This lecture is from Proceedings of the Royal Institution of Great Britain, 67, 239 - 275 (1996). It was also given at Westminster School as the 1997 Sir Henry Tizard Memorial Lecture.
England's Leonardo: Robert Hooke (1635-1703) and the
art of experiment in Restoration England
No portrait or contemporary visual likeness survives of Robert Hooke, though when the German antiquarian and scholar Zacharias von Uffenbach visited the Royal Society in 1710, he specifically mentioned being shown the portraits of 'Boyle and Hoock', which were said to be good likenesses. Though Boyle's portrait survives, we have no idea what has happened to that of Hooke. It is curious, furthermore, that when Richard Waller edited Hooke's Posthumous Works for the Royal Society in 1710 he did not have this picture engraved to form a frontispiece to the sumptuous folio volume.
On the other hand, we do possess two detailed pen-portraits of Hooke written by men who knew him well. The first was that recorded by his friend John Aubrey, and describes Hooke in middle life and at the height of his creative powers:
He is but of midling stature, something crooked, pale faced, and his face but little below, but his head is lardge, his eie full and popping, and not quick; a grey eie. He haz a delicate head of haire, browne, and of an excellent moist curle. He is and ever was temperate and modera.te in dyet, etc.
The second is that by Richard Waller, whose forthright account of the elderly Hooke can scarcely be said to err on the side of flattery:
As to his Person he was but despicable, being very crooked, tho' I have heard from himself, and others, that he was strait till about 16 Years of Age when he first grew awry, by frequent practicing, with a Turn-Lath ... He was always very pale and lean, and laterly nothing but Skin and Bone, with a Meagre Aspect, his Eyes grey and full, with a sharp ingenious Look whilst younger; his nose but thin, of a moderate height and length; his Mouth meanly wide, and upper lip thin; his Chin sharp, and Forehead large; his Head of a middle size. He wore his own Hair of a dark Brown colour, very long and hanging neglected over his Face uncut and lank... 
The context of the New Science
Before examining Hooke's life and researches, however, it is important to look at their context, for his contributions to physical science came at the end of a period of a century and a half during which the once-coherent structures of classical science had received one blow after another. The explanations of the natural world which medieval European scholars had inherited from classical Greece had been static in character and based on a series of apparently self-evident principles. All changes of matter could be explained by the interaction of the four elements, Earth, Water, Air and Fire, as the principles of solidity, wetness, volatility and heat endlessly mixed and separated.
These four elements also lay at the foundation of all living things. The hearts of all living creatures generated a spontaneous, or innate, heat, that was radiated throughout the body by the blood, while the life-principle of air, or pneuma, both intermingled with the blood in respiration, and also helped to cool the heart. Heat rose, cold congealed, 'grass became flesh', and flesh decayed when its life principle had departed.
While this flux of elements prevailed on the earth, the heavens were made from a perfect, stable fifth element. Because they were made of one single and changeless substance, the stars and planets moved with a geometrical precision that nothing on earth could ever emulate, thus exemplifying that deep dichotomy between terrestrial and celestial that lay at the heart of all classical science.
The ancient Greeks, and most significantly Aristotle, had devised a complete taxonomy of nature based on these principles by 350 BC, and for the next 1900 years it proved capable of answering most of the questions that could be addressed to it. It was a magnificent intellectual achievement, though it embodied a conservative approach to knowledge, and like librarianship or museum curation, saw its first duty as absorbing, classifying and preserving the known rather than exploring pastures new.
But after 1492, the assaults on its all-encompassing explanatory credibility began to increase. The discovery of America fundamentally discredited ancient geography. Tycho Brahe's supernova of 1572 and Galileo's telesc9pic discoveries after 1609 similarly shook classical astronomy. Rapid developments in optics and mechanics, moreover, seemed to indicate that phenomena could be studied amidst the four chaotic elements of the Earth that were just as mathematically exact as those observed in the heavens, while in 1628 William Harvey discovered that the heart was a pump, and not a furnace. All of these discoveries flew in the face of the classical writings, and showed that the 'moderns' might well know more than the 'ancients'. None of these discoveries, moreover, were the fruits of speculative philosophies; they were physical discoveries. Passive observation could classify, but experiment could break into realms of new knowledge. In the words of Sir Francis Bacon, who more than anyone else championed the cause of experiment, and whose writings directly inspired Robert Hooke and the early Fellows of the Royal Society, nature must be 'put to the torture', and made to yield its reluctant secrets to the astute investigator. It was not for nothing that Bacon's distinguished legal career took place in one of the most sanguinary periods of English constitutional history!
And as the judicial inquisitor needed his special tools of assault and persuasion to make his victim speak, so the scientific experimentalist needed his, for the laboratory-which included the newly invented telescope, microscope, airpump, thermometer, and many other instruments that refined the perceptions-was the torture chamber wherein long-secretive nature would be cross-examined.
The radical reappraisal of how nature worked that was taking place in the early seventeenth century was also rich in perceived religious implications. Far from being persecuted by the Church, indeed, we must not forget that the Scientific Revolution was seen as fulfilling Old Testament prophecies. Hooke expressed the prophetic character of the New Science very succinctly in the Preface to Micrographia in 1665:
And as at first, mankind fell by tasting of the forbidden Tree of Knowledge, so we, their Posterity, may be in part restor'd by the same way, not only by beholding and contemplating, but by tasting too those fruits of Natural Knowledge, that were never yet forbidden. 
In the spirit of Bacon's and the Royal Society's motto, Nullius in Verba, it was not only to be by passive word-exercises that mankind would reach a profounder understanding of the Divine Creation, but also by action and experiment.
Yet this new mastery of nature to which the age was laying claim had a darker (or more ecstatic) dimension, depending on one's perspective. After the Fall of Mankind as a result of excessive curiosity, as recounted in Genesis, humanity had been bounded within a fixed scheme of knowledge, though ancient prophecies had indicated that new enlightenment would come to man shortly before the end of the world. Many seventeenth-century scholars had computed from the prophetic books of the Bible that Armageddon was now at hand, and no prophecy fitted the age better than that from the Book of Daniel, XII. 4:
Many shall run to and fro, and Knowledge shall be increased.
The geographical discoveries, the religious wars of the Reformation, numerous new inventions, supernovae, Jupiter's moons, the execution of King Charles I, the discovery of the microscopic realm and of the vacuum, and the refutation of the truths of Aristotle's science: all were clear fulfilments of Daniel's and many similar prophecies. The search for religious meaning lay at the heart of seventeenth-century intellectual culture, and to dismiss it from our understanding of their science produces a picture as lopsided as that which would result if a historian in 300 years' time wrote of the twentieth century in a way that dismissed the significance of economics.
Hooke's origins and early career
Robert Hooke was born on 18 July (Old Style) 1635 at Freshwater, Isle of Wight, where his father John was curate in charge of the parish. The Reverend John Hooke had fulfilled a variety of curacies on the island since at least 1610, and had been at Freshwater since 1626. As a boy Hooke was not strong, and his father, who was reluctant to subject him to the rigours of a boarding school, educated him at home. It was during these early years, however, that his talents began to manifest themselves. He was an extraordinarily quick learner, possessed a manual dexterity which enabled him to build an impressive array of mechanical devices, and his untrained draughtsmanship so struck the visiting artist John Hoskins that he advised Mr Hooke to settle upon an artistic career for Robert. In 1648, the Reverend John Hooke died, and while Robert was believed to have received a legacy of £100, the recent discovery of John Hooke's Will indicates that he only received £40, a wooden chest, and some supplementary payments .
Though we have no idea who made the arrangements, the thirteen year-old Robert now went up to London for 'tryall' in the studio of Peter (later Sir Peter) Lely, the leading portrait painter of the age. According to the account that the middle-aged Robert Hooke gave to his biographer friend, John Aubrey, he 'quickly perceived what was to be done' in painting, and complaining that the oils and varnishes irritated his chest, left the studio to be enrolled at Westminster School under Dr Richard Busby. One presumes that the thirteen-year-old must have had friends, and a patron in London, for these adroit social moves would have been extraordinary, even for a youth of Hooke's precocity, unless he had received help.
At Westminster, Robert Hooke found his feet in the city that was to provide the theatre of operations for the greater part of his career. Dr Busby, who was still within the first decade of his fifty-five-year reign as Head Master of what he was turning into the most intellectually distinguished school of the seventeenth century, quickly recognized Hooke's genius. At Westminster, Hooke acquired a mastery of ancient languages, learned to play the organ, contrived flying machines and mastered the first six books of Euclid's Elements in a week. He remained on warm terms with Dr Busby for the rest of the Head Master's eighty-nine-year life, undertaking architectural design work for him and for Westminster Abbey, and mentioning him in his Diary.
In 1653, Robert Hooke left Westminster to take up a poor scholar's place at Christ Church, Oxford, where he was described as 'Servitor' to a Mr Goodman. He was also to have been a Singing Man in the Cathedral, though as the abolition of the Anglican Church between 1643 and 1660 would have closed down the liturgical choirs, one presumes that Hooke received a Singing Man's modest endowment by way of a scholarship. But Robert Hooke was clearly a man who possessed musical abilities.
