Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley. John Gribbin
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.
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.fn1 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.fn2
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