food preservation and ventilation, and published Philosophical Experiments (1739), A Description of Ventilators (1743) (Fig 3,), and A Treatise on Ventilators (1758).


‘Natural history of vegetation’

His work first came to public notice when he read a paper to the Royal Society on the effect of the sun's rays on the sap of plants. His book, the Vegetable Staticks: or an account of some statical experiments on the sap in vegetables: being an essay towards a natural history of vegetation was published in 1727. In this book Hales related experiments using his ‘pneumatic trough’ to collect gases given off by plants; he tried to discover how plants take up and expel water.

He measured the water vapour emitted by plants known as transpiration, showing that it was the leaves that transpired and that this caused a continuous upward flow of water and nutrients from the roots. He determined the upward


Fig 4. Stephen Hales: leaves as lungs.

direction of sap flow, and measured its pressure. He also measured the rates of growth of shoots and leaves. (Vegetable Staticks, 1727) Hales compared the respiration of plants’ leaves with those of animals’ lungs (Fig 4). Some of his

conclusions were remarkably prescient; he postulated:

Light also, by freely entering the expanded surfaces of leaves and flowers, contribute much to the ennobling the principles of vegetables…

This followed Newton’s speculations, plainly hinting at photosynthesis years before its physiology was understood.

Not only did he experiment on plant physiology, but in 1758 he helped the Royal family in planning a heated greenhouse, 120 feet long, to be built at Kew gardens. A shrub found in Carolina by Alexander Garden was called Halesia carolina (silverbell or snowdrop tree): the genus was named after Hales by John Ellis in  Linnaeus's Systema Naturae in 1759.



Infections were recognized as common causes of illness and deaths in the foul-smelling, confined spaces of poorhouses, convict ships, and prisons where the condition was named ‘pestilential, infectious, jail distemper’. Hales invented a ventilator that used bellows designed to draw fresh air into such confined spaces. It was widely used and successfully fitted: on ships, in the House of Commons, the courts of justice, Drury Lane Theatre, and in hospitals, workhouses, and prisons throughout the country. After diligent research he published A Description of Ventilators (1743) (Fig 3), and A Treatise on Ventilators (1758).[iv] He advocated the regular burning of brimstone and cleansing with vinegar to eliminate contagion. The mortality rates fell dramatically in these places, although the bacterial nature of infection was not then known. He produced distilled fresh water from Seawater, by blowing [with his ventilator] Showers of Air up through the Distilling Liquor.


Fig 3. Stephen Hales. A description of
Ventilators, London 1743

Electrical conduction in nerves

The nervous system as the regulator of physiological functions had been suspected since the time of Descartes. As a result, the postulated spiritus vitalis was located in the nerves. In the 3rd edition of Haemastatics, 1740,  (p.55.) Hales made the startling suggestion that perhaps nerves functioned by conducting ‘electrical powers.’ This was based on his experiments in which he decapitated a frog and thirty hours later observed it moved its body when stimulated, and convulsed when he thrust a needle down the spinal marrow. This was the first observation of spinal reflexes and their abolition by destruction of the cord. It was quoted in  Robert Whytt’s (1714-66) Physiological Essays, 1755 before Whytt’s own  description of reflex action in 1757. [v],[vi] Proof of the electrical nature of nerve conduction and muscle contraction came later from Luigi Galvani's De viribus electricitatis in motu musculari commentarius, 1791.  

Yet another diversion arose when he was involved in the claims of a Mrs. Joanna Stephens for curing victims of the bladder complaint known as ‘the stone’. This led Parliament to set up a group of trustees to appraise the effectiveness of her nostrum. Hales, one of the trustees, set to work, but failed to find a solvent for stones. Paradoxically this led to his award of the Copley medal by the Royal Society in 1739. He had been elected FRS (Fellow of the Royal Society) on 13 March, 1718.

As a total abstainer he supported the unpopular Gin Act of 1736, attempting to restrict the sale of gin through penal excise duty. Warning notices had appeared all over London, for example:  "Drunk for a penny, dead drunk for tuppence, clean straw for nothing". Hales wrote A Friendly Admonition to the Drinkers of Brandy and other Distilled Spirituous Liquors (1734). He was much more vehement as an opponent of slavery, and was a trustee of the ‘non-slave colony’ of Georgia.