It is not known whether Hooke was especially inspired or encouraged by John Owen, the new Dean and Oliver Cromwell's ex-Chaplain, who had been imposed on once-Royalist Christ Church by the Puritan Commissioners after the Civil War, but he fulfilled the necessary requirements to take his BA and MA degrees. However, Oxford inspired Hooke in other, non-curricular ways, for it was in the University city that he fell in with that group of men who within a decade would form the original Fellowship of the Royal Society. It was in Oxford that Robert Hooke began his apprenticeship to science and formed a collection of influential and creative friendships. Dr John Wilkins, Warden of Wadham College, who was the leader of the Oxford scientific 'Club', encouraged him in astronomy, mathematics, and mechanics, as did the young Christopher Wren. Hooke was employed as chemical assistant by the distinguished anatomist Dr Thomas Willis and from him, and from Richard Lower, he probably acquired those dissection skills which would be essential in his own later researches into respiration. But his
own most important single contact in Oxford was with Robert Boyle, whose Assistant he became around 1658, after leaving the employment of Dr Willis. From Boyle, he gained a thorough mastery of chemistry and practical laboratory skills, while Hooke in his turn contributed his own talents as a mechanician to Boyle's own researches into air. But we will return to Boyle when looking at Hooke's experimental researches. 
As an intelligent and experienced 'operator' to Willis and Boyle, and as a man of acknowledged talent as a practical exponent of the 'new philosophy', Robert Hooke was the obvious choice for the office of 'Curator of Experiments' to the newly chartered Royal Society of London in 1662. But we must not forget that at this early stage in his career he did not sit with the Fellows-Boyle, Wren, Wilkins and others-as an equal, but as an employee (or 'servant' in seventeenth-century language) with a salary yet to be paid. Unlike them, he was not a 'Gentleman, free, and unconfin'd' possessing independent means, and the original terms of his employment required him to produce demonstrations at Royal Society meetings, as well as receive 'orders' for the undertaking of particular research investigations.  In 1665, however, his academic standing became more regularized when he was appointed Professor of Geometry at Gresham College, with rooms and accommodation in the same building wherein the Royal Society held its meetings; by then he was already a full Fellow of the Royal Society. By 1665, therefore, Hooke had acquired those physical circumstances that would see him through the remaining thirty-eight years of his life. They also made him the first salaried research scientist in Britain. 
Hooke's scientific ideas
Mid-seventeenth-century Europe was a veritable market-place of competing philosophies of nature in the wake of the confusion that followed the eclipse of Aristotelianism. Though historians of science generally speak of the rise of the 'mechanical philosophy' at this period, one should remember that this is a portmanteau designation for several quite distinct 'systems' that shared the speculative premise that energy was transmitted by particulate collision. The most uncompromising of mechanists was Thomas Hobbes (better known today as a political philosopher) who argued that matter and the laws of motion could be made to explain everything, from celestial mechanics to the appearance of ghosts. Rene Descartes saw all physical, but not spiritual, phenomena as occasioned by an endlessly agitated aether, the vortices and swirls of which carried along the particles that produced physical motion. Pierre
Gassendi revived the once-called Godless doctrine of atomism, and conceived of matter in terms of the geometrical arrangement of fundamental particles guided by the hand of God. And especially popular in England were the ideas of Francis Bacon, which were concerned less with the inner-most structures of matter in motion and more with developing the correct experimental method and arranging the results into taxonomic schemes.
Robert Boyle, in his chemical investigations, was drawn to a Christianized version of the atomic theory, where the geometrical arrangement of the atoms defined the chemical characteristics of the substance. And as a Baconian, Boyle devised meticulous courses of experiments by which he hoped to test these ideas. As Boyle's assistant, one can expect Robert Hooke to have been influenced by his master's ideas, though there are important points of divergence.
While it goes without saying that Hooke was an experimentalist in the Baconian tradition, it is obvious to anyone who reads Hooke's writings that he was no methodological purist. As every modern scientist now knows, no original investigator can be the rigid adherent of a pre-determined method, for creativity in science is more than recipe-following. Robert Boyle, and Robert Hooke, however, were probably the first scientists to encounter this fact of life, for while they were not the first men to perform experiments, they were the first to undertake whole courses of experiments and, in Hooke's case, conduct them in disciplines as diverse as physics and physiology. Micrographia, which published the results of a series of observations and experiments conducted between 1661 and 1664, should be required reading for every science undergraduate, for it amply demonstrates how brilliantly eclectic, yet how tightly controlled, a series of physical investigations can be. It showed how the microscopical examination of ice crystals could lead to a discussion of atomic structures; how the first recognition of the cellular structure of wood initiated research into the role of air in combustion; and how the anatomical description of a fly developed into an experimental essay in aerodynamics, acoustics, and wave-patterns.
In published researches covering nearly forty years, Hooke was constantly casting around for a consistent, underlying principle that could be shown to bind the whole of nature together: a 'Grand Unified Theory', as it were. That nature did contain common lucid principles would have been taken as axiomatic by Hooke, for as the entire universe was the product of divine intelligence, it was inconceivable that God could be inconsistent in His grand design. And as human intelligence was congruent with that of God, it stood to reason that the key should be within man's reach. As Kepler had said, science was thinking God's thoughts after Him.
Though Robert Hooke never came up with a Grand Unified Theory that could be made to stand experimentally in all cases, one can extract a series of principles which run as a thread through his thought. One of these was a version of the atomic theory of matter, though he was careful not to push it too far, for lack of clear experimental evidence. Yet Hooke's atomism is more dynamic and rooted in motion than that of his master. Boyle, as we have seen, held to a broadly geometrical concept of atomic arrangement, whereas Hooke's was more kinematic and based on energy, or pressure, constantly exciting a medium so that the atoms became the efficient causes of all things. 
And when one came to the medium, or aether, in which the atoms were suspended and through which they received their powers of impulse, Hooke seems to have held different ideas at different times, depending on the results of particular researchers. Was the aether itself a 'stagnant', passive agent through which atomic collisions took place (in the way that railway lines are passive agents down which colliding waggons move), or was it the aether that originated the motion? Hooke considered that the primary forces of nature, such as light, magnetism, and gravity, might act through aethers, or parts of the aether, that were peculiar to themselves. [81
As a mechanist, Hooke needed a medium of some sort if a cause were to produce an effect, for without a physical connecting agent, no matter how tenuous, one was no better off than the magicians who happily explained cause and effect by means of occult sympathies. One fundamental way in which Hooke differs from modern (or post-Newtonian) scientists is in his concern with active principles and connecting mediums, for like most other seventeenth-century researchers, he was still a 'philosopher' who was interested in the causes of things. Though he, like Boyle, Descartes and Gassendi, had abandoned the Greek qualitative approach to nature, in favour of a mechanical, quantitative approach, his thought processes were still haunted by the sources of cause and effect, albeit re-dressed in mechanical garb. It took Newton and the scientists of the eighteenth century to bequeath causes to the metaphysicians, and concentrate on expressing the nature of effects in precise mathematical terms.
If there were one single mechanical principle which Hooke saw as present in most parts of physical nature, it was vibration. In many branches of research, he saw vibration as the thing which moved from an active source, through its appropriate aether, to produce a measurable effect. We will return to Hooke and vibration when looking at his work on spring and the elasticity of bodies.
Though Hooke might have been happy to entertain the presence and characters of atoms and aether when speculating about an ultimate metaphysic for science, he had a clear understanding of what had made the scientific discoveries of the age possible: an enhanced ability to perceive and quantify nature by means of instruments. It is in the long Preface to Micrographia in 1665 that he sets out most clearly his scientific manifesto and speaks of instruments as devices which lend new investigative power to relatively imprecise human sense-perception:
The next care to be taken, in respect of the Senses, is a supplying of their infirmities with Instruments, and, as it were, the adding of artificial Organs to the natural. 
In his stress on the primacy of the senses in all perceptions of nature, from everyday experience to sophisticated research, Robert Hooke became one of the founders of the British empirical tradition and an influence on figures like John Locke. Hooke, moreover, did not merely talk about sense-knowledge, but made it the very king-pin of his experimental technique, realizing that, if one were going to investigate the 'animalcules' in water, the surface of the moon, or the vacuum, then the senses would need artificial enhancement by means of instruments. The invention and use of instruments, indeed, runs through his entire career, from his first devising of an airpump for Boyle in 1659 to his last recorded scientific utterance in December 1702, when, according to the Posthumous Works (p. xxvi), he tried to devise an improved instrument to measure the horizontal solar diameter, 'but discovers not the way'. It is true that Aristotle had placed an emphasis on the senses when examining natural phenomena, but to him the reality of a thing was defined in the totality of its parts as perceived by the gross senses. But what the new, instrument-based, experimentalists introduced into sensory perception were new options whereby a scientific reality could be defined. Was the correct definition of a horse that of a large quadruped, or was it the mechanics of its skeleton and muscles, its heart-rate related to body weight, or particular characteristics of cells and blood as seen under the microscope?
