Читать книгу Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley - John Gribbin - Страница 9

At the end of the 1650s, England was once again plunged into political turmoil. Oliver Cromwell died on 3 September 1658, and was succeeded by his son Richard, a less competent administrator unable to cope with a Commonwealth that was already in difficulties, with mounting debts and rival factions. In April 1659 Richard was pushed aside and the army took over, raising the prospect of another civil war. Many people who were in a position to do so, Hooke among them, started to make contingency plans. Hooke’s youthful imagination had been caught by the sight of the ships entering and leaving Yarmouth, and he now began to consider life as an adventurer and explorer travelling to the Far East. In May 1659, still not yet twenty-four years old, he read a book, Itenerario, written by a Dutch traveller, Jan van Linschoten, and made notes, which survive, about the kind of life he could expect if he followed in van Linschoten’s wake. He took particular note of the attractions of China, where ‘Schollars are highly esteemed’. But before Hooke’s plans could come to fruition – if they were ever more than a pipe dream – in the spring of 1660 Charles II was welcomed back to England, and the monarchy was restored. A wave of optimism swept the country, and Hooke, from the staunchly Royalist Isle of Wight, abandoned his plans to travel and looked forward to a future in England, where he was securely established with Boyle and had a growing reputation among the wider circle of experimental philosophers. He published his first scientific paper (as we would now call it), on capillary action, in 1661. But by then, the centre of experimental philosophy was shifting from Oxford to London.

More precisely, the scientific activity was centred around an institution known as Gresham College, in the City of London (the edifice known as Tower 42 now stands on the site, between Broad Street and Bishopsgate). In Hooke’s day, the building on that site was a large Elizabethan mansion, once owned by a wealthy merchant, Thomas Gresham. A range of buildings surrounded a square courtyard roughly a hundred yards across. Gresham had died in 1579, and left the income from his investments to have the house converted into a college and to pay for the appointment of seven ‘professors’ in perpetuity. The professors would be provided with an income of £50 a year for life, and rooms in the college, in return for giving lectures in their specialist subjects once a week in term time. The specialist subjects chosen by Gresham were law, physic (medicine), divinity, rhetoric, music, chemistry and astronomy. The professors were also required to be celibate, although as we shall see the interpretation of this term was rather loose. The status of these posts has waxed and waned over the years, but there are still Gresham Professors giving lectures, even though they no longer have a college to live in.

Hooke’s Oxford friend, Christopher Wren, had become the Gresham Professor of Astronomy in 1657, a post he held until 1661, when he returned to Oxford as Savilian Professor of Astronomy. Other experimental philosophers based in, or visiting, London (and crucially including Wilkins, who had become the Master of Trinity College in Cambridge in 1659, but was ejected when the Royalists returned to power, and was now lodging with a friend in Gray’s Inn) used to attend Wren’s lectures, and got into the habit of meeting up afterwards to discuss the topics raised and other scientific matters. On 28 November 1660, after one of Wren’s lectures, the group decided (clearly by prior arrangement) to formalise these gatherings. A record in the Royal Society archive reads:

Memorandum November 28, 1660. These persons following according to the custom of most of them, met together at Gresham College to hear Mr Wren’s lecture, viz. the Lord Brouncker, Mr Boyle, Mr Bruce, Sir Richard Moray, Sir Paule Neile, Dr Wilkins, Dr Goddard, Dr Petty, Mr Ball, Mr Rooke, Mr Wren. And after the lecture was ended they did according to the usual manner, withdraw for mutual converse.

That ‘mutual converse’ led to the resolution that they would form an association ‘for the promoting of Experimentall Philosophy’ and:

That this company would continue their weekly meetings on Wednesday, at 3 of the clock in the term time, at Mr Rooke’s chamber at Gresham College; in the vacation at Mr Ball’s chamber in the Temple, and towards the defraying of expenses, every one should, at his first admission, pay downe ten shillings and besides engage to pay one shilling weekly … Dr Wilkins was appointed to the Chair, Mr Ball to be Treasurer, and Mr Croone, though absent, was named the Registrar.

This was the beginning of the Royal Society, whose members became known as ‘Fellows’. Because of Wilkins’ reputation as a Parliamentarian, it became politic for him to take a back seat (at least formally), and Sir Robert Moray was installed as President of the fledgling association on 6 March 1661. In no small measure thanks to his skill at political wheeling and dealing, the Society gained its first Royal Charter in 1662, with Brouncker now named as President, but this Charter proved unsatisfactory (for obscure reasons), and was replaced by a second Charter in 1663, formalising the name as ‘the Royal Society of London for Promoting Natural Knowledge’.^fn1 The Society had a coat of arms, and a motto, Nullius in Verba, which can be translated as ‘take nobody’s word for it’. In other words, carry out experiments and test hypotheses for yourself, do not rely on hearsay. It would be Hooke who soon put that fine sentiment into practice. We shall always refer to the institution as the Royal Society (even for the period before the award of the first charter), the Royal, or the Society; one of the aims of seeking royal status was to get financial support from the King, which was never forthcoming, but the status did encourage rich dilettantes to offer their support, if only by becoming Fellows and (sometimes) paying their subscriptions.

As early as December 1660, the Society laid out the ground rules for doing experiments, and recognised the need for ‘curators’ who would carry out the experiments. At first, this role was carried out by the most expert Fellows (known as virtuosi), but this was not a success, and it became clear that they needed somebody who could do the job full time. In the early 1660s, Boyle was spending some of his time in Oxford and part at his sister’s house in London, where he had a laboratory. Hooke accompanied him and was well known to the Fellows (his little paper on capillary action is mentioned in their records). By 1661, Boyle and Hooke were developing an improved air pump, and Boyle gave their original pump to the Royal, where it languished with nobody able to operate it satisfactorily. This was another indication of the need for a skilled curator who could make things work. And who better than the man who had designed and built that pump?

So on 12 November 1662 Sir Robert Moray proposed, and the Fellows accepted, that Hooke should be appointed Curator of Experiments ‘to furnish them every day when they met, with three or four considerable Experiments’, as well as following up topics for investigation suggested by the Fellows. The only snag was, the Royal did not have any funds with which to pay him. The solution was that in effect Boyle ‘lent’ Hooke to the Royal Society until 1665, when a combination of circumstances (not all of them straightforwardly honest) stabilised the situation.

The Royal had notionally set Hooke’s salary as £80 a year, even though they were not paying it. Nor were they able to provide him with accommodation, so he had to make do with temporary lodgings. Partly as compensation, in recognition of his value he was elected as a Fellow of the Royal Society on 5 June 1663, with all the usual fees and subscriptions being waived. The prospect of establishing the relationship on a proper basis came in May 1664, when Isaac Barrow (the successor to the Laurence Rooke in whose rooms the Royal had its early meetings) resigned his post as Gresham Professor of Geometry to become the first Lucasian Professor of Mathematics in Cambridge (where he came across a student called Isaac Newton, who later became the second Lucasian Professor). Before he left for Cambridge, Barrow had been giving some of the astronomy lectures in place of Dr Walter Pope, Wren’s successor, who was temporarily away from London. After Barrow left, Hooke took on those temporary duties, and received the appropriate stipend, while Pope was away. Who better to be Barrow’s replacement?

There were two candidates for the post: Hooke, who had strong support from the Royal, and a physician, Arthur Dacres. On 20 May 1664, a committee (‘The Court’) met to decide between them, and duly announced their verdict:

two learned persons viz. Dr Arthur Dacres and Mr Robert Hooke being suited for the same, their petitiones being Read their ample Certificates considered and the matter debated The Court proceeded to election and made thereof the said Dr Dacres to supply the said place of Geometry Reader in the College.

A few days later, perhaps while drowning his sorrows, Hooke bumped into a wealthy merchant, Sir John Cutler, in a public house. He knew Cutler through a mutual friend, and gloomily recounted the tale. Cutler’s response was to tell Hooke to cheer up, because he, Cutler, would provide the financial support Hooke needed by creating a post for him to lecture on the History of Trades, at the same remuneration as a Gresham Professor – £50 a year. Before the arrangement could be formalised, however, the Royal Society got wind of some irregularities surrounding the appointment of Dacres. It turned out that the actual committee had voted for Hooke by five to four, but that the Lord Mayor of London, Sir Anthony Bateman, who was present as an observer but not a member of the committee, then voted for Dacres, making a tie, and followed this up by claiming the right to a casting vote in favour of his man. Bateman’s term as Lord Mayor came to an end shortly after this fiasco, and he was succeeded by Sir John Lawrence, a more straightforwardly honest man who knew Hooke’s abilities. Following formal representations by the Royal, a committee of investigation chaired by Sir John met on 20 March 1665 and concluded:

that Robert Hooke was the person legally elected and accordingly ought to enjoy the same with the Lodgings profits and all accommodation to the place of Geometry Reader appertaining.

In the months before the appeal was heard, the Royal acted with underhand cunning to secure the benefits of Cutler’s offer for themselves. On 27 July 1664, the Council of the Royal formally voted to appoint Hooke as Curator of Experiments with a salary of £80 a year, but kept this secret while they negotiated ‘on Hooke’s behalf’ with Cutler. It was agreed that Hooke would give what became known as the Cutlerian Lectures, on practical applications of science ‘to the advancement of art and nature’ but on specific topics chosen by the Royal. And Cutler’s money would be funnelled to Hooke through the Royal. So when Hooke was formally appointed as Curator on 11 January 1665, the Royal only had to add £30 a year for his income to be made up to the promised £80. The situation was compounded when Cutler (possibly piqued by this, or maybe just unreliable) failed to pay his share most of the time, leading to tedious legal hassles only resolved in Hooke’s favour after Cutler’s death, in 1696 (for the first ten years, the Royal also had trouble finding the money to pay their contribution to his salary). But still, as he did get the Gresham chair Hooke was reasonably comfortable from the time he was installed as Gresham Professor in March 1665 (he had actually been lodging in rooms in the College since the previous September). As well as the income, he had a parlour, library and two smaller rooms in a first-floor apartment, a workshop on the ground floor, cellar rooms providing further space for his experimental work, and a garret for a servant. He was able to keep at least one servant, usually a girl, and usually on more than friendly terms, as we discuss later. He was a gregarious and friendly man (at least until old age and infirmity made him more grumpy), who welcomed visitors to his home, as well as mingling with his friends in the coffee houses. At the age of twenty-nine, he was settled for life, with no need of patronage.

Hooke was a diligent lecturer, unlike many of his fellow Gresham Professors. Some didn’t even live at the College, but let out their rooms and enjoyed a quiet life in the country, or even in another country. Hooke’s duties (in addition to his work for the Royal, remember!) were to give his lectures on Thursdays in term time,^fn2 in Latin between 8 a.m. and 9 a.m. and the same lecture in English between 2 p.m. and 3 p.m. He seems to have always had the lectures prepared and been available to do his duty, but very often, as his diary records, nobody turned up to listen to them. He also gave the Cutlerian Lectures, officially during the vacations but sometimes on other occasions; many of these were collected and published in 1679. These wandered far from the original brief, which makes them much more interesting to us even if it helps to explain Cutler’s reluctance to pay Hooke.

But that is getting ahead of our story.

The year 1665 was a turning point for Hooke in other ways, but before we discuss the changes in his life that took place in the second half of the 1660s, we should go back to look at his scientific achievements in the first half of that decade.

Some idea of the breadth of Hooke’s activities can be gleaned from a ‘wish list’ he wrote at the beginning of the 1660s of the projects he had in mind:

Theory of Motion:

of Light

of Gravity

of Magneticks

of Gunpowder

of the Heavens

Improving shipping

– watches

– Opticks

– Engines for trade

– Engines for carriage

Inquiry into the figures of Bodys

– qualitys of Bodys

Hooke worked on many of these projects (and others) in parallel.

We can only pick out the highlights, and describe them consecutively, even when two or more of them overlapped chronologically. The extraordinary fact is, though, that Hooke worked on an array of subjects at the same time, while also giving his lectures and doing more experiments at the behest of the Royal Society. But let’s begin with some of his first work for the Royal, using the air pump that Boyle had given to the Society, and which only Hooke could operate effectively. With that tool, he carried out the two duties that were the key to the survival of the Royal Society, a survival that he alone ensured. First, he entertained the Fellows with dramatic demonstrations. The importance of this cannot be overemphasised. It was this kind of showy demonstration that fascinated the more dilettante Fellows and which brought in a flow of subscriptions to keep the Royal afloat, even if that flow was sometimes only a trickle. Secondly, and much more important to us, he carried out experiments that advanced scientific knowledge profoundly.

A good example of Hooke’s skill as a showman, and the way this linked up with his scientific studies, is provided by his work with hollow glass balls. He delighted his audience with demonstrations in which the balls ‘exploded’ as they cooled down after being blown from molten glass, and the way air rushed into them when they were placed under pressure in the chamber (receiver) of the vacuum apparatus and cracked open. Among other things, though, this set Hooke thinking about the strength of arches and other curved structures, so the experiments fed directly into his later work as an architect.

It also seems that Hooke was not afraid to experiment on himself. In his diary entry for 7 May 1662, John Evelyn (himself a Fellow) describes a meeting of the Royal Society attended by the King’s cousin, Prince Rupert:

I waited on Prince Rupert to our Assembly, where were tried several experiments of Mr. Boyle’s Vacuum: a man thrusting in his arme, upon exhaustion of the ayre, had his flesh immediately swelled, so as the bloud was neere breaking the vaines, & insufferable: he drawing it out, we found it all speckled.

There is little doubt that the experimental subject was Hooke himself. Some years later, he built a receiver large enough to sit in, and did so while an assistant pumped the air out. He described how this caused pain in his ears, deafness and giddiness, before he decided enough was enough and the air was let back in. But a discussion of Hooke’s most important work with the vacuum pump can wait until we discuss his great book, Micrographia.

Although he was not afraid to experiment upon himself, Hooke was far more reluctant than most of his contemporaries to experiment on other animals, at least when it clearly caused them pain. At the beginning of the 1660s, nobody knew exactly what the importance of breathing was in sustaining life. One school of thought held that although the circulation of the blood was clearly important, the role of breathing was simply to act as a pumping mechanism, by which the in and out motion of the thorax stirred up the blood and kept it flowing. The idea that something from the air mixed with blood in the lungs and was essential for life was a minority view. In one indecisive experiment at the beginning of 1663, Hooke placed a live chick and a burning lamp in a sealed chamber to see which one lasted longer. The lamp went out, but the chick survived. This, however, neither proved nor disproved the hypothesis. It was not until November 1664 that Hooke, possibly at Boyle’s suggestion, conceived of an experiment on a living dog, which could be dissected ‘displaying his whole thorax, too see how long, by blowing air into his lungs, life might be preserved, and whether anything could be discovered concerning the mixture of the air with the blood in the lungs.’

The gruesome experiment was carried out on 7 November. With the dog cut open and all its organs exposed, unable to breathe of its own volition, air was pumped into the lungs of the dog by a pair of bellows through a hollow cane stuck into a hole in the dog’s windpipe. The experiment was a success, in that the dog lived during it. As Hooke wrote to Boyle on 10 November 1664:

at any time, if the bellows were suffered to rest . . the animal would presently begin to die, the lungs falling flaccid, and the convulsive motions immediately seizing the heart and all the other parts of the body; but upon renewing the reciprocal motions of the lungs, the heart would beat again as regularly as before, and the convulsive motions of the limbs would cease.

But in the same letter, Hooke confessed that although the experiment suggested several other lines of investigation:

I shall hardly be induced to make any further trials of this kind, because of the torture of the creature: but certainly the enquiry would be very noble, if we could any way find a way so as to stupefy the creature, as that it might not be sensible [conscious].

Three years later, Hooke was asked to repeat the demonstration, but initially refused. Two doctors, who were less squeamish about such matters, tried to replace him, but made such a mess of the operation that Hooke, by then an employee of the Royal, was ordered to do it and repeated his earlier success.

At the end of 1662 in another series of experiments, he demonstrated how a hollow glass ball that would float on top of cold water gradually sank to the bottom when the water was warmed, or could be made to ‘hover’ partway up the vessel if the temperature conditions were just right. He correctly suggested that the heat ‘loosened’ the water (that is, reduced its density), which was another step towards an understanding of matter as made up of atoms and molecules. He also invented (at least in principle; we are not sure if he made it) an efficient water heater in which a heated piece of copper at the bottom of a tub of water would heat the whole vessel as the warm, loosened water rose to the top and was replaced by descending cooler water. He had ‘discovered’ convection – but he went too far when he speculated that this might make it possible to manufacture a perpetual motion machine in which the water circulated endlessly through a system of pipes without any further heating once it had been started. More practically, he pointed out that because the cold sea at high latitudes could support heavier ships than the ‘loosened’ water closer to the equator, ships setting out from polar latitudes to the tropics should not be fully laden. Much later, starting in the late nineteenth century, merchant ships were marked with ‘Plimsoll lines’ showing exactly how far they could be safely loaded, depending on the waters they were visiting.

Hooke’s investigations of pressure, density and convection fed directly into another lifelong interest of his: the weather, and the possibility of forecasting the weather. This became a major thread of his work in September 1663, when Wilkins, on behalf of the Royal, asked Hooke to collect daily records of the weather, in the hope that these might reveal patterns that could be used in prediction. Wilkins probably had in mind a simple note of whether it was sunny or cloudy, rainy or dry, and so on. But Hooke never did anything by halves, and he began by setting out a systematic schedule of everything scientific weather observers should take note of (wind speed and direction, temperature, humidity, air pressure, the appearance of the sky, and so on) before he put those principles into practice. He said that the weather observer should also note what illnesses (human and animal) were rife at the time, what diseases and pests were affecting the crops, and many other items. All of this was to be recorded in a standard format, so that the data for each month could be scanned at a glance. Among these details, Hooke was the first person to establish a standard list of terms to describe different kinds of cloud cover.

The project soon developed far beyond the simple record keeping envisaged by Wilkins. You can’t keep reliable records unless you have reliable instruments to measure with, and a reliable scale against which to calibrate those measurements. It was Hooke who defined the freezing point of distilled water as the zero of temperature, marked on sealed glass thermometers, an idea enshrined in later temperature scales with the boiling point of water set as the second fixed number, though by then nobody remembered it had been Hooke’s idea. He realised that thermometers were affected by the expansion and contraction of the glass as it warmed and cooled, and studied the effect. To measure humidity, he observed the way the ears of the wild oat and wild geranium bent more or less as the humidity changed, and adapted this for use in a hygroscope.^fn3 But he made perhaps his most significant weather discovery in September 1664, just after he first moved into rooms at Gresham College.

This harked back to his work with Boyle on ‘Mr. Townly’s hypothesis’. It used a portable barometer shaped like a letter J, as in that work, but this time with the long end of the tube closed and the bottom (the short limb of the J) open to the air. Mercury in the U-bend of the J would be pushed down more when the atmospheric pressure was higher, forcing the mercury on the other side further up the long arm of the tube. Similarly, when the pressure fell, the mercury in the long arm fell. By the end of 1663, Hooke had converted this into a ‘wheel’ barometer, with a pointer that moved around a dial like the face of a clock to show how the pressure was changing. He did this by twisting a thread around the axle of the pointer, with the other end of the thread attached to a weight floating in the mercury in the open end of the tube, and a counterweight on the other side of the axle hanging free in the air. As the mercury moved up and down, the thread tugged the pointer round the dial one way or the other. And if the friction of the axle made it stick, all you had to do was to tap the barometer to get it to unstick and move to the appropriate position.

On 6 October 1664, Hooke wrote to Boyle to tell him of a great discovery he had made using one of these barometers:

I have also, since my settling at Gresham college, which has been now full five weeks, constantly observed the baroscopical index … and have found it most certainly to predict rainy and cloudy weather, when it falls very low; and dry and clear weather, when it riseth very high, which if it continues to do, as I have hitherto observed it, I hope it will help us one step towards the raising a theoretical pillar, or pyramid, from the top of which, when raised and ascended, we may be able to see the mutations of the weather at some distance before they approach us, and thereby being able to predict, and forewarn, many dangers may be prevented, and the good of mankind very much promoted.

