Stephen Hales: A Life in Five Chapters

Chapter 1 · Fifty Years in One Parish

A curate who applied Newton to a cabbage

1677 – 1718 · Bekesbourne · Cambridge · Teddington

Stephen Hales was born at Bekesbourne in Kent in September 1677, into a landed family, and went to Corpus Christi College, Cambridge, intending the Church.

Cambridge in the 1690s was Newton's Cambridge. Hales attended lectures, worked through the *Principia*, and took from it not a set of results but a *method*: that nature is to be understood by measuring quantities and finding the relations between them. He also built apparatus — an orrery, a machine for demonstrating planetary motion — with William Stukeley, later Newton's biographer and the man who recorded the apple story.

He was ordained and in 1709 became perpetual curate of Teddington in Middlesex, a small parish on the Thames. He stayed for over fifty years, until his death, refusing preferment. He looked after the parish conscientiously — he supervised the building of a new water supply for the village, extended the church, and kept the registers carefully.

And in the garden and the parsonage he did his science.

The idea he brought to it was simple and, at the time, almost nobody had tried it. Newton had shown that forces and motions can be measured. Hales asked whether the same could be done to living things: whether a plant or an animal has quantities in it — flows, pressures, rates — that can be put in numbers.

He called this *statical* investigation, and he was elected to the Royal Society in 1718 for it.

“The great importance of the doctrine of statics in the animal and vegetable oeconomy.”

— Stephen Hales, Vegetable Staticks (1727)

Why this matters

Hales's contribution was to apply Newtonian measurement to living organisms, which nobody had systematically attempted before.

You have the curate with a balance and Newton on the shelf. What would you ask?

Ask Hales

• “What did you take from Newton that you could use on a plant?”
• “Why stay fifty years in one small parish?”
• “How did the parish work fit around the experiments?”
• “What does 'statical' mean in your sense of it?”
• “Did your faith and your measuring ever conflict?”

Chapter 2 · Weighing a Sunflower Every Day

How much water a plant loses, and where it goes

1719 – 1727 · Teddington

Hales took a sunflower growing in a pot, sealed the surface of the soil with lead sheet and putty so that water could escape only through the plant itself, left a small tube for watering, and weighed the whole thing every day.

The loss in weight is the water the plant has given off. He measured it repeatedly, in different weather, by day and by night.

A sunflower about three and a half feet high lost, on a warm dry day, somewhere around a pound of water — many hundreds of grams — in twelve hours. He then measured the surface area of the leaves and the surface area of a man's skin, and calculated that weight for weight the plant loses water something like seventeen times faster than a person perspires.

This is the quantity now called *transpiration*, and Hales established it: not that plants lose water, which anybody could guess, but *how much*, in units, compared with something else.

He went on to measure how the loss changes: greater in sunlight, greater in dry air, greater in moving air, less at night. Those are the conditions every school potometer experiment is still designed to test.

And he asked the obvious next question. If this much water leaves the top of the plant, something must be pulling or pushing it up from the roots. He set out to measure that too.

Why this matters

Hales gave transpiration a number and a comparison, which turned a vague observation into a quantity that could be explained.

You have the sealed pot and the daily weighing. What is your question?

Ask Hales

• “Why seal the surface of the soil with lead?”
• “How much water does a sunflower lose in a day?”
• “What conditions made the loss go up?”
• “Why compare a plant with a perspiring man?”
• “Where does all that water actually go?”

Chapter 3 · The Push From Below

Glass tubes on a cut vine, and a failure worth studying

1720s · Teddington

Hales cut a vine in spring, fitted a glass tube to the stump with a bladder seal, and watched sap rise up it. He then attached a mercury manometer and measured the *pressure* the roots were exerting.

It was substantial — enough to raise mercury by several inches, which corresponds to a column of water many feet high. This is *root pressure*, and Hales discovered it and measured it.

He also measured the pressure in the branches and stems of living trees, the rate at which sap flows, and the way the flow varies with the season and the time of day.

And then he did something admirable: he worked out that his own discovery could not explain what he wanted it to explain.

Root pressure raises water some feet. Tall trees are a hundred feet high or more, and some far higher. The push from the roots is nowhere near enough, and it also disappears in summer, when transpiration is at its greatest. Hales stated this plainly. He had found a real force and had shown, by his own measurements, that it was not the answer.

He suspected the leaves were doing the pulling, and he was essentially right, but he had no way to explain how a column of water could be pulled from above without breaking. The cohesion–tension mechanism — water molecules holding to each other strongly enough to be dragged up in an unbroken thread under tension — was not proposed until the 1890s.

Why this matters

Hales discovered root pressure and then published the demonstration that it is far too weak to lift sap up a tall tree — a model of reporting against your own result.

You have the pressure measured and the problem unsolved. What would you ask?

Ask Hales

• “How did you measure the push coming from the roots?”
• “Why is root pressure not enough to supply a tall tree?”
• “Why publish a result that undermines your own explanation?”
• “What did you think was pulling from the top?”
• “How can a column of water be pulled without snapping?”

Chapter 4 · A Tube in a Horse's Artery

The first measurement of blood pressure, 1733

1733 · Teddington

Having measured pressure in a plant, Hales did the same in an animal.

He tied down a horse, opened the crural artery, and inserted a brass pipe connected to a glass tube some nine feet long, held vertical. Blood rose in the tube to a height of about eight feet three inches, and rose and fell with each heartbeat.

That is the first direct measurement of arterial blood pressure in history, published in *Haemastaticks* in 1733. Every sphygmomanometer descends from it, and blood pressure is still expressed as the height of a column — millimetres of mercury — for exactly this reason.

He went further. He measured the output of the heart per beat, estimated the total volume of blood, calculated the velocity of blood in the arteries, and worked out the resistance offered by the fine vessels. He noted that the arteries expand elastically with each beat and recoil between them, smoothing a pulsed flow into a steadier one — the *Windkessel* effect, still the standard account.

The experiments were performed on living animals — horses, dogs, deer — and were criticised for cruelty in his own century, not only afterwards. His neighbour and friend the poet Alexander Pope, who lived at Twickenham, is reported to have found them distressing and said so about Hales personally, while otherwise liking the man.

Hales does not appear to have thought there was a difficulty. That his contemporaries did is part of the record.

Why this matters

Blood pressure is still expressed as the height of a column of liquid because that is literally how Hales first measured it.

You have the nine-foot tube and the horse. What is your question?

Ask Hales

• “How high did the blood actually rise?”
• “Why is blood pressure still given as a height?”
• “What do the arteries do between heartbeats?”
• “Pope thought these experiments cruel — what do you say?”
• “Could you have got the number any other way?”

Chapter 5 · Ventilators, Gin and a Long Old Age

A clergyman who counted deaths and changed them

1741 – 1761 · Teddington · Newgate · London

In 1741 Hales designed a *ventilator*: a large bellows, worked by hand or by a windmill, that forced fresh air into an enclosed space through ducts.

He had them installed in ships, in hospitals, in granaries and in prisons. The results were dramatic and were counted. At Newgate gaol, where typhus — *gaol fever* — killed prisoners constantly, deaths fell sharply after installation. On slave ships and naval vessels, mortality dropped. He did not know that the diseases were caused by micro-organisms; he thought foul air itself was the poison. The intervention worked anyway, because reducing crowding-borne infection does not require knowing what causes it.

He also campaigned against gin, which in the 1730s and 1740s was destroying poor communities in London, and gave evidence supporting the Gin Acts.

He was a trustee of the new colony of Georgia, was awarded the Royal Society's Copley Medal, refused a canonry offered by the Prince of Wales, and accepted only a chaplaincy to the Dowager Princess of Wales. Foreign academies elected him.

His wife Mary died young, after about five years of marriage, and he did not remarry. He worked into his eighties, latterly on distilling fresh water from sea water and on preserving meat.

He died at Teddington in January 1761, aged eighty-three, still curate of the same parish, and is buried under its church tower.

Why this matters

Hales's ventilators reduced deaths in prisons and ships decades before anybody knew what caused the diseases — an intervention that worked without a correct theory.

You have the bellows, the gaol and the counted deaths. What would you ask?

Ask Hales

• “How did you know the ventilators were working?”
• “What did you think was wrong with the air?”
• “Why campaign against gin?”
• “Why refuse advancement in the Church?”
• “What were you still trying to measure at eighty?”

What Hales changed

Hales turned the plant into something that could be measured — flow, force, quantity, rate — and every potometer and transpiration experiment descends from what he did in a parsonage garden with glass tubes and a balance. He made the first direct measurement of blood pressure, which is still expressed as the height of a column because of how he took it, and his ventilators reduced deaths in ships, prisons and hospitals decades before germ theory.

A debate that continues

Hales's experiments on living animals were criticised for cruelty in his own lifetime, including by his neighbour Alexander Pope, and he does not appear to have regarded the objection as serious.

Keep exploring — ask Hales

• “What else in a living thing would you have measured?”
• “Can an intervention be right for the wrong reason?”
• “What defeated you about the tall tree?”

Related lives

• Jan Ingenhousz — The One Who Found the Light
• Henri Dutrochet — Who Watched Water Cross A Living Skin
• Joseph Priestley — Discoverer of Dephlogisticated Air
• William Harvey — Discoverer of the Circulation

Related themes

Transpiration · Transport in plants · Blood pressure