Having looked at the principles that underlay Hooke's scientific thought and approach to knowledge, it is now time to examine some of his researches in more detail.
Robert Hooke's researches
1. Air and combustion
For two thousand years up to the seventeenth century, air had been regarded as a stable, simple, vital force of nature, and one of the four elements. According to Aristotle, it must invade all spaces not occupied by one of the other three elements, for the universe was full and intact, with no unoccupied interstices. Nature, therefore, abhorred the concept of the vacuum, for in a balanced universe, actual vacuums could not exist. It was seriously disconcerting for the loyal adherents of Aristotle's physics after 1643, therefore, when Galileo's pupil, Evangelista Torricelli, found an empty space in the sealed glass tube, above the mercury, in his newly invented barometer. Philosophers across Europe tried to devise ways of establishing the properties of the 'Torricellian vacuum', and the anti-Cartesian Frenchman, Giles Persone de Roberval, had the idea of inserting a deflated and sealed carp's bladder into the barometer tube before it was filled with mercury and inverted. The apparently deflated bladder suddenly inflated in the vacuum, as the residual air inside it expanded to fill the vacant space. 
To investigate this new, instrument-generated phenomenon of nature, however, it was necessary to make a vacuum that was physically larger and more accessible than that inside a barometer tube. Otto von Guericke in Germany pumped air out of a sealed barrel, invented the famous 'Magdeburg Hemispheres' which needed teams of horses to separate when evacuated, and showed that air pressure bore down intensely upon an evacuated space.
It was when Robert Boyle began his own researches into air and vacuum around 1658 that Robert Hooke made his debut on the scientific stage, for while he had previously worked for Thomas Willis in Oxford, it was with Boyle that his creative scientific genius first found expression. Originally, Boyle had gone to Ralph Greatorex for the construction of an airpump which was to be more versatile than von Guericke's, but it had been a failure. How Hooke, as an entirely academically trained individual, without any formally imparted craft skills behind him, had built or perfected a successful pump is hard to explain, for Greatorex was the leading pumping engineer in England, and a man who had made a considerable amount of money in draining the Fens. [ii] But Boyle gave Hooke acknowledgement in his published researches, though it is clear that even with Hooke's machine, vacuums were difficult to maintain, and pistons, cylinders and valves were liberally coated with 'Sallad Oyl' to make better seals. It took about three minutes of hard pumping to get a good vacuum, and some additional action was probably necessary if it were to be kept for several minutes, as the air squeaked and whistled through the imperfect seals. Hooke does not seem to have done the manual work during the experiments, for Boyle refers to 'our pumper' as a third party. 
Apart from any new features that were special to the pumps or valves, Hooke's machine contained three design features that were of the greatest significance. The first of these was a large glass vessel some fifteen inches in diameter, called the 'Receiver', as the space to be evacuated. Secondly, a brass stopper some four inches in diameter set into the top of the receiver made it possible to gain easy access to the experimental area, and seal everything up before the pumper set to work. Thirdly, an ingenious secondary brass stopper with conical sides passed through the large stopper, so that when liberally coated with salad oil, it could be turned around without breaking the air seal. This rotating stopper could be used to pull a thread to actuate some experiment in vacuo. With Hooke's machine, therefore, the experimenter had easy physical access to a fairly large experimental site that was entirely visible through the glass receiver. It was to be used to conduct a series of experiments which needed clear vision and the ability to ignite and move things. 
Candles, glowing coals, and slow-match all went out in the evacuated receiver, though the coals could be revived to glow again spontaneously if air were re-admitted in time. Boyle and Hooke were especially interested in the behaviour of smoke that rose from the extinguished candle wicks. Did some all-pervading aether that was much more subtle than air still bear the smoke upwards before it touched the inner walls of the receiver and descended? Was the Aristotelian doctrine true in so far that all fire-products rose upwards-even in vacuo? But it was impossible to be certain about the true cause of the smoke's behaviour, for both Boyle and Hooke realized that they did not have perfect vacuums, and a very small quantity of residual air was always likely to be present. 
What the airpump experiments did demonstrate beyond all doubt, however, is that the element air was greatly elastic and capable of much rarefaction and compression. Hooke found, for instance, that when a burning candle was placed inside a sealed vessel of a particular size, the flame went out in about three minutes. But when the air was compressed by pumping into the same vessel, it burned for fifteen minutes, thereby showing that the active principle in air 'which sustained a flame in any given volume could be altered mechanically. When Boyle repeated Roberval's carp bladder experiment with that of a lamb in the capacious receiver of the Hooke airpump, he concluded that air could expand by a factor of 152 to fill the vessel. 
But in many ways, Boyle's interests in air differed from those of Hooke, for while Boyle was primarily interested in the physical properties of aerial elasticity, and how it related to his ideas on atomic structures, his assistant's interests tended to be more chemical. In particular, they related to Hooke's interests in combustion and in what he conceived as a corrosive property in air.
Almost certainly, Hooke's interests in air and its role in combustion stemmed from Boyle's original researches into saltpetre, or nitre, as the seventeenth-century chemists called potassium nitrate. Boyle had conducted these experiments around 1655, several years before the airpump was built, as part of an investigation into the nature of saltpetre's inflammable principle' and its relation with air. A crucible of saltpetre, or nitre, had been heated until it melted. Pieces of charcoal were then dropped into the fused mass, and each piece immediately combusted into flames and smoke. After enough charcoal had been dropped in, however, no more combustion took place, and the washed residue was found to be inert. But when this residue was mixed with nitric acid, Boyle observed that the easily identified crystals of saltpetre began to 'shoot' in it once again, and when prepared and dried, could be heated so as to consume more charcoal. 
It seemed, indeed, that the so-called 'inflammable principle' could be driven off by burning, and then restored by a 'nitrous' chemical substance. Was burning, therefore, not an innate force of nature, but a chemical process where ingredients were exchanged between the air and chemical substances? If this were the case, it seriously challenged Aristotle's doctrine of combustion as caused by the element fire, and further implied that Aristotelian air was not a pure element, but possessed chemically reactive properties.
The challenge to Aristotelian ideas was strengthened when Thomas Willis, who was Hooke's first Oxford employer, began to experiment with a new preparation named aurum fulminans, or exploding gold (gold fulminate), which could explode without fire. All that one needed to do was to place a little aurum fulminans on to a spoon, and cover the spoon with a heavy coin. If one gently tapped the spoon on to a table top, the chemical exploded violently and blew the coin up to the ceiling. 
If aurum fulminans could create fire without a spark, it was shown during the winter of 1659-60 that gunpowder, which is rich in saltpetre, could be fired in the evacuated receiver of the airpump without the need for air. The low winter light and the irregularities in the glass made it difficult to ignite the gunpowder by means of a burning-glass focusing the sun's rays. Instead, Hooke devised a frame that could be secured inside the receiver upon which was fastened a cocked pocket pistol with a pinch of gunpowder in its flashpan. By turning the airpump's secondary brass stopper, with its well-oiled airtight seal, a piece of string could be used to pull the trigger of the pistol. The ensuing discharge of the priming powder in vacuo clearly suggested that air was not necessary for the discharge of gunpowder. 
By the early 1660s, Robert Hooke had developed a theory of combustion in which the elastic medium of air possessed two quite separate properties. It contained a 'nitrous' part, which was capable of reacting with substances to produce combustion or explosion, and a 'fixed' or inert part. He outlined the theory and the experiments by which he tried to substantiate it in 'Observation 16; Of Charcoal' (p. 103) in Micrographia. Hooke envisaged air as a powerful dissolving agent, or, in the nomenclature of the early chemists, a menstruum. Whenever 'sulphureous', or potentially inflammable, substances like wood were heated to a particular point, their atoms were furiously descended upon by the menstruum of 'nitrous air' or 'aerial nitre'. The potentially volatile 'sulphurs' within the substance were thereby released, and an inert ash remained. Commonly combustive materials like wood and candle wicks could not burn in vacuo because even when locally heated by a burning-glass no aerial nitre was present to dissolve them. Gunpowder exploded in vacuo, however, for it contained its own inbuilt supply of 'fixed' nitre, in its saltpetre.
In his Cutlerian Lecture Lampas, delivered to the Royal Society in 1677, Hooke developed his ideas on combustion when he analysed the parts of a lamp or candle flame. He noticed that the point of combustion appeared to be at the bottom part of the conical flame, where the oil rising up the wick became excited by the heat above it. At a critical point, it was devoured by the aerial nitre, and produced the tulip-shaped inner flame, where the rising sulphurous particles or atoms made contact with the aerial nitre to produce a glowing combustive interface. He also realized that the interior of the flame did not emit light, but only the tulip-shaped, combustive interface around it. The interior consisted of heated but non-luminous sooty particles that had failed to go off, as it were, and simply rose as greasy smoke. It was within this dark, sooty interior that the non-light-emitting part of the wick lay, and Hooke noticed that when this spent wick fell over, and broke through the combustive interface, it glowed red, as it entered the aerial nitre that surrounded the flame. 