Hooke’s vision was not immediately fulfilled: too many other elements, not least rapid communication systems to enable the collation of data from widespread observers, would be required before the vision became reality. It would be two centuries before Admiral Robert FitzRoy ‘invented’ the weather forecast, but when he did so the kind of links between atmospheric pressure and weather that Hooke had discovered were a key ingredient. And, as FitzRoy’s rank highlights, among the ‘many dangers’ Hooke referred to were the hazards of storms at sea.

Although this particular development was of no immediate benefit to mariners, as we mentioned in connection with Hooke’s work on timekeepers, maritime matters were of vital importance to England in the second half of the seventeenth century, and therefore they were of vital importance to the Royal Society as a means of proving its worth to the King. Naval wars with the Dutch involved fleets as far away as America, the Caribbean, West Africa and even the East Indies. It was during a lull in these activities (under the Treaty of Breda, also known as the Peace of 1667) that England formally gained the former Dutch colony of Nieuw Amsterdam, which they had captured in 1664, and promptly renamed it New York. Hooke invented several devices for studying the sea, or working under the waves. One was a depth sounder, which worked by dropping a hollow ball attached to a heavy weight into the sea. When the weight hit bottom, it released the ball, which floated to the surface. By timing how long it took before the ball surfaced, the depth could be calculated. At least, it could in a flat calm with good seeing conditions. In practice, under less than ideal conditions, from the small ships of the seventeenth century the balls could not be spotted as soon as they surfaced (if at all) so the technique was impractical. In the nineteenth century, however, the same idea was dreamed up, independently, by an American, J. M. Brooke, and was used to measure the depth of the sea bed when the first transatlantic telegraph cables were laid in the middle of that century.

Another of Hooke’s devices was more immediately successful. This was a bucket on a long line, with hinged lids that allowed it to fill with water at depth, but closed when it was pulled to the surface. This was effective in bringing back samples, which could be studied to measure such things as the saltiness and (with luck) the creatures that lived at depth.

In February 1664 (still before he was being paid by the Royal), Hooke served on a committee that investigated the practical possibilities of diving. He devised a system where a diver working on the bed of a river, or in shallow water at sea, could be supplied with a succession of air-filled lead boxes lowered from the surface, from which he could breathe through a tube. This was reasonably successful during trials in a large tub set up outside the Royal and in the Thames. These and other ideas, including diving goggles, a life jacket, and plans for a submarine, were summed up in an account Hooke published in 1691, but they are only tangentially of interest to our story of Hooke the scientist, as another example of his versatility and capacity for hard work.^fn4

But another aspect of Hooke’s maritime work ties in more closely with the main thread of our story. This was his interest in the use of astronomy for navigation, which led him to design and manufacture more accurate instruments for measuring the height of the Sun and stars above the horizon – a key to determining latitude, but also a key to measuring the positions of the stars relative to one another more accurately for other astronomical purposes. This involved better sights (in effect, little telescopes), and instruments calibrated and marked to exquisite precision. One of Hooke’s instruments (a quadrant), presented to the Royal in February 1665 (while in the middle of the hassles concerning his appointments as Cutlerian Lecturer and Gresham Professor), was just seventeen inches across, but could measure angular distances as small as ten seconds of arc. Since there are 60 seconds in a minute of arc, 60 minutes in a degree, and 360 degrees in a circle, this means that the instrument could measure precisely angles that are only 1/360th of a degree, or 0.0000077 of a circle. The unprecedented accuracy of Hooke’s instruments led to an argument with the much older astronomer Johannes Hevelius of Danzig, who could not believe the superiority of Hooke’s designs; the controversy, detailed later, also brought in Edmond Halley, in one of his first missions as a Fellow of the Royal Society.

In much of his astronomical work, especially in the first half of the 1660s, Hooke collaborated with his friend Christopher Wren, who was based in Oxford but still in communication with the Royal. Astronomers of the time were lucky enough to see several comets, and in December 1664 the Royal asked Hooke and Wren to make observations and report on a new comet that had become visible.^fn5 Hooke observed from London, Wren from Oxford, and their results plus measurements from other observers were combined and reported by Hooke. Pepys attended a lecture at Gresham College on 1 March 1665 and tells us that on that day (a couple of weeks after he had demonstrated his quadrant), Hooke talked about:

the late Comett, among other things proving very probably that this is the very same Comett that appeared before in the year 1618, and that in such time probably it will appear again – which is a very new opinion.

New to Pepys, and to Hooke, although we now know that the English clergyman and astronomer Jeremiah Horrocks had speculated along the same lines – that comets follow closed orbits around the Sun – three decades earlier. It happens that Hooke was wrong about this particular comet: it was not the same one that was seen in 1618, and it did not return in 1711. But the improving telescopic technology of the time was starting to show astronomers that comets did not move in straight lines, but followed curved paths through space; this was the beginning of the idea that led Halley, before too long, to make the prediction of the return of the comet that now bears his name. The significance for Hooke’s story is that it seems that, by the mid-1660s at the latest, he was already thinking about the possibility that comets (and therefore the planets) were under the influence of some kind of force, reaching out to them across space from the Sun itself. He realised that comets are part of the Sun’s family, not something weird or magical. This was among the insights that led him to carry out several experiments to investigate the nature of gravity, which we describe later. It is worth getting slightly ahead of our story, however, to highlight one of Hooke’s most important insights (perhaps the most important), which (like so many of his ideas) has been misattributed for hundreds of years.

Going back into the mists of time, it had been assumed by natural philosophers that the ‘natural’ motion of objects such as planets unaffected by friction or other forces was circular. This had to be so, they reasoned, because circles are perfect, and only perfection could be at work in the heavens. They interpreted the seemingly irregular motion of planets in terms of epicycles, where the planets were constrained to move in small perfect circles around points which themselves moved in perfect circles around the Earth, or the Sun. When, only a short time before Hooke was born, Galileo carried out experiments involving balls rolling down inclined planes, he found that the balls rolled off the end of the ramp horizontally – literally, towards the horizon – and he realised that if there were no friction they would keep rolling for ever. But he knew that the Earth was round, so to him ‘horizontal’ motion meant always moving towards an always receding horizon, in a circle around the Earth. It was Hooke who realised, partly from his studies of comets, that any object that is not acted upon by an external force will keep moving in a straight line. Does that sound familiar? It should. It is something we all learn in school, where it is called ‘Newton’s First Law’ of motion. But it was Hooke who came up with it, and who (as we shall see) explained it to Newton.

