Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley

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,

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.

CHAPTER THREE (#ulink_30c95a88-b674-5583-8521-66a831e7b784)

MONUMENTAL ACHIEVEMENTS (#ulink_30c95a88-b674-5583-8521-66a831e7b784)

In the spring of 1665, following the publication of Micrographia, Hooke’s prestige was higher than ever, and he was, at the age of twenty-nine, at the peak of his abilities as a scientist and ‘mechanick’. But his immediate plans, and those of the Royal Society, were disrupted by the severe outbreak of plague that affected London as the weather got warmer. Plague was far from being unknown, and there had been lesser outbreaks from time to time, but this occasion was different. Cases had been reported in Holland in the spring of 1664, resulting in ships from the Netherlands being quarantined in the Thames; in February 1665, war broke out between England and the Dutch, so all trade ceased. In any case, the cold winter of 1664–1665 had slowed the spread of the disease, but the death toll began to rise in March 1665.

At the time, the Lord Mayor was Sir John Lawrence, who that same month had been instrumental in rectifying the irregularities involving Hooke’s election as Professor of Geometry, and he stayed in the City to keep control throughout the months that followed; Henry Oldenburg, the Secretary of the Royal, also stayed at his post, acting as a conduit (the seventeenth-century equivalent of the Internet) for the flow of information between scientists in Europe (including Holland, in spite of the war) and England.

Samuel Pepys, of course, was another who stayed. But those who could leave the city did so. The King and his Court moved to Oxford, followed by many of the Fellows of the Royal, including Boyle. Queen Henrietta went to Paris, accompanied by a large party including Christopher Wren, whom the King had instructed to study the buildings programme of Louis XIV. But after the Royal suspended its Wednesday meetings following 28 June, its three leading experimenters – Hooke, Sir William Petty and Dr John Wilkins – moved (with a lot of experimental apparatus, and an assistant, known as an ‘operator’) to Durdans, an estate near Epsom in Surrey, owned by George, Lord Berkeley, himself an FRS. There, they carried out a full programme of experiments on behalf of the Royal, and many others of their own, particularly Hooke’s, devising.

The work that was of greatest interest to the Royal is of no interest to us. Always eager to demonstrate the practical value of their research, the Society urged the experimenters to devise improved, lightweight carriages that would be both fast and comfortable for their passengers. Their designs were successful, but did nothing to advance the progress of science. A related development, which Hooke worked at on and off for years, was a ‘waywiser’ to measure how far a coach had travelled, based on counting the rotations of a wheel attached to the vehicle. Another potentially practical (and lucrative) project commissioned by the Royal was a series of experiments aimed at improving marine timekeepers. But as we have mentioned, these ultimately failed to solve the longitude problem, partly because the pressure of other work prevented Hooke from concentrating on clocks, and partly because of the disagreements about patent rights.

Boyle visited Durdans in July 1665, and carried out some experiments with Hooke (that is, as usual Boyle and Hooke together devised the experiments, then Hooke and the operator did the work). These helped him to finish his book Hydrostatical Paradoxes, but he left early in August, and Petty went on to Salisbury a day or so later. Hooke had increasing freedom to carry out his own experiments. These included astronomical observations and the development of new (or improved) astronomical instruments, building on his work with Wren. One of the motivations for this work was again practical: if measurement could be made accurate enough then it would be possible in principle to determine longitude by timing the exact moment when the edge of the Moon passed in front of (occulted) stars whose positions on the sky were already known.

But for us the most interesting work that Hooke carried out while at Durdans involved his continuing investigation of gravity, making use of two deep wells on nearby Banstead Downs. First, he investigated claims made by Henry Power, three years earlier, that the weight of an object is less underground than at the surface of the Earth. But as Hooke wrote to Boyle on 15 August 1665:

I have made trial since I came hither, by weighing in the manner, as Dr. Power pretends to have done, a brass weight both at the top, and let down to the bottom of a well about eighty foot deep, but contrary to what the doctor affirms, I find not the least part of a grain difference in a weight of half a pound between the top and bottom. And I desire to try that and several other experiments in a well of threescore fathom deep, without any water in it, which is very hard by us.

The other well turned out to be blocked at a depth of 315 feet, and the experiments tried there also failed to show any change in the weights. But this was not unexpected: Hooke knew that much more sensitive methods would be needed to measure any changes, and when he presented his findings to the Royal in March 1666, he offered some suggestions for the kind of instruments that would be required (the designs were sound, but beyond the technology of the time). But the key passage of the paper On Gravity that he presented to the Royal that month shows his understanding of the nature of gravity:

A body at a considerable depth, below the surface of the earth, should lose somewhat of its gravitation, or endeavour downwards, by the attraction of the parts of the earth placed above it.

This is another version of Hooke’s realisation of the universal nature of gravity. It was not a mystic force, pulling things only to the centre of the Earth (or, indeed, the Moon), but a property of all matter, with the material above the object pulling upwards just as the material below the object pulled downwards. Twenty years later, Hooke’s priority in understanding this would play a key part in his dispute with Newton (see Chapter Seven).