Hooke obtained this information by inserting thin plates of glass and 'Muscovy glass' (mica) in the flame, both from above and bisecting the flame sideways, to reveal the light and dark zones of the interior. He also used powerful sunlight to project an image of a candle flame on to a whitewashed wall, whereby he could discern the dark interior and heat zones in the resulting shadow.
Hooke's researches into combustion and the airpump naturally led him to the physiology of respiration. In this immediately post-Harveian age, physiologists were investigating the relationship between blood circulation and respiration, and the deaths of birds and small animals
that were placed in the receiver of the airpump demonstrated the importance of 'vital' air to life in a way that we take for granted today. The circulatory physiology of William Harvey recognized that the lighter colour of arterial, as opposed to venous, blood had something to do with its having picked up air in the lungs. In 1667, Hooke suggested that the blood might pick up 'aerial nitre' in the lungs. Boyle noticed that fresh lamb's blood inside the airpump receiver frothed as the pressure fell, and John Mayow repeated and refined the experiment. When blood from the veins was put into the airpump, however, nothing happened. 
The asphyxiating properties of the newly discovered vacuum were sufficiently impressive that when a visiting dignitary from Denmark was entertained by the Royal Society, around 1663, an anonymous wit wrote the following verse:
To the Danish Agent late was showne,
That where noe Ayre is, theres noe breathe;
A glass this secret did make knowne
Where[in] a Catt was put to death.
Out of the glass the Ayre being screwed,
Pusse died, and ne're so much as mewed. 
To study the effect of low atmospheric pressure on a human being, Hooke devised a sealed chamber in 1671, out of which the air could be evacuated. He sat in the chamber himself, while his 'pumper' took the pressure down to well below normal atmospheric pressure. This courageous experiment, in which Hooke experienced pains in his ears and deafness, probably made him the first person to experience 'high altitude' sickness, and monitor its effects. 
Within a decade, the Aristotelian explanations for burning and breathing, and the plausibility of two of the four elements, had been fundamentally challenged. In the early 1670s, moreover, Dr John Mayow, of Wadham College, Oxford, had started to take Hooke's ideas one stage further, when he discovered that burning and breathing competed with each other for the same active ingredient in an otherwise inert volume of air.  But it would be historically incorrect to see Hooke with his aerial nitre as a proto-discoverer of oxygen, for seventeenth-century researchers had no real concept of chemically specific gases, and still couched their ideas in terms of 'vital principles' and what we might call allotropic states of air. Yet the very fact that air might have allotropic states, that it was vastly elastic, and that it might somehow be able to fix its vital parts in stable chemical substances, signalled a fundamental shift in ideas about the natural world. And all these new ideas had been gained not by speculation, but by 'putting nature to the torture' in a carefully planned enquiry that hinged on a newly invented piece of apparatus.
The microscope was invented some thirty years before Robert Hooke was born. The Yorkshire scientist Henry Power had published microscopical observations before Hooke, and in 1661, Marcello Malphigi had used the instrument to provide clinching evidence in favour of Harvey's theory of blood circulation when he discovered the capillary vessels in the lungs of a frog. Yet for over half a century after its invention, the microscope had been a poor relation to the telescope in terms of its ability to produce fundamental scientific discoveries. Not until Robert Hooke published his own microscopical researches, in 1665, was it made manifest to the scientific world that the microscope revealed an organized realm of nature that was as diverse in its structures and as vast in its scale as the telescopic universe. For centuries, indeed, and long before the invention of the telescope, philosophers had speculated about the vastness of space, though no one had thought seriously about the existence of living creatures that were smaller than cheese-mites or inanimate objects smaller than dust particles. It is true that the atomists had conjectured about the existence of the minuscule particles that composed matter, but these had been objects of a philosophical character, which held out no hope of physical detection.
When Hooke's Micrographia first appeared in the bookshops in January 1665 at a lavish thirty shillings per copy, therefore, it had a quite sensational impact. It bowled Samuel Pepys right over and transfixed him in his chair until two o'clock in the morning; 'the most ingenious booke that ever I read in my life'.  More than anything else, it whetted Pepys's appetite for the New Science. He subsequently bought instruments, joined the Royal Society in February 1665, and in 1684 became its President. Micrographia was one of the formative books of the modern world, and like all influential pieces of writing, was capable of triggering responses on many different levels of understanding.
Within the scientific community, it provided one of the most articulate and beautifully presented justifications for experimental science ever produced. Mere observation, after all, could take one no further than Aristotle had gone in his descriptions of animals and natural forces, but when observation was refined by means of specially designed instruments, and used to 'put nature to the torture' in a context of addressed questions, then remarkable discoveries could be made. Micrographia not only provided a wealth of new data for science to consider, but showed how experimental investigations could be built upon them. A seemingly simple observation of a piece of charcoal under the microscope, for instance, could lead to a recognition of the presence of cells, to an investigation into burning, and to Hooke's work on the dissolving properties of aerial nitre. None of the Observations in Micrographia are simple; all of them are detailed starting-points for further physical investigations in one way or another. Hooke showed that sense knowledge could be reliable when used within the correct disciplinary restraints, and what the body could physically perceive via its 'artificial Organs' left little doubt that the experimental method actually worked.
If Micrographia was so important within the scientific community, it must be remembered that its influence on the cultured laity, like Pepys, was equally profound. The book was written in an easy style that would have been accessible to any innumerate who could read Shakespeare or the Bible, for Hooke could write vivid and powerful prose. It was, moreover, the first proper picture-book of science to come off the presses, for its sixty Observations were accompanied by fifty-eight beautiful engravings of the objects seen beneath the microscope. Hooke's artistic gifts had been essential to Micrographia, for only a man who could faithfully interpret and delineate the awkward images that were produced by the compound microscopes of the 1660s could envisage such a book in the first place. Modern science is replete with visual images, and the televisual image is the most powerful medium through which its ideas are now communicated to the lay public. We must not forget that this tradition of visual communication largely begins with Hooke's Micrographia.
Part of the popular fascination of Micrographia lay in the arresting new perspective that it cast on to common and familiar objects: a fine needle point looked like a rough carrot (Observation 1, p.1), delicate silk looked like basket-work (Observation 4, p.6), and extinguished sparks resembled lumps of coal (Observation 8, p.44). But it was the observations of moulds of various kinds, 'Eels in Vinegar', and of insects that were the most sensational (Observations 53 and 54, pp.210-il). That a flea could be depicted with the anatomical precision of a rhinoceros was quite shocking, and one wonders how many nightmares were occasioned by Micrographia in that unbathed age. In the late twentieth century we have become blas6 about the impact of scientific discovery, generally communicated by means of visual images, and it is hard for us to imagine the fascination value of a book like Micrographia, which opened up a hitherto invisible universe to the reading public.
One of the hallmarks of an outstanding scientific discovery in our own time - from black holes to DNA - is its influence on popular entertainment. Micrographia influenced the creative imagination of the Restoration in a variety of ways, but nowhere more embarrassingly, for Hooke, than in Thomas Shadwell's box-office success The Virtuoso, 1676. This play used the recently published discoveries and activities of the Royal Society to provide part of the plot motive and a main ingredient of the comedy in this farce of duplicity, seduction, and experimental philosophy. The butt of most of the jokes was Sir Nicholas Gimcrack, a foolish amateur scientist, or Virtuoso, who wasted his energies and fortune on seemingly absurd enterprises. Sir Nicholas 'spent two thousand pounds on microscopes to find out the nature of eels in vinegar, mites in cheese, and the blue of plums which he has subtly found out to be living creatures'. On 25 May, Hooke was told about the Virtuoso, then recorded in his Diary, on 2 June 1676, 'With Godfrey and Tompion at Play. Met Oliver there. Damned Doggs. People almost pointed.'  Such was the price of scientific fame.
Micrographia, therefore, was far more than a collection of careful observations made, as the title said, 'by the aid of magnifying glasses'. It was one of the first fruits of the new science to strike deep into the non-scientific imagination, and show how a cardboard tube containing two lenses could produce images of a vast new realm of knowledge. And when this realm was communicated through the medium of clear English and beautiful engravings, it could keep senior civil servants from their beds, and provide material for popular plays. Second only to Newton's Principia, which was a very different type of book that was published twenty-two years later, Micrographia was one of the formative books of the age, and assured Robert Hooke's reputation as a scientist of genius.
3. Of flight and of spring
Artificial flight, by means of mechanical contrivances, formed one of Hooke's most enduring fascinations, and could well have gone back to those childhood days in Freshwater when he devised clocks, model ships and other machines. 'At schoole he was very mechanical, and (amongst other things) invented thirty several ways of flying',  which must have amazed Dr Busby. And as in several other ways, he found an ideal metier at Oxford, for Dr Wilkins, the Warden of Wadham, had already published books on the use of flying machines, for both terrestrial and celestial journeys. With Dr Wilkins, Hooke 'made a module, which, by the help of springs and wings, rais'd and sustained itself in the Air'.  This led Hooke to consider a topic to which he would return at other times in his subsequent career: elasticity and spring.
After abandoning hope of using human muscle power in the propulsion of a flying machine, Hooke attempted to devise an artificial muscle substance, though we do not know what he used. As no rubber-based elastic materials were known in the seventeenth century, it is likely that his principal experimental agents were metal springs and gunpowder, which sometimes figure in the seventeen references to flight that occur in his Diary between 1673 and 1679.  But one Diary entry for 11 February 1675 not only makes a distinctly boastful claim for the invention of an artificial muscle, but indicates that his quest for such a material went back to his Oxford days:
Dr Croon at Royal Society read of the muscles of birds for flying. I discoursed much of it. Declared that I had a way of making artificial muscule to command the strength of 20 men. Told my way of flying by vanes [wings] tryd at Wadham. 
Hooke's references to the use of gunpowder probably relate to the agent that was to be used to compress the artificial muscle prior to its activation. Various scientists around Europe were considering the force provided by controlled explosions as an agent that might perform mechanical tasks.
Robert Hooke left no clear drawings or even descriptions for either a complete flying machine or for any kind of 'artificial muscle'. His most detailed treatment of actual flight came from his descriptions of insects in Micrographia. In his account of the Blue Fly (Observation 38, p.172) one finds Hooke's genius for observation, experiment, and mechanical analysis at its finest. What especially interested Hooke were the different wing-velocities of flies and bees on the one hand, and butterflies and moths on the other. Under the microscope, those insects that buzzed as they flew had hard 'glassy' wings, whereas moths were silent and had downy, or 'feathered' wings. These 'feathers', Hooke considered, could trap air and help the creature to float, so that less energy was needed to fly, and they made no sound.
The polished surfaces of bee and fly wings, however, were incapable of trapping air, so that only an exceedingly rapid motion could sustain them in flight, in consequence of which they buzzed. From these preliminary observations, Hooke went on to investigate the aerodynamics of the Blue Fly. Securing a lively fly upon the end of an unsharpened quill by means of a spot of glue, he first examined it with a powerful magnifying glass. When he tilted the quill, so that the fly might sense that it was flying in forward or vertical directions, he observed, from the blurred
shape that they made, that the wings beat in arcs of different amplitude with relation to the direction of the fly's body. As the amplitude changed with assumed flying direction, so did the musical pitch of the buzz. Hooke suggested that a stringed instrument might be tuned in unison with this pitch, and that the ensuing note or vibration could be expressed in mathematical terms.
Very frustratingly, Micrographia takes this point of physical vibration and musical pitch (which was two centuries before Helmholtz) no further, other than to conclude that a fly's wings beat 'many hundreds if not some thousands of vibrations in a second minute of time'  and that it was the most rapid mechanical action in nature. But Hooke does appear to have come up with a relatively precise value within eighteen months of Micrographia's publication, for Samuel Pepys, who was still very much of a scientific beginner, recorded in his Diary a meeting with Hooke on 8 August 1666:
He did make me understand the nature of Musicall sounds made by strings, mighty prettily; and told me that having come to a certain Number of Vibracions proper to make any tone, he was able to tell how many strokes a fly makes with her wings (those flies that hum in their flying) by the note that it answers to in Musique during their flying.
Though Hooke does not explicitly state how he believed the wing-action of a fly kept it airborne, he seems to have related it to the spring, or elasticity, of the air, as the down-beat of the wings created a somewhat compressed pocket of air on which the creature rode, while the up-beat created a form of suction that lifted it up. One can see how these concepts related to his earlier work, conducted when he was Boyle's assistant, into the 'spring' of air.
It has already been mentioned that vibration was widely used as an explanatory mechanism by Hooke. Vibration was intimately related, in Hooke's thought, with elasticity, spring, and resonance, while these in turn were seen as mechanical responses to more fundamental agencies such as 'force', light, weight, and gravity.
While Hooke possessed no coherent idea of how springy bodies differed in structure from inert ones, it is clear that his scientific interest in elasticity went back to his Oxford days. It was in the company of Wilkins and Boyle that he had first encountered ideas of making artificial muscles for flying machines, and he recalled a curious pneumatic fountain that Dr Wilkins had built in the gardens at Wadham College, whereby
the Spring of the included Air to throw up to a great height a large and lasting stream of water: which water was first forced into the Leaden Cistern thereof by two force pumps which did alternately work, and so condence the Air included in a small Room. 
As this device had been operational before Hooke had built the airpump for Boyle, one wonders at the range of experiments with compressed air that were going on in the 1650s, before it was possible to work properly with vacuums.
Like many of Hooke's researches, his work on spring and elasticity was done at scattered intervals stretching over several decades. In the late 1650s, he had been experimenting in Oxford with spring-regulated timepieces, and his 'pendulum watches' attempted to apply the isochronal principle of the pendulum clock to a portable timepiece in 1660.  His fullest discussion of the use of springs to produce isochronal swings within a watch balance was published in 1676, and was intended to develop a timepiece whereby a ship could find its longitude at sea. Though Hooke's timepieces were not sufficiently accurate to 'be used as marine chronometers (nor would such a device be practicable for nearly a century), his application of spring tension and release to produce equal rotations of a watch balance-wheel provided the fundamental principle on which all portable time-keepers would be based down to the invention of electronic chip watches in our own time.
It was in his Helioscopes in 1676 that Hooke followed the popular seventeenth-century conceit of announcing a discovery in an anagram: cediinnoopsssttuu. He published its key two years later, in his most complete treatment of elasticity, in De Potentia Bestitutiva, or Of Spring. Here Hooke enunciated the original formulation of the law that bears his name: Ut Pondus sic Tensia, or 'the weight is equal to the tension'.  As the tension was seen as the product of an increasing series of weights in pans suspended on coiled springs, it is easy in this pre-Newtoniangravitation age to understand how Hooke spoke of the pondus, or weight, as acting on the spring. The formulation of 'Hooke's Law' with which we are more familiar today is Ut Tensia, sic Vis, or 'the tension is equal to the force'.
In De Potentia Bestitutiva, Hooke presented his most complete treatment of vibration and elasticity, as well as expressing his concept of an aether that pervaded 'the whole Universe' and in which particles moved continually. This concern with a vibrative agency that could express the motions of a fly's wings and also the propagation of light, or gravity, in space, was the nearest that Hooke came to a 'Grand Unified Theory' in the Mechanical Philosophy. And considering his fundamental concern with mechanism in nature, one can understand the resentment that Hooke felt when Newton presented the very different unifying theory of Universal Gravitation to the Royal Society in 1686.
Before considering Hooke's ideas on gravity, however, it is important to look at his wider work in astronomy, and the way in which instrumentation and mechanism lay at the heart of his thought processes in this as in all the other branches of science that he investigated.
4. Astronomy and gravitation
Of all the individual branches of science to which Robert Hooke made significant contributions, astronomy was the most extensive. Astronomical matters concerned him, in one way or another, for well over forty years, as he dealt with the subject theoretically, observationally, and from the viewpoint of instrumentation. Of the twenty-one papers that he contributed to Philosophical Transactions, over a dozen deal with astronomy. Two astronomical observations appear in Micrographia, and during his frenetic decade of architectural activity, the 1670s, he produced his most significant astronomical publications, while his last recorded scientific investigation, in 1702, was an attempt to measure the solar diameter more accurately.
Hooke was an assiduous collector of data relating to the natural world, and he took particular pleasure in using the telescope and the microscope to add to it. The surfaces of planetary bodies were of great interest to Hooke, especially during the 1660s, when he was using various long telescopes (including those of twelve and sixty feet in focal length) to observe them. Robert Hooke, along with Cassini and Huygens, was among the first astronomers to carefully observe the surface of Jupiter. In May 1664, he reported a small round spot on the biggest Jovian belt, which he believed, unlike a satellite shadow, to be a permanent feature. It moved over two hours, and while Hooke later claimed to have used it to measure the planet's period of axial rotation, it was Cassini, in fact, who first published a value for Jupiter's rotation. In June 1666, however, Hooke reported another permanent spot, and differentiated its appearance from that of a satellite shadow. 
Observation 60 (p.242) in Micrographia consists of an examination of a group of lunar craters made with a thirty-foot telescope in October 1664. In addition to the very considerable amount of detail that Hooke includes in his survey of that part of the lunar surface which Riccoli had named Hipparchus, he follows this with a discourse on lunar geology. As in most things, Robert Hooke is not satisfied with simply describing something, but is impelled to construct a plausible experiment and find an explanation. From his observations of the concave shadows produced inside the 'pits' or craters, Hooke proposed that they were probably the products of Earthquake (or 'Moonquake') generating processes within the body of the satellite. He likened the 'pits' to those found on the surface of a pot of boiling alabaster, or the outlines of the bubbles that remained if one blew air through a nearly solidified mixture of pipe-clay and water (p.243).
Hooke's astronomical observations contain references to the characteristics of the telescopes with which he made them, as one might expect from a man who possessed such a thorough-going instrumental and sensory approach to research. But it is in his observations of the Pleiades star cluster, also recorded in Micrographia (Observation 59, p.241), that he initiated discussion into what modern astronomers refer to as the resolving power' of the telescope. Though Hooke was aware of the higher magnifications obtained with object-glasses of longer focal length, and that with a telescope of twelve feet he could see seventy-eight stars in the Pleiades, whereas Galileo had only been able to see thirty-six, he recognized the crucial principle that it was object-glass diameter that was of primary importance in seeing faint objects. He experimented with a series of object-glass stops, and noticed that he saw the maximum number of stars through an entirely unstopped lens, though, surprisingly, Hooke never gives his object-glass apertures in inches.
During the winter of 1664-5, the skies of the northern hemisphere were dominated by a brilliant comet, which was the most conspicuous since that of 1618. Hooke observed it with his long telescope from London, and conducted the first detailed investigation into cometary nuclei and tails. It was not until a second brilliant comet appeared in 1677, however, that he published his researches. From studies of the two comets, Hooke concluded that their nuclei were solid bodies. A combination of their movement through the aether, and internal agitations within, caused the nuclei to be gradually eroded away, to form tails and streamers across space. He also concluded that comets did not merely reflect sunlight, but generated some light of their own, for there was never any sign of shadow in a cometary nucleus, even in those parts that were not facing the sun. 
The cause of cometary attrition and light-generation puzzled Hooke, and while he saw the aether as playing a role in this process, it was not so straightforward as that played by aerial nitre in the burning of a candle flame. In a cometary nucleus, observed Hooke, the brightest part was at the very centre, whereas in a candle or lamp flame (and here he drew attention to his work in Lampas), the centre was always dark, and the greatest light around it. 
When Hooke made his first observations of the comet of 1664, he lacked an effective micrometer whereby he could measure the angular diameter of the nucleus. His genius for improvization was displayed when he watched the comet low in the sky, and compared its nucleus diameter with the apparent width of an ornamental iron rod supporting a weather-cock on the roof of a distant building behind which the comet passed.  By later measuring the iron rod, and its distance from his telescope, he was able to calculate that the comet's nucleus was about 25 arc seconds, and its coma 4½ minutes in diameter.
How to obtain accurate angular measurements ran through almost everything Hooke did, for astronomy was the most intensely instrumental of the sciences, and astronomers across Europe realized that it was on refined angles that arguments and theories must stand or fall. Hooke's lack of a micrometer was solved in 1667, when he saw Richard Towneley's instrument, which was based on a prototype of 1640 invented by the Yorkshireman William Gascoigne. This instrument used a pair of fine-pitched screws to move two pointers in the focal plane of a Keplerian telescope. By enclosing the object to be measured between the pointers, its angular diameter could now be computed to within a few arc seconds, if one knew the exact focal length of the telescope, and the pitch of the screw which moved the pointers. Hooke published an engraving of the instrument to accompany Towneley's description in 1667. Its principle was to lie at the heart of astronomical measurement down to the twentieth century. 
Hooke quickly developed the concept of using screw turns to measure angles in the remarkable quadrant that he described to the Royal Society in 1674. In this instrument, he attempted to avoid the problems of unequally drawn degrees on the scale of an astronomical instrument by cutting fine teeth into the brass edge of the quadrant. By rotating a precision tangent screw along these teeth, he hoped to be able to express degrees, minutes, and seconds in full and part turns of the screw. It was a brilliant and portentous idea and one of the earliest attempts to apply precision mechanics to astronomy. Unfortunately, like so many of Hooke's inventions, it went beyond the current skills of manufacture and failed to work properly. But one Hooke invention that was originally intended to form part of an astronomical instrument was his celebrated 'Universal Joint', which was devised to operate an adjusting arm of his Helioscope apparatus in 1676. 
One man who tried to use a Hooke screw-edge angle-measuring instrument for regular astronomical research was the first Astronomer Royal, John Flamsteed, for Hooke had been brought in to provide designs for some of the original instruments of the Greenwich Royal Observatory in 1675. But it is clear from his remarks that Flamsteed did not find Hooke's prototype instrument to be very successful in practice, complaining that he was 'much troubled with Mr. Hooke who, not being troubled with the use of any instrument, will needs force his ill-contrived devices upon us'.  Robert Hooke was the brilliant deviser of machines, who could detect a way whereby a principle in mathematics could be expressed in metal to produce an experimental instrument. Flamsteed, on the other hand, was the painstaking working astronomer, going through his nightly slog of measuring star positions on top of Greenwich Hill, and he did not wish to be burdened with experimental instruments, no matter how cleverly they were conceived.
But John Flamsteed owed more debts to Robert Hooke than he cared to acknowledge. He made extensive use of telescope micrometers at Greenwich, while Hooke's devising of a thirty-six-foot zenith instrument in 1669, in his attempt to measure a stellar parallax, provided an interesting application of telescopic mechanics to trying to prove the Copernican theory. Hooke's endeavour to demonstrate the motion of the earth failed, but this only led Flamsteed to try for himself, and also fail; and he further took up Hooke's suggestion that zenith star images could best be observed from the bottom of deep shafts in the ground, setting up his 'well telescope' at Greenwich. 
Gravitation was a subject which occupied Hooke's interests for well over twenty-five years before Newton published his Principia. According to Aristotle, the Earth could be the only gravitational body, as it drew 'heavy' things towards itselL By the 166()s, however, astronomers were considering the possibility that gravity could be possessed by other bodies as well, such as the planets. The nature of gravitation was in itself mysterious, though in his lunar observations in Micrographia (p.245) Hooke had argued that the even, spherical shape of the moon indicated that it must possess a gravitational power which caused everything to fall evenly around its centre. The same went for the planets.
As a mechanist, Hooke looked for physical connections in nature through which gravity might operate, and this led him to believe that an aether must exist whereby it (along with light and magnetism) could move or resonate. His thorough-going experimentalism was always leading him to new 'tryalls', and in 1662 and 1665 he reported to the Royal Society experiments conducted on the towers of Westminster Abbey and old St Paul's Cathedral. Identically heavy weights on identically heavy lines were prepared. One was rolled up and placed in one pan of a balance, while the other was set free down the tower while attached to the other balance arm. Would the weight that was now 90 feet closer to the earth become heavier against its rolled-up companion? He found no appreciable change, so he tried it in reverse, as it were, down a deep well on Banstead Downs, in Surrey. Once again, no decisive results could be obtained. 
By the 1670s, he was trying to find the operation of gravity in space. He argued that the motions of comets, whereby they were pulled out of once stable orbits to move near the sun and earth, must be occasioned by the 'vortices' through which gravity operated. But it was in his Attempt to prove the Motion of the Earth in 1674 that Hooke made some of the most pertinent remarks about gravitation that were made before Newton. He summed them up under three headings: (1) that gravitation exists towards the centres of all bodies, and between all bodies; (2) that all bodies will move in straight lines under their own impulse, but can be disturbed into orbits by other gravitational bodies; (3) that gravity acts more powerfully when bodies are closer together than when further apart, but 'what these several degrees are I have not yet experimentally verified'. 
He was clearly much closer to a solution by 6 January 1680, when
he told Newton that gravitational attraction 'is always in a duplicate proportion to the Distance from the Center Reciprocall', and 'as Kepler Supposes Reciprocall to the Distance'. Hooke does not tell us exactly how he had come to these all-important conclusions, though it is likely that it was by a combination of astronomical observations and the experiments with iron balls which he mentions in the same letter. He also told Newton in the same letter that Edmond Halley, who had recently returned from St Helena, had been puzzled by the fact that his pendulum clock ran slower up the mountain than it did when lower down, 'But I presently told him [Halley] that he had solved me a query I had long Desired to be answered ... that ... gravity did actually Decrease at a height from the Center.' 
It was Newton's refusal to acknowledge Hooke's insight into this Inverse Square Law of Gravitation, following the publication of Principia in 1687, that led to the appalling debacle which broke out in the Royal Society. Yet with a knowledge of Newton's work to hand, one can see how very differently the two men approached the problem of gravitation, and science in general. Hooke was the mechanist, constantly searching for physical connecting agents d~at could be demonstrated experimentally. Newton was the mathematician, willing to accept the presence of force that acted at a distance, provided that it was amenable to precise mathematical expression.
If astronomy, in its various branches, displayed Hooke's creative genius and powers of investigation at their highest, it was the practice of architecture that made him a rich man.
Robert Hooke, architect and City surveyor
It may seem strange to us today that a man without any practical training in building or architecture should have been appointed by the City of London authorities to be their Surveyor following the Great Fire in 1666. We know that Hooke was quick off the mark in presenting a proposed new ground-plan for the City, almost as soon as the embers were cold, but we do not know who pleaded his case for the Surveyorship. His old Oxford encourager, Dr John Wilkins, could well have been one of his backers, for Wilkins was well connected in the City, but we have no certain knowledge.
But the appointment of a non-professional like Hooke would not have seemed so absurd as it would do today. There was, after all, a tradition of 'gentleman' architects three centuries ago, with figures like Sir Roger Pratt, and especially Hooke's friend, Christopher Wren, who was appointed to the parallel post of Surveyor to the King. Between them, these two scientists, Hooke and Wren, were to be responsible for designing most of the principal Royal and City buildings in the metropolis. In Wren's case, the commitment of time was to shift away from mathematics and science to architecture almost full-time by 1675, though Hooke ran his Surveyorship and architectural practice in conjunction with an unabated commitment to scientific research. 
What made the untrained 'gentleman' architect a serious figure in the seventeenth century was the nature of the academic education that he would have received. Classical architecture, in many ways, is very formulaic, with its Orders, Cubes and proportions. A man with a knowledge of Latin and Euclidean geometry could master Vitruvius' De Architectura Libri Decem along with the sixteenth-century Italian architectural writers like Palladio. If he had a good eye for proportion, and a natural good taste that he had perfected by the close study of engravings of Greek, Roman and Renaissance buildings, he could learn to produce elegant designs. And if he had the imaginative powers of Wren and Hooke, combined with their scientific grasp of thrusts and forces, then he could produce ardhitecture of genius. While Hooke's architecture may not have been quite so brilliant as Wren's (and far fewer examples of it survive), engravings of Physicians' College, Bedlam Hospital, and other edifices none the less show that he was genuinely gifted as an architect. These gifts also found concrete expression in the very substantial fortune that he made from architecture and City planning.
Robert Hooke's Diary in the 1670s gives one some indication of his commitment to the Surveyorship. He had to authorize the safety aspects and street widths of other mens' designs, to try to make the new City less fire-hazardous than the old one, while he accepted commissions to provide drawings of his own. The rebuilding of London must also have put Hooke on his mettle as an administrator, as he tried to implement safety regulations, design buildings that pleased fee-paying clients, and organize tradesmen and contractors to see that things took physical shape. 
In London, Hooke designed many Livery Company Halls, part of the Thames and Fleet waterfronts, private residences in St James' Square, and great mansions, like that for Lord Montague. Outside London, he designed Ragley Hall, Warwickshire, almshouses, and parts of the Tangier Mole: numerous buildings of virtually every architectural rank. He sometimes worked in conjunction with his friend Wren, most notably on the design of the Royal Observatory, Greenwich, and the Fire Monument, or 'Piller', in Fish Hill Street. But his greatest independent creations were the magnificent buildings for the Royal College of Physicians, completed in 1679, and Bethlehem Hospital, or 'Bedlam'. 
Physicians' College, with its 'gilded pill'-surmounted dome, later came to be popularly ascribed to Wren, until Hooke's long-lost Diary came to light in Guildhall Library around 1890. Bedlam Hospital, with its French chateau style, pavilions, and 540-foot facade, was magnificent by any standards. The wits of the day made the remark that the English housed their lunatics in buildings such as those in which the French housed their Kings! In the original design, Bedlam represented advanced thinking in its accommodation of the mentally ill, with an individual cell for each inmate, which led the comic writer Ned Ward to comment upon the hospital's governors 'I think they were mad that built so costly a College for such a crack-brained Society'.  In Bedlam, Hooke had not only produced his own architectural masterpiece, but had created one of the landmarks of London for the next century. It figures in Pope's Dunciad, foreign visitors went to see the antics of the unfortunate patients, and William Hogarth set his last scene of the Rake's Progress in its now squalid and over-crowded interior. It was demolished in the early nineteenth century, and all that now survives are the two statues that once flanked the entrance: the frightening images of the 'crazed brothers', Raving Mania and Melancholy Mania, who now reside in the Victoria and Albert Museum.
Robert Hooke's buildings have had a disastrous survival record in general, which is probably one reason why we generally do not speak of him in the same context as Sir Christopher Wren. Most of what had survived into the nineteenth century perished in the wholesale remodelling of Victorian London. Even the Second World War took its toll, when the magnificent wooden screen which he designed for the Company of Merchant Taylors was destroyed by a bomb. The only Hooke building that survives intact is the beautiful red-brick parish church at Willen, Buckinghamshire, which he designed in 1680 for his old Head Master, Dr Busby, who was patron of the living. It is well worth a visit, in greater Milton Keynes.
In the 1670s Hooke showed himself to be a skilful architect, capable of working in several styles. He was also a competent administrator and planner, and played a major part in raising London from the ashes of 1666. And all of this was going on at the same time as he was measuring the stellar parallax, experimenting on respiration, flames, and vibrations, improving the design of watches, and formulating Ut Pondus, sic Tensio.
Hooke's character and friends
Robert Hooke never married, and on being appointed to his Gresham Professorship in 1665 was happy to live in College to the end of his days.
Though his first biographer, Waller, presented him as a melancholic, 'monastick' old bachelor, one must remember that this was the elderly, embittered Hooke who felt cheated by Newton. For most of his life, as his Diary vividly exemplifies, he was a highly clubbable man with a large social circle, though the mainsprings of his life, and of his friendships, were intellectual.
Though he never married, Hooke's Diary makes it clear that he had fleeting liaisons with a succession of seamstresses and maidservants.  But he was generous to them, and occasionally helped them to find husbands. Indeed, generosity seems to have been a clear mark of the younger Hooke, combined with a genuine kindness towards those who were dependent on him. These include the orphan boy, Tom Gyles, whom Hooke took in and educated, the young Harry Hunt, who later became Curator of Experiments at the Royal Society, and his ward, Grace Hooke. His friendships, moreover, seem to have covered a large social spectrum, and included enterprising tradesmen as well as Fellows of the Royal Society. Hooke's endlessly hungry mind crossed all social barriers in search of nourishment.
John Aubrey was a long-standing friend. Like Hooke, he had a roving intellectual appetite, but unlike him, was a child in the ways of the world. The Wiltshire landed gentleman, sliding inexorably into bankruptcy, contrasted sharply with the curate's son who was also an astute man of business, but they got on excellently. Sir Christopher Wren was also deeply valued and admired by Hooke, as was Boyle. Robert Hooke was a man of passionate likes and dislikes and he never forgot a friend or benefactor. He lamented the death of Dr Wilkins in 1672, and stayed in contact with Dr Busby down to his old Head Master's death in 1695.
His highly strung, prickly pride made it very natural for him to snap and scratch if he felt slighted or patronized. He disliked Henry Olden-burg ('Kindle-Cole', or mischief-maker) in the Royal Society, while he and Flamsteed were like cat and dog with each other. His traumatic relationship with Newton after 1687 seemed to overshadow the rest of his life, making him 'Melancholy, Mistrustful, and Jealous, which were increas'd upon him with his years'. [so] This was most unfortunate, for by temperament Hooke was an open and direct man.
During the last couple of years of his life, Hooke's health deteriorated rapidly. He suffered from swollen legs, chest pains, dizziness, insomnia, extreme emaciation, blindness, and what was possibly gangrene of the feet. Though it is impossible to diagnose causes of death across nearly three centuries, one suspects the presence of cardiovascular disorders and possibly diabetes as contributory factors. Hooke made no will, though he left £9580 in money and goods, along with some small property on the Isle of Wight. It was an estate that many country squires would have been proud to leave. He was buried in St Helen's Church, Bishopsgate, in the City, with all resident Fellows of the Royal Society in the cortege .
Robert Hooke was a figure of extraordinary and diverse creativity. With his grasp of ancient languages, the quality of his draughtsmanship as shown in the plates of Micrographia, and his success as an architect, he clearly possessed high artistic talents. And his craft skills enabled him to build an airpump where the country's leading pumping engineer had failed. But most of all, he was the man who showed that the 'experimental philosophy' actually worked and could be used to extend the bounds of natural knowledge. He was Europe's last Renaissance man, and England's Leonardo.
I wish to thank Mr A. V. Simcock, of the Museum of the History of Science, Oxford, for his assistance with books and early printed sources. I also thank Mrs Sheila Edwards and her staff at the Royal Society Library, and Mrs Irene McCabe and Dr Bryson Gore of The Royal Institution for their assistance with the Discourse.
Notes and References
1. Z. C. von Uffenbach, Merkwiirdige Reisen durch ... Ulm, 1753. Translated as London in 1710 (trans. W. H. Quarrell and M. Mare), London, 1710, p. 102. Aubrey's Brief Lives (ed. O. L. Dick), London, 1975, p.165. The Posthumous Works of Robert Hooke (ed. R. Waller), London, 1705, p. xxvi.
2. Robert Hooke, Micrographia, or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries thereupon, London, 1665, 'Preface', unpaginated, pp.7-8 from beginning.
3. Hideto Nakajima, 'Robert Hooke's family and his youth: some new evidence from the will of the Rev. John Hooke', Notes and Records of the Royal Society, 48 (1); (1994), 11-16.
4. Most of Hooke's early biographical information comes from Aubrey's and Waller's accounts (see ref. 1). See also R. E. W. Maddison, The Life of the Honourable Robert Boyle, London, 1969, pp. 92ff.
5. Thomas Sprat, A History of the RoyaPSociety of London, 1667, 2nd edn, London, 1702, p.67.
6. Margaret 'Espinasse, Robert Hooke, London, 1956, p.83.
7. Robert G. Frank Jr., Harvey and the Oxford Physiologists, Berkeley, CA,
1980, pp.133, 135.
8. Hooke discusses his 'aether' in a variety of places; e.g. Posthumous Works (ref. 1), pp.172, 174, 184. For a modern study, see John Henry, Robert Hooke, incongruous mechanist, in Robert Hooke, New Studies (ed. M. Hunter and S. Schaffer), Woodbridge, 1989, pp.157-62.
9. Micrographia (ref. 2), 'Preface', unpaginated, p.3 from beginning. One of the most significant studies on Hooke and the role of instrumentation in his science is by J. A Bennett, Robert Hooke as mechanic and natural philosopher, Notes and Records of the Royal Society, 35 (1), (1980), 33-48.
10. Oxford Physiologists (ref. 7), p. 116. Charles Webster, The discovery of Boyle's Law, and the concept of the elasticity of air in the seventeenth century, Archive of the History of Exact Sciences, 2 (1965), 441-502. Dr Webster's article provides one of the most thorough studies of Boyle's pneumatic work. For his treatment of Roberval, see pp.448-51.
11. Robert Boyle, New Experiments Physico-Mechanical, Touching the Spring of the Air, and its Effects (Made, for the most part, with a New Pneumatical Engine)..., 2nd edn, Oxford, 1662, p.4. Waller, in Posthumous Works (ref. 1), iii, states that Greatorex's machine had been 'too gross to perform any great matter'.
12. New Experiments (ref. 11), p.8.
13. New Experiments (ref. 11), pp.5-6.
14. New Experiments (ref. 11), pp.10-1l, 40-1.
15. Thomas Birch, History of the Royal Society, Vol.11, London, 1756, p.10, 25 January 1665. New Experiments (ref. 11), Experiments 4 and 6, p.32.
16. R. Boyle, Certain Physiological Essays..., London, 1661, pp.107-10.
17. J. R. Partington, A History of Chemistry, Vol.11, London, 1961, p.308. Also R. J. Frank Jr., John Aubrey F.R.S., John Lydall, and science at Commonwealth Oxford, Notes and Records of the Royal Society, 27 (1973), 193-217; for aurum fulminans, see pp.197-8.
18. New Experiments (ref. 11), pp.48-50. When Boyle did succeed in focusing sunlight on gunpowder, however, he found that it did not combust as rapidly as it did in air, but that aurum fulminans, when dropped onto a heated plate inside the airpump receiver, dissolved in a flash: History of Chemistry, Vol.11 (ref. 17), p. 527.
19. R. Hooke, Lampas: or, Descriptions of some Mechanical Improvements of Lamps & Waterpoises. Together with some other Physical and Mechanical Discoveries, London, 1677, pp.4-8. Hooke had already described his work on flame in February 1671, in 'An Experiment to prove the substance of a Candle or Lamp is dissolved by the Air, and the greatest part thereof reduced into a Fluid, in the Forme of Air', 14 March 1671-2, Royal Society MS, RBC 3, 201-3.
20. Robert Boyle, Philosophical Transactions, 5 (1670), 2043. For John Mayow see History of Chemistry, Vol.11 (ref. 17), p.602.
21. From The Ballad of Gresham College (anon.), a Broadsheet of c. 1663, printed with critical notes by Dorothy Stimson, Isis, 18 (1932), 103-17.
22. Robert Hooke (ref. 6), p.51. Hooke also reported the effects of air deprivation on fish, on 20 May 1663, Royal Society MS, RBC 2, 31-32, and on 24 October 1667 on a dog, Philosophical Transactions, 2 (28), (1667), 539-40.
23. This was in John Mayow's Tractatus Quinque, Oxford, 1674; see History of Chemistry, Vol.11 (ref. 17), pp.587-613.
24. The Shorter Pepys (ed. Robert Latham), London, 1986, 21 January 1665, p.464.
25. Thomas Shadwell, The Virtuoso, 1676 (ed. Marjorie Hope Nicholson and David Stuart Rhodes), London, 1966; Act 1, Scene II, line 6, p.22. (In addition to microscopy, The Virtuoso made fun of 'the art of flying', blood-transfusion, and other Hooke interests.) The Diary of Robert Hooke, M.A., M.D., F.R.S., 1672-1680 (ed. H. Robinson and W. Adams), London, 1935, 25 May and 2 June 1676.
26. Brief Lives (ref. 1), p.165.
27. John Wilkins, Mathematical Magick, London, 1648. Posthumous Works (ref.1), p. iv.
28. The best study of Hooke's work as represented by the Diary is in Richard Nichols, The Diaries of Robert Hooke, The Leonardo of London, 1635-1703, Lewes, 1994, pp.169-72.
29. The Diary of Robert Hooke (reL 25), 11 February 1675.
30. Micrographia (ref. 2), p. 173. Hooke also looked at the relation between vibration and music in 'A Curious Dissertation concerning the Causes of the Power & Effects of Musick...', an undated paper posthumously communicated by Dr W. Derham, 14 December 1727, Royal Society MS RBC 13.3. In this paper, Hooke argued that an awareness of the vibrations of music precedes an awareness of language, as babies respond to music (p.6).
31. R. Hooke, De Potentia Restitutiva, or of Spring Explaining the Power of Springing Bodies, London, 1678, p.23.
32. De Potentia Restitutiva (ref. 31), p. 5. See also Michael Wright, Robert Hooke's Longitude Timekeeper, in Robert Hooke, New Studies (ref. 8), pp. 63-118.
33. Robert Hooke, A Description of Helioscopes and some other Instruments, London, 1676, p. 32, item 9. Also De Potentia Restitutiva (ref 31), p.5.
34. See Philosophical Transactions, 1 (1), (1665-6), 3; Philosophical Transactions, 1 (14), (1665-6), 239-42 and 245-6 (mis-numbered 145). Also R. Hooke, Cometa, or Remarks about Comets, London, 1678, Supplement 'The Period of Revolution of Jupiter upon its Axis .. .', pp.78-80.
35. Cometa (ref. 34), pp.9-10, 32-4.
36. Cometa (ref. 34), p.47. Also 'Of Comets and Gravity', in Posthumous Works (ref. 1), pp. 166ff.
37. Cometa (ref. 34), pp.4-5.
38. Richard Towneley, A Description of an Instrument for dividing a Foot into many thousand parts, and thereby measuring the diameters of planets to great exactness, Philosophical Transactions, 2 (1667), 541-4.
39. R. Hooke, Some Animadversions on the First Part of Hevelius, his 'Machina Coelestis', London, 1674, p.55. Helioscopes (ref. 33), plate 11, and p.18.
40. Letter, J. Flamsteed to R. Towneley, 3/7/1675, Royal Society MS, 243 Fl. 8.
41. The 'Preface' to John Flamsteed's 'Historia Coelestis Britannica 1725', edited and introduced by Allan Chapman, based on a translation by Alison Dione Johnson, National Maritime Museum Monograph No.52, 1982, pp.179-80.
For Flamsteed's 'Well Telescope' see Derek Howse, Francis Place and the Early History of The Greenwich Observatory, New York, 1975, p.58, plate XII.
42. R. Hooke, 'Of the Difference of Gravity by removing the body further from the Surface of the Earth', 24 December 1662, Royal Society MS, RBC 1. 288-291. Also R. Hooke, 'Of Gravity', 21 March 1665-6, Royal Society MS, RBC 2.223.
43. R. Hooke, Attempt to prove the Motion of the Earth, London, 1674, p.23. The Essay 'Of Comets and Gravity' in Posthumous Works (ref. 1) provides a vibrative model of gravity, pp.184-5.
44. R. Hooke to Isaac Newton, 6 January 1679-80, in The Correspondence of Isaac Newton, Vol.11, Cambridge University Press, 1960, p.309. In his Brief Lives (ref. 1), Aubrey clearly stated Hooke's priority in recognizing the Inverse Square Law of Gravity, p.166.
45. Robert Hooke (ref. 6), pp.83-105.
46. The Diary of Robert Hooke (ref. 25) contains numerous references to architectural work during the 1670s. See also The Diaries of Robert Hooke (ref. 28), pp.101-10.
47. For the most detailed study of Hooke's architecture, see Marjorie Isabel Batten, The architecture of Dr Robert Hooke, F.R.S., Walpole Society (London), 25 (1936-7), 83-113.
48. Ned Ward, The London Spy, London, 1703, Folio Society Edition, London, 1955, p.48.
49. Lawrence Stone, The Family, Sex, and Marriage in England, 1500-1800, London, 1977, pp.561-3.
50. Posthumous Works (ref. 1), p. xxvii.
51. 'Hooke's possessions at his death: a hitherto unknown inventory', in Robert Hooke, New Studies (ref. 8), p.294.
Born 1946 in Manchester, obtained a first Degree from the University of Lancaster, then went to Wadham College, Oxford, for postgraduate work. A historian by training, working in the history of science, his particular research interests are in scientific biography and astronomy. He teaches the history of science in the Faculty of Modern History, Oxford. In addition to published research, he lectures extensively in the history of science in England and abroad and, in January 1994, gave the Royal Society's triennial Wilkins Lecture in the History of Science, on Edmond Halley as a historian of science.
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