Henry Guerlac gives an excellent, comprehensive account of Hales’s diverse experiments. [vii]



From his portrait in the National Portrait Gallery dated 1759, Hales appears a kindly man with handsome features. In old age he was revered as a national figure. He became ‘the greatest physiologist since Harvey.’2 But in the course of his scientific explorations dissent and argument were no strangers to him. Yet this did nothing to alter what Peter Collinson called his ‘native innocence and simplicity of manners’, or ‘to awaken in him ambition either for honours or preferment.’ (Collinson,1 pp. 273–8).  There is a contrast between his objective scientific methods, his search for truths behind natural phenomena and his almost mystical beliefs. Amongst several esteemed scientists of The Enlightenment  (late 17th and early 18th century) such belief in miracles was common. Discussing earthquakes, Hales related in The Philosophical Transaction Royal Society (April, 1750) that ‘the ordinary course of nature is as much carried on by the divine agency, as the extraordinary and miraculous events’: a view shared by Linnaeus; and that God sometimes ‘changes the Order of Nature, with Design to chastise Man for his Disobedience and Follies…’

To the evangelical Methodist Charles Wesley, Hales was ‘a truly pious, humble Christian’ (The Journal of the Revd. Charles Wesley, 1849).  His brother John Wesley, also visited him, was shown several experiments, and noted in his diary:

How well did science and religion agree in that man of sound understanding.

Alexander Pope, a great dog lover and opponent of vivisection, was Hales's friend in Middlesex, but criticized him for his experiments on live animals:

Yes he is a very good man, only - I'm sorry - he has his hands imbrued with blood.…Indeed, he commits most of these barbarities with the thought of it being of use to man. But how do we know that we have a right to kill creatures that we are so little above as dogs, for our curiosity, or even for some use to us?
(The Works of Alexander Pope, ed. J. Wharton, 1797, 3.222)

However Hales was well aware of the suffering caused by his vivisection experiments, but although he believed they would have therapeutic benefits for man:

I… did not pursue the Mater any further being discouraged by the Disagreeableness of Anatomical Dissections. (Introduction of 3)

After the publication of Haemastaticks in 1733 he is said to have stopped animal experiments.

Gilbert White noted that:

‘His whole mind seemed replete with experiment which of course gave a tincture and turn to his conversation often somewhat peculiar, but always interesting’.

His last act of benevolence’ that White described was in a street in Farringdon, where Hales was ‘busy painting white the tops of the foot-path posts, that his neighbours might not be injured by running against them in the dark’.

After a brief illness he died at Teddington on 4 January 1761, aged 83, and was buried according to his wishes beneath the tower of Teddington Church. A monument in Westminster Abbey, paid for by the Princess Augusta, carved by Wilton bears the inscription:

 ‘Well versed... in ministering to the ills of mankind, in exploring the works of the Lord’ and proclaims that ‘the passing of ages shall not diminish your praise and renown, O Hales. It is England’s boast to number you among her distinguished sons, alongside her dear Newton: O proud land of England.’


[1] Staticks and statical referred to Hales’s insistence on objective measurements.



1. Collin son, P. Some Account of the Life of the Late Excellent and Eminent Stephen Hales D.D., F.R.S. Gentleman's Magazine 1761; p. 32, 44; 1764, p. 273-278.
2. Clark-Kennedy, A.E. Stephen Hales, D.D. F.R.S. Cambridge University Press. 1929
3. Hales, S. Statical Essays, 2 vols. (London, 1738–1740), vol. I is 3rd ed. of Vegetable Staticks and vol. II is 2nd ed., “corrected,” of Haemastaticks; and Statical Essays, 2 vols. (London, 1769), vol. I is 4th ed. of Vegetable Staticks, and     vol. II is 3rd ed. of Haemastaticks. Statical Essays containing Haemastatics, Volume 2, 2nd edition p. 1. 1740
4. Allan D. G. C.  Hales, Stephen (1677–1761), Oxford Dictionary of National Biography, Oxford University Press, 2004; online edn, Oct 2006 [, accessed 29 Nov 2016] see also: D G C Allan & R E Schofield, Stephen Hales Scientist and Philanthropist, Scolar Press, 1980
5. Pearce JMS. Robert Whytt and the stretch reflex. J, Neurol Neurosurg Psychiatry 1997;62:484
6. Becker RO,  Marino AA. Electromagnetism & Life 1982.
7. Guerlac H. Hales, Stephen. Complete Dictionary of Scientific Biography. . Retrieved December 06, 2016 from



JMS PEARCE, MD, FRCP, is an Emeritus Consultant Neurologist at Hull Royal Infirmary.