In the middle of these experiments, Hooke visited the Isle of Wight in the autumn of 1665. His mother had died in June, but at that time travel to the island was restricted in the hope of preventing the spread of plague. It was only in October that Hooke was able to visit his childhood home to settle family matters. It was at this time that he made a more careful investigation of the fossils that had intrigued him as a boy, making notes and sketches as he walked around the south-west corner of the island. This led to the first of his ‘Discourses on Earthquakes’, presented in lectures starting in 1667, but only published after his death; we shall save these to describe together later (Chapter Nine). Hooke was back at Durdans by January 1666, and returned to Gresham College the following month, just before Wren returned from Paris and the King from Oxford. But the City had just six months to get back to normal life before a disaster in some ways worse than the plague struck.

During these months Hooke was active and prolific. As well as his discovery of the Great Red Spot of Jupiter, which had been made earlier but was published in March 1666, he measured the rotation of Mars, continued his work on timekeepers, carried out attempts at blood transfusions with dogs (some more successful than others) and gave lectures. Pepys mentions a ‘very pretty’ lecture on the trade of felt-making, a reminder to us of the practical side of Hooke’s work. But we want to focus on one achievement in particular from that summer, yet another invention that should have made Hooke even more famous than he was but was overlooked; this time it was neglected not least because of the fire which broke out on Sunday 2 September 1666. At the meeting of the Royal scheduled for 12 September (which was, of course, abandoned in the aftermath of the fire) Hooke had been intending to describe what was then called a reflecting quadrant, but developed into the instrument which we now know as a sextant.

This was another astronomical instrument that doubled as an aid to navigation, measuring not longitude (the distance east or west of the home port) but latitude (the distance north or south of the equator). Latitude can be determined by measuring the height of the Sun above the horizon at noon; in astronomical work the same kind of instrument can be used to measure the height of a star above the horizon, or the angular distance between two astronomical objects. Prior to Hooke’s invention, this involved looking through a telescope or open sight at the horizon, holding the instrument steady (easy for astronomers on solid Earth, less easy for mariners on the deck of a heaving ship), and moving a second telescope (or sight) on an arm hinged to the first one on to the target star (or Sun), then measuring the angle between the two jointed arms. Hooke’s ingenious idea was to attach a small mirror to the moveable arm of the instrument, which reflected an image of the object being observed (the target star, or the Sun) on to a second mirror and along the first (indeed, now the only) sighting arm, while this was pointed at the horizon. The operator could then see both the horizon and an image of the target along the same sight, and the angle of the moveable arm could be adjusted until the image of the target seemed to be sitting on the horizon. The angle when this occurred could be read off from a curved, graduated scale on the instrument.

Hooke’s main interest in developing the instrument was astronomical – he told the Society, in the run-up to the intended lecture, that he had a method to determine accurately the angular distances of stars relative to the Moon, which had a direct bearing on the longitude problem. But had he not been distracted by other work, and been able to develop the idea, it would surely soon have been applied to navigation. Hooke’s instrument was so completely forgotten that in 1691 Edmond Halley reinvented it, but quickly withdrew his claim for priority when it was pointed out that his friend Hooke had come up with the idea twenty-five years earlier. And in 1699, Isaac Newton claimed to have invented such a device, which roused the by then ailing Hooke to drag himself along to the next meeting of the Royal to remind them of his priority. The idea only really took off, however, after 1731, when John Hadley demonstrated his own version of the sextant to the Royal Society.

In September 1666, the City of London was, as far as fire was concerned, an accident waiting to happen. Many of the lanes were only five or six feet wide, and the upper storeys of the buildings hung over the lanes so that they almost touched, perhaps with a foot of space through which a strip of sky might be visible. The houses themselves were timber-framed, with a lathe and plaster infill. And everything was tinder dry, following a long, hot summer. In addition, on 2 September, when fire broke out in a bakehouse in Pudding Lane, there was a strong north-easterly wind, which fanned the flames and swept them westward and down to the Thames. The conflagration raged for three days before a slackening wind enabled the firefighters to get on top of it, and by Thursday most of the City was a smouldering ruin. The Royal Exchange, source of Sir Thomas Gresham’s wealth, was gone, but the fire had stopped about 200 yards from Gresham College, in the north-eastern (upwind) quarter of the City. Only about 75 acres out of just over 400 acres within the City walls survived, while a further 63 acres to the west, covering Holborn and Fleet Street, had also gone. The old St Paul’s Cathedral was lost, together with 90 churches, 52 Livery Company halls and more than 13,000 houses.

An estimated 100,000 people lived in the City at the time, and three-quarters of these were now forced to camp outside in the open spaces beyond the walls, but fortunately there was only relatively minor loss of life, perhaps a few hundred people.