On 21 March 1666, when nobody outside Cambridge and few people inside Cambridge had heard of Isaac Newton, Hooke gave a lecture to the Royal about gravity, where he presented some of these ideas. He described several experiments involving his study of gravity, which he stated was ‘one of the most universal active principles in the world’ and set out his ambition to determine:

whether this gravitating or attractive power be inherent in the parts of the earth [and] whether it be magnetical, electrical, or of some other nature distant from either

as well as ‘to what distance the gravitating power of the earth acts’.

On 23 May that year he presented his big idea to another meeting of the Royal, and in a paper entitled ‘Inflexion of a Direct Motion into a Curve by a Supervening Attractive Principle’. In that lecture (and many times afterward) Hooke used a long pendulum, with the bob moving in a circle, or (crucially, in terms of understanding the motion of the planets) an ellipse, not just to and fro; this demonstrated the nature of orbital motion, which, he pointed out, required a force (in this case, supplied via the string of the pendulum) to keep the bob ‘in orbit’. By attaching a secondary, shorter string, with its own bob, partway down the pendulum he could also demonstrate the motion of a ‘moon’ around a ‘planet’. The idea he presented to the Fellows (which really was ‘a very new opinion’) was that the natural motion of a planet is in a straight line – a tangent to its orbit – and that it is deflected from this tangential path by a force of attraction stemming from the centre of the planetary system – that is, a force emanating from the Sun. As he explained to the Fellows:

I have often wondered why the planets should move about the Sun according to Copernicus’s supposition, being not included inn any solid orbs^fn6 … nor tied to it, as their centre, by any visible strings.

He stressed that ‘all bodies, that have but one single impulse’ ought to move in straight lines, and inferred that there must be another ‘impulse’ acting on the planets. If that impulse were a force of attraction from the Sun then:

all the phenomena of the planets seem possible to be explained by the common principle of mechanic motions [and] the phenomena of the comets as well as of the planets may be solved.

These two ideas, ‘Newton’s’ first law and the force of attraction between the Sun and planets (an inward, or centripetal, force), are the keys to the ‘Newtonian’ revolution in science that took place two decades later. It might have happened sooner, and had a different name, if Hooke’s attention had not been diverted by dramatic developments in England in 1665 and 1666. Conveniently for us, however, he had summed up what he described as his ‘first endeavours’ in a book published just before those changes took place.

Micrographia, Hooke’s great book, was written and published on the instructions of the Royal Society as a deliberate attempt to promote the Society and its aims. Hooke has been described as a ‘reluctant author’,^fn7 and almost all of his published work resulted from his contractual obligations, primarily to the Royal Society and to a slightly lesser extent to John Cutler and in connection with his role as a Gresham Professor. But the background to Micrographia predates Hooke’s appointment as Curator of Experiments.

At the beginning of the 1660s, Christopher Wren was supposed to be preparing a book of microscopical observations for presentation to the King, who had seen some of his drawings of microscopic objects and been impressed by them, but the newly appointed Savilian Professor of Astronomy found that he had too much on his plate, and passed this task on to Hooke, who took over the work in September 1661. The design and manufacture of optical instruments – telescopes and microscopes – was improving dramatically at this time, and although Hooke was involved in developing some of the ideas that went into these instruments, he relied on expert craftsmen, notably Richard Reeve, for the tools of his trade. As he put it in his book: ‘all my ambition is that I may serve to the great Philosophers of this Age, as the makers and grinders of my Glasses did to me’.

By the end of 1662, Hooke was presenting some of his microscopic studies to the Royal. The first of these observations, presented in December that year, dealt with the patterns of ice crystals seen in ‘frozen urine, frozen water, and snow’. The Fellows were sufficiently impressed that at the Council meeting of 25 March 1663 Hooke was ‘solicited to prosecute his microscopical observations, in order to publish them’. In the months that followed, Hooke made many specific observations at the behest of individual Fellows, as well as following up his own interests. The Council kept a keen eye on the progress of the work, with the book intended to provide an example of the experimental method, which was at the heart of their philosophy, and which they explicitly took from Bacon. In the book, Hooke emphasises the need ‘to begin to build anew upon a sure Foundation of Experiments’, and explicitly cites the ‘Noble and Learned’ Bacon as an inspiration. The book was partially intended as propaganda for the Society itself and for the new way of studying the world. It succeeded dramatically on both counts, thanks to Hooke’s known genius as a scientist and his perhaps unexpected skill as a writer. But it only got into print after some heart-searching by the Council, which has been detailed by John Harwood.^fn8

Hooke had more or less enough material for his book by March 1664, a year after he had formally been instructed to carry out the work. By then, the Royal had chosen a printer and discussed such details as the official Royal Society imprimatur to go in the front of the book. This emphasised in the clearest way that it was a Royal Society book, stating that:

By the Council of the Royal Society of London for Improving of Natural knowledge.

Ordered, That the Book, written by Robert Hooke, M.A. Fellow of this Society, Entitled, Micrographia, or some Physiological Descriptions of Minute Bodies, made by Magnifying Glasses, with Observations and Inquiries thereupon, Be printed by John Martyn and James Allestry, Printers to the said Society

Novem. 23.

1664. Brouncker. P.R.S

But in the interval from March 1664 to November 1664, the contents of the book had been carefully vetted and discussed by selected Fellows. This caused them some disquiet, because – strictly speaking, exceeding his brief – Hooke did not restrict himself to presenting the observations that he had made with the microscope, but also offered theoretical explanations for why things might be the way they are. He also professes a mechanistic view of Nature, pointing out in the Preface that the reason why we may hope to use mechanical techniques – experimental science – to reveal the workings of the world is that the world operates on the same principles as a machine:

We may perhaps be inabled to discern all the secret workings of Nature, almost in the same manner as we do those that are the productions of Art [artifice], and are manag’d by Wheels, and Engines, and Springs, that were devised by humane Wit.

All of this elevated Hooke’s perceived status to that of a natural philosopher, rather than a ‘mere’ mechanical experimenter. But if his ideas were wrong, the Royal did not want to be seen to endorse them. Ultimately, the Council decided to allow Hooke’s speculations to appear in the book, but only if it was made clear that they were his alone, and not the official view of the Society. They ordered:

That the president be desired to sign a licence for the printing of Mr. HOOKE’S microscopical book: And, That Mr. HOOKE give notice in the dedication of that work to the society, that though they have licensed it, yet they own no theory, nor will be thought to do so: and that the several hypotheses and theories laid down by him therein, are not delivered as certainties, but as conjectures; and that he intends not at all to obtrude or expose them to the world as the opinion of the society.

Hooke complied, and one result of all this is that we can be sure the book is all his own work, enhancing his reputation even more. And he wrote in English, in the first person, making his ideas widely acceptable. The book was the first scientific best-seller. Samuel Pepys saw the sheets being prepared when he happened to visit the bookbinders on other business, and promptly ordered a copy of the book. He received it on 20 January 1665, and the next evening ‘sat up till 2 a-clock in my chamber, reading of Mr. Hooke’s Microscopicall Observations, the most ingenious book that ever I read in my life’.^fn9 A couple of weeks later, Pepys was himself admitted as a Fellow of the Royal Society, and noted in his diary the luminaries present at the meeting. ‘Above all,’ he tells us, ‘Mr Boyle today was at the meeting, and above him Mr Hooke, who is the most, and promises the least, of any man in the world that I ever saw.’ In other words, in spite of Hooke’s unprepossessing appearance, Pepys rated him above Boyle as a scientist. Clearly, this was at least partly thanks to the impression made by Micrographia.

To us, the speculations that gave the Royal cold feet are more significant than the illustrations that were the original raison d’être for the book, astonishing though they were at the time, and still are, considering the difficulties Hooke had to cope with. Remember, for example, that the only light sources he had were the Sun, candles and simple oil lamps. In a standard setup, light from an oil lamp was focused first through a globe containing a transparent solution of brine, and then through a lens on to the specimen he wanted to study. Straining his eyes to concentrate on the image, he then had to draw what he saw with meticulous precision. Micrographia^fn10 contains sixty illustrated ‘observations’, fifty-seven of them microscopic and three astronomical, made with the aid of a telescope. In a demonstration of his skill as a communicator and his methodical way of working as a scientist, Hooke begins with observation ‘of the Point of a sharp small Needle’. ‘As in geometry,’ he writes, ‘the most natural way of beginning is from a Mathematical point.’ He goes on to describe, with illustrations,^fn11 how even the smoothest, sharpest needle looks rough and rounded under the microscope, and he makes a digression to describe the appearance of full stops, both printed and handwritten, which were abundantly ‘disfigur’d’ even when they appeared perfectly round to the human eye. And he is not averse to a pun, saying after a digression ‘But to come again to the point …’ The style is easy and accessible even to modern eyes, and the illustrations still stunning. Although in modern times some critics have suggested Hooke could not possibly have seen the detail he claimed, Brian J. Ford, an expert in the history of microscopy, found that by using similar instruments and making careful adjustments of light and focus he could indeed reach the level of detail reported by Hooke. We shall not, however, describe each of the sixty observations in detail. Instead, we shall follow the example of Hooke’s biographer Margaret ‘Espinasse in picking out four key topics that helped to revolutionise seventeenth-century science.

The first highlight is Hooke’s work on light and optics, which is doubly important because it would lead to an intense disagreement with Newton, and one of the most misunderstood comments in the history of science (see Chapter Four). Observation 9 of the Micrographia deals with ‘the colours observable in Muscovy glass, and other thin bodies’. This ‘glass’ is a mineral that is ‘transparent to a great thickness’, but is made up from many thin layers discernible under the microscope. Hooke was intrigued by the way this material converted white light into a rainbow pattern of colours, and discovered microscopic flaws in the layers of the material: ‘with the Microscope I could perceive, that these Colours were ranged in rings that incompassed the white speck or flaw.’ Newton, of course, is today remembered as the man who discovered that white light could be split into rainbow colours, and these rings are known, of course, as ‘Newton’s rings’. Hooke explained the phenomenon as a result of the combination (we would now say interference) of light reflected from the upper and lower surfaces of the thin layers, and described how the effect was only produced if the layers were thinner than a critical thickness; his explanation was based on the idea that light is a form of wave, in his words ‘a very short vibrating motion’, but incorrectly suggested that red and blue are the primary colours from which others are derived by ‘dilutions’.

Even here, though, Hooke’s reasoning was sound, given the state of knowledge at the time, and based on an experiment that clearly intrigued the young Isaac Newton. Hooke allowed a narrow beam of sunlight to enter the top of a conical flask filled with water, striking the surface of the water at an angle. He saw how the beam of light was spread out as it entered the water, producing a band of colour with red (he called it scarlet) on one side and blue on the other, with other fainter colours in between. It was this that led him to infer that white light is a mixture of colours (which is correct) and that red and blue are the primary colours, which are mixed together in different amounts to produce different colours (which was wrong, but not stupid). This experiment, described in Observation 9, is what pointed Newton towards his experiments with prisms, for which he is credited for the discovery that white light is a mixture of colours.

But the breadth of Hooke’s interests and the depth of his theorising (the things that worried the Council of the Royal) can be seen in his summing up at the end of the Observation:

I think these I have newly given are capable of explicating all the Phenomena of colours, not only of those appearing in the Prisme, Water-drop or Rainbow, and in laminated or plated bodies, whether in thick or thin, whether transparent, or seemingly opacous.

The whole Observation amounts to what we would now call a scientific paper, and as ‘Espinasse points out it is ‘a progression of precise observation, masterly analysis and induction, and speculation’.

In Observation 58, one of the three astronomical observations, Hooke returns to optics to discuss the phenomenon of refraction, starting out from the by then well-known telescopic observation that ‘the Sun and Moon neer the Horizon, are disfigur’d (losing that exactly-smooth terminating circular limb, which they are observ’d to have when situated near the Zenith)’. After discussing several other phenomena, notably ‘that both fix’d Stars and Planets, the neerer they appear to the Horizon, the more red and dull they look, and the more they are observ’d to twinkle’, he concludes:

First, that a medium, whose parts are unequally dense, and mov’d by various motions and transpositions as to one another, will produce all these visible effects upon the Rays of light, without any other coefficient cause.

Secondly, that there is in the Air or Atmosphere, such a variety in the constituent parts of it, both as to their density and rarity, and as to their divers mutations and positions one to another.

By Density and Rarity, I understand a property of a transparent body that does either more or less refract a Ray of light.

And

The redness of the Sun, Moon and Stars, will be found to be caused by the inflection of the rays within the Atmosphere … it is not merely the colour of the Air interpos’d.

In other words, the colour is inherent in the original white light and is not some kind of pollution, or corruption, caused by the passage of light through the intervening medium – another discovery later attributed to Newton.

The second great insight in Micrographia comes in Observation 16, where Hooke presents his ideas on combustion. The microscopic justification for including these ideas comes from his studies of charcoal and burnt vegetables, but the experiments from which his most impressive insights are drawn do not really involve the microscope at all. These included his observations of the way flames went out when a lit candle was shut in a sealed chamber, how small animals collapsed and died after a certain time in such a chamber, the gruesome vivisection of a dog, and the experiments with candles and living things involving the air pump. Having already, in Observation 9, asserted that heat is ‘a motion of the internal parts’ of a substance (also mentioned in Observations 7 and 8), he now draws a clear distinction between heat and combustion. ‘This Hypothesis,’ he says, ‘I have endeavoured to raise from an Infinite of Observations and Experiments, the process of which would be much too long to be here inserted.’ But as he tells us, the idea ‘has not, that I know of, been publish’d or hinted, nay, not so much as thought of, by any.’ He was right.

One of the key series of experiments that he hints at here was carried out as demonstrations at the Royal in January and February 1665. In a beautiful example of the scientific method at work, he showed first that gunpowder would still burn in the absence of air, and then that neither of two of the three ingredients of gunpowder, charcoal and sulphur, would burn on their own in the absence of air. But each of them could be reignited by adding the third ingredient, which he knew as saltpetre but which we call potassium nitrate. As Hooke says in Micrographia, it is clear from these experiments that combustion involves ‘a substance inherent, and mixt with the Air, that is like, if not the very same, with that which is fixt in Salt-peter.’ That substance is, of course, oxygen; the chemical formula for potasssium nitrate is KNO₃.

Hooke’s idea is that something in the air is essential to combustion, which takes place when that something combines with something in the burning object. ‘There is no such thing as an Element of Fire’, he asserts, dismissing the idea that had held sway since the time of Ancient Greece. A flame ‘is nothing else but a mixture of Air and volatile sulphureous parts of dissoluble or combustible bodies, which are acting upon each other whilst they ascend, that is, flame seems to be a mixture of Air, and the combustible volatile parts of any body’. Further, the component of air that is essential for combustion is also, Hooke tells us, essential for life. In Observation 22, almost as an aside, he mentions that there is a ‘property in the Air which it loses in the Lungs, by being breath’d’. In being so close to the discovery of oxygen, Hooke was nearly a century and a half ahead of his time; right up until the end of the eighteenth century, the phlogiston theory of combustion (which said, flying in the face of experiments like those Hooke carried out with the air pump, that burning substances released phlogiston, rather than absorbing something from the air) held sway, and Hooke’s ideas were forgotten. In 1803, chemist John Robison wrote:

I do not know of a more unaccountable thing in the history of science, than the total oblivion of this theory of Dr. Hooke, so clearly expressed, and so likely to catch attention.

But it did catch the attention of one person, the serial plagiarist Isaac Newton. In an appendix to his book on optics, hurried into print immediately after Hooke’s death (see postscript to Chapter Seven), Newton presented a suite of ideas about combustion that chemist Clara de Milt has described, with admirable academic restraint, as ‘very, very much like those of Hooke’. As Private Eye might put it, could they by any chance be related?

The third great insight presented in Micrographia comes in Observation 17: Of Petrify’d wood, and other Petrify’d bodies. The petrified objects he refers to are what we now call fossils. Before Hooke, it was widely thought that these were, in his words, ‘Stones form’d by some extraordinary Plastick virtue latent in the earth’. In other words, that these were just curious stones that happened to resemble the forms of living things. But he dismissed this notion, and stated unequivocally (‘I cannot but think’) that they were ‘the Shells of certain Shel-fishes, which, either by some Deluge, Inundation, Earthquake, or some other such means, came to be thrown to that place’. ‘That place’, he was well aware, was high up in a mountain, or on the cliffs that he had walked as a boy on the Isle of Wight. So how did such things as wood and shells become petrified, or fossilised? Hooke’s description of the process could almost come from the pages of a modern textbook of geology:

this petrify’d Wood having lain in some place where it was well soak’d with petrifying water (that is, such water as is well impregnated with stony and earthy particles) did by degrees separate, either by straining and filtration, or perhaps, by precipitation, cohesion or coagulation, abundance of stony particles from the permeating water, which stony particles, being by means of the fluid vehicle convey’d, not onely into the Microscopical pores, and so perfectly stoping them up, but also into the pores or interstitia, which may, perhaps, be even in the texture or Schematisme of that part of the Wood, which, through the Microscope, appears most solid.

And as for shells, they must have been:

fill’d with some kind of Mudd or Clay, or petrifying Water, or some other substance, which in tract of time has been settled together and hardened in those shelly moulds.

Hooke clearly understood two things: that there were geological processes that transformed once-living things into ‘petrified’ rock, and that there were geological processes that transformed the structure of the Earth’s crust. Implicit in this was the understanding that the timescales involved (‘tract of time’) were far greater than the ‘official’ chronology of a few thousand years derived from the Bible.

Hooke even begins to hint at the kind of investigations that would lead to the idea of evolution:

It were therefore very desirable, that a good collection of such kind of Figur’d stones were collected; and as many particulars, circumstances, and informations collected with them as could be obtained, that from such a History of Observations well rang’d, examin’d and digested, the true original or production of all those kinds of stones might be perfectly and surely known.

Soon after the publication of Micrographia, a Dane, Niels Steensen (who used the Latinised version his name and is remembered as Steno), publicised very similar ideas, and suggested that different rock strata, containing fossils such as sharks’ teeth, had been laid down under water, far from the present-day seas, at different times during Earth’s history by a succession of floods. Coincidence? Hooke didn’t think so. He had developed these ideas further in his Cutlerian Lectures which we discuss later. Henry Oldenburg, the Secretary of the Royal Society and someone who often rubbed Hooke up the wrong way, was in correspondence with scientists across Europe as part of his job. Steno published his ideas in 1669, in Latin. Oldenburg promptly made the Royal aware of the book, and arranged for it to be translated into English, which helped to ensure that Steno became remembered as the inventor, or discoverer, of these ideas. Hooke was not exactly pleased and tried unsuccessfully to get recognition that he at the very least had the idea first. When it was suggested that he had borrowed his ideas from Steno, rather than the other way around, he was moved to write a letter, read to a meeting of the Royal on 27 April 1687, in which he said:

I must now add in my own vindication that I did long since prove Steno had much of his treatise from my Lectures, which some time before that I had read [in Gresham College] which Lectures Mr Old. Borrowed and transcribed and by Divers circumstances I found he had transmitted the substance of if not the very Lectures themselves [to Steno]. And he did as good as own it, and upon my challenging him with it he did in two of his transactions publish that I had Read A great part of that Doctrine & hypothesis in my Lectures in Gresham Colledge Some time before Mr Steno had published his Booke.

There is no reason to doubt Hooke’s version of affairs, and there is no doubt at all that his work preceded that of Steno, whether or not Steno got word of it via Oldenburg. Steno, by the way, never gave a clue one way or the other: he disappeared from the scientific scene after writing his book. He became a Catholic priest in 1675, and was ordained as a bishop in 1677, inflicting on his body such a harsh regime of fasting and self-denial that he died in 1686, at the age of forty-eight.

Hooke’s more extensive ideas about earthquakes, Earth history and geology will be covered in Chapter Nine. Now, we still have a fourth great insight from Micrographia to discuss, although here we diverge from ‘Espinasse’s assessment of which of the ideas Hooke presented there were most significant. She picks out his discovery of the structures he named cells (after the rooms occurred by monks in a monastery) in thin slices of cork (Observation 18). As Hooke puts it, no ‘Writer or Person’ had ‘made any mention of them before this’. But although the name was taken up and used by later biologists, it was in a slightly different context. The ‘pores’, as he also called them, that Hooke had found are not living cells, but non-living structures left over from the growth of the plant. The first person to see and study live cells under a microscope was Hooke’s Dutch contemporary Antoni van Leeuwenhoek. In 1674 he described an algae, Spirogyra, and other organisms that moved of their own volition; he named them animalcules (‘little animals’). In this area, Hooke’s work was important, but not as important as the work of van Leeuwenhoek and others. In our estimation, his astronomical Observations were of far greater importance.

Hooke was a serious and highly respected astronomer. On 9 May 1664, using a twelve-foot-long refracting telescope, he had discovered the Great Red Spot of Jupiter, and used it to measure the rotation of the giant planet. Contemporary (and now more famous) astronomers such as the Italian Giovanni Cassini picked up on the discovery, and referred to the phenomenon as ‘Hooke’s Spot’. But it was observations of something much closer to home that led Hooke to important insights that appeared in Observation 60: Of the Moon. This is a short contribution that to a casual glance looks like a mere filler. That couldn’t be more wrong.

Observation 60 provides a nice example of the scientific mind – Hooke’s scientific mind – at work: making observations, devising hypotheses, testing them by experiment and further observation, and drawing general conclusions from specific cases. Remember that this was less than sixty years after Galileo, with the aid of one of the first astronomical telescopes, discovered that the Moon is not a perfect sphere but pockmarked with craters and scarred by mountain ranges. Hooke was intrigued by the nature of these craters, and puzzled over their origin. He described them as ‘almost like a dish, some bigger, some less, some shallower, some deeper, that is, they seem to be a hollow Hemisphere, incompassed with a round rising bank, as if the substance in the middle had been digg’d up, and thrown on either side’. Which establishes, as if we did not already know, that he was a good and accurate observer.

How could such craters be formed? Hooke came up with two hypotheses and set out to test them. The first was that the craters were caused by impacts. To test this, Hooke made a mixture of water and pipe-clay, ‘into which, if I let fall any heavy body, as a Bullet, it would throw up the mixture round the place, which for a while would make a representation, not unlike these of the Moon.’ So incoming objects (bodies) would do the trick. But Hooke found it ‘difficult to imagine whence those bodies should come’, so he turned to his other idea. In this experiment he heated a pot of alabaster to the boiling point, and then, while it was still bubbling, took it off the fire and allowed it to set. Then ‘the whole surface, especially that where some of the last Bubbles have risen, will appear all over covered with small pits, exactly shaped like these of the Moon, and by holding a lighted Candle in a large dark Room, in divers positions to this surface, you may exactly represent all the Phenomena of these pits in the Moon, according as they are more or less enlightened by the Sun’.

So Hooke plumped for volcanic activity as an explanation of lunar cratering, rather than impacts. This was a perfectly reasonable conclusion to draw at the time, and for the next four hundred years volcanic activity remained a viable explanation for lunar cratering. The idea was only finally laid to rest, in favour of the impact hypothesis, when astronauts visited the Moon and its geology could be studied first hand. We now know that the craters were indeed made by impacts, in which ‘the substance in the middle had been digg’d up, and thrown on either side’. But if it was thrown up, either by impacts or by volcanic activity, something must have pulled it back down on to the surface of the Moon to make the circular ramparts surrounding the craters. That something, Hooke reasoned, must have been gravity – the Moon’s own gravitational pull.

Developing his idea, Hooke said that it ‘is not improbable, but that the substance of the Moon may be very much like that of the Earth’ (which would have amounted to heresy a few decades earlier). And then he goes beyond Galileo, who noticed the imperfection of the Moon, to draw attention to the remarkable roundness of the Moon in spite of the small irregularities we see on its surface. The Moon, he points out:

we may perceive very plainly by the Telescope, to be (bating the small inequality of the Hills and Vales in it, which are all of them likewise shaped, or levelled, as it were, to answer to the center of the Moons body) perfectly of a Spherical figure, that is, all the parts are so rang’d (bating the comparatively small ruggedness of the Hills and Dales) that the outmost bounds of them are equally distant from the Center of the Moon, and consequently, it is exceedingly probable also, that they are equidistant from the Center of gravitation; and indeed, the figure of the superficial parts of the Moon are so exactly shap’d, according as they should bye, supposing it had a gravitating principle as the Earth has.

This is mind-blowing stuff. At a time when other people talked about vortices and whirlpools being responsible for the shape of the planets and their orbits, and Isaac Newton was an unknown student who would soon be eagerly devouring Hooke’s book,^fn12 Hooke is suggesting the universal principle of gravitation (he can hardly have failed to notice that Jupiter and the other planets are also round!), that all objects possess this property, which makes the moons and planets round and (although he discusses this elsewhere) holds them in their orbits around the Sun. The very last paragraph of Micrographia begins with the words: ‘To conclude, therefore, it being very probable, that the Moon has a principle of gravitation … whereby it is not only shap’d round, but does firmly contain and hold all its parts united, though many of them seem as loose as the sand on the Earth’.

We emphasise that the idea of universal gravity is of key importance. This is the beginning of an understanding that the laws of physics which operate in the Universe at large – in the Heavens – are the same as the laws that apply here on Earth. That idea is often traced back to Newton; it should be traced back to Hooke. It’s a long way from the study of the point of a needle!

Any of these four ideas, or indeed his ideas about planetary orbits and gravity, which we have already discussed, should have ensured Hooke’s status as one of the greatest scientists of all time. And remember that there were dozens of lesser ‘observations’ in Micrographia, including some of the first observations of the tiny creatures that live in water and other liquids – the Fellows were particularly intrigued by the discovery of the creatures we call nematodes, but were referred to then as ‘eels’, living in vinegar (Observation 57). Perhaps Hooke would have been suitably recognised by posterity if he had been able to develop his ideas more fully, which he clearly intended to do. In his book, he says (especially in reference to his ideas about combustion, but undoubtedly with broader relevance):

In this place I have only time to hint at an Hypothesis, which, if God permit me life and opportunity, I may elsewhere prosecute, improve and publish.

But Hooke’s opportunities to ‘prosecute, improve and publish’ his revolutionary ideas were almost immediately restricted by plague, the Great Fire of London, and a change of career that occurred in the aftermath of the fire. While he was otherwise engaged, at least some of those ideas were taken up and developed by Isaac Newton, who had an early copy of Micrographia which he read and annotated extensively (the copy still exists), having ample opportunity to study it while he was away from Cambridge during the plague year of 1665 when he was twenty-two. He was particularly inspired at that time by Hooke’s ideas about light and colour, developed in Observation 10. Newton’s variation on this theme would soon come to the attention of the Royal and lead to a lifelong bitterness between Hooke and Newton. But Hooke would have ten more years – the happiest years of his life – before that controversy reared its head.