The City was governed by a Court of Aldermen, or corporation, under the Lord Mayor; having lost their Guildhall home, the Court moved into Gresham College, meeting there for the first time on Thursday 6 September, and forcing all the professors and their tenants (legal and illegal) out, except for the household of one man – Robert Hooke, who had good City connections and was seen as a valuable person to have on hand for advice and help. The Royal was able to find accommodation in Arundel House on the Strand, and continued to function – which meant that Hooke and his operator had to trek about a mile and a half across the ruins and up Fleet Street with the equipment for the demonstrations he was still required to perform. The astonishing thing is that he did continue to carry out a full programme for the Royal, and give his Gresham and Cutlerian lectures, even though he now undertook a task that to anyone else would have been a full-time occupation: supervising the rebuilding of London.

While the fires were still smouldering, several people presented plans for the rebuilding of the city to the King. The City

had moved with impressive speed to restore order, issuing instructions for clearing the rubble, setting up temporary markets, and so on. They recognised the need for swift action on a formal rebuilding programme to prevent unregulated reconstruction higgledy-piggledy across the city, and on 13 September in a wide-ranging proclamation in support of their actions, Charles II instructed the Lord Mayor and City to:

cause an exact survey to be made and taken of the whole ruins occasioned by the late lamentable fire, to the end that it may appear to whom all of the houses and around did in truth belong, what term the several occupiers were possessed of, and what rents, and to whom, either corporations, companies, or single persons, the reversion and inheritance appertained: that so provision may be made, that though every man must not be suffered to erect what buildings and where he pleases, he shall not in any degree be debarred from receiving the reasonable benefit of what ought to accrue to him … we shall cause a plot or model to be made for the whole building through those ruined places: which being well examined by all those persons who have most concernment as well as experience, we make no question but all men will be pleased with it, and very willingly conform to those orders and rules which shall be agreed for the pursuing thereof.

The initial hope of the King and the City was to rebuild London on a grand scale as a completely new city, but they realised very quickly that speed was of the essence, and that the rebuilding programme must start as soon as possible (which meant in the spring of 1667, when the winter was over) or haphazard illegal construction would begin, and the homeless citizenry would become restless. It was with this in mind that the plans submitted while the ground was still hot were, though widely admired, rejected. First off the mark was Christopher Wren, who presented his idea for a new city to the King on 11 September; he was quickly followed by John Evelyn, who presented his own proposal to Charles two days later. These direct approaches to Charles upset Oldenburg, who felt that an opportunity to promote the Royal Society had been missed. Both Wren and Evelyn were Fellows, and if the plans had gone first to the Royal and then on to the King in the name of the Royal it would have benefited the Society. Hooke was more diplomatic. His plan, probably endorsed by Sir John Lawrence, was shown first to the City and then to the Royal, at a meeting held on 19 September. Only then was it presented to the King. Hooke’s proposal, we are told by Richard Waller, was for a rectangular layout of streets, a grid like the layout of many modern American cities, but the original plan has been lost. In any case, all these ideas (and a couple of others) came to naught. Because of the need for speed, it was decided to rebuild the city essentially along the old lines, literally building on the old foundations, but with some streets being widened and proper provision being made for facilities such as markets. The joint responsibility for getting this done was shared by the Privy Council (on behalf of the King) and the Court of Aldermen, representing the City. On 4 October 1666, a meeting of the Court considered a report from the Privy Council, and made the declaration that since:

for the better and more expedition of this work [the King] hath pleased to appoint Dr Wren Mr May and Mr Pratt to joyne with such Surveyors & Artificers as should be appointed by the City to take an Exact & speedy survey of all Streetes Lanes Aleys houses & places destroyed by the late dismall Fire That every particular Interest may be ascertained & provided for & the better Judgment made of the whole Affaire This Court doth therefore Order that Mr Hooke Reader of the Mathematicks in Gresham Collidge Mr Mills and Mr Edward Jermyn do joyne with the said Dr Wren Mr May & Mr Pratt in taking the said Surveigh …

There was clearly a delicate balancing act going on here. The three Royal appointees, known as ‘the King’s Commissioners’,

had all been advising Charles, before the fire, on plans to repair the old St Paul’s Cathedral, which was now beyond repair and would have to be replaced by a new cathedral. The three City appointees, known as ‘the City Surveyors’,

were chosen to match the number of King’s Commissioners, and perhaps with an eye on matching them intellectually, or academically, as well. Peter Mills was an obvious appointment; he was already the City Surveyor, and one of the people who had drawn up a plan for the rebuilding. Edward Jermyn (or Jerman) turned down the appointment (probably because he had a busy and lucrative private architectural practice); the fact that he was not replaced suggests that, indeed, the City had only appointed three men in the first place because the King had three. Hooke was a less obvious choice, in that he had only limited experience of building work, but he was known as a very able man who got things done – the only Gresham Professor who had been allowed to stay in residence, precisely because he might come in useful. He was also a friend and academic partner of Wren, his intellectual equal; it seems likely that these factors encouraged the City to appoint him, partly as a counterweight to Wren who could argue the City’s case if necessary, partly that their friendship might make such arguments less likely. Michael Cooper, the leading authority on Hooke’s work as City surveyor, describes the appointment as ‘remarkable’, highlighting that: