by John Gribbin
Halley and Hevelius clearly got on very well with one another during the weeks they were observing together, and in mid-July, shortly before Halley left Danzig, Hevelius asked him to provide what amounted to a testimonial to the older man’s observing technique. The document Halley provided makes fascinating reading, and suggests that Halley was a natural diplomat, tactful to the point of being disingenuous:
I have seen with my own eyes not one or two, but several observations of the fixed stars made with your large brass sextant by different observers, and some of them by myself, which even when repeated did most accurately and almost incredibly agree …
Hardly surprisingly, Hevelius referred to Halley as ‘a very pleasant guest, a most honest and sincere lover of the truth’. But look carefully at what Halley actually said, both in the testimonial and in the letter to Flamsteed. Nowhere does he provide a comparison with measurements made using other instruments, and nowhere does he say that the measurements made by Hevelius are as accurate as Hevelius claimed. He says that repeated measurements made with the same instruments gave the same answer. Flamsteed and Halley were pioneers of the application of statistical methods in astronomy to determine what we now call systematic errors – that is, errors that are built into particular instruments, so that although they always give the same answer it is not necessarily the right answer. And it seems Halley said one thing for public consumption and another in private, since in his diary entry for 14 August 1679 Hooke wrote: ‘Halley returned this day from Dantzick (Hevelius rods in pisse).’ The tact, if you like to call it that, would serve Halley and the Royal Society well when the time came to cajole Newton into publishing his masterwork. In spite of his efforts, however, the controversy concerning Hevelius continued to rumble on until it died with him in 1687.
A couple of years before his death, Hevelius published a book, Annus Climactericus, which was riddled with errors about Halley’s work. He said that Halley had been sent to St Helena by the Royal at Hevelius’ suggestion, when in fact he had gone on his own initiative; he described Halley’s instruments incorrectly; and generally downplayed Halley’s work. On 26 March 1686, Halley wrote to a colleague in Dublin, William Molyneux, that:
the Controversy between Mr Hevelius and Mr Hook, as you very well observe does, as Hevelius manages the matter, affect all those observers that use Telescopic sights, and myself in particular, and it is our common concern to vindicate the truth from the aspersions of an old peevish gentleman, who would not have believed it possible to do better than he has done, and for my own part I find myself obliged to vindicate my observations made at St Helena and to rectify some mistakes, whether wilful or not I cannot say; first he sais I was sent by R.S. to St Helena at his request to observe the Southern Starrs (pag. 14) whereas it is very well known to all our Astronomers that at my own motion and charge I undertook that voiage above two years, before I had the honour of being a member of the R. S. …
In a second letter to Molyneux, dated 27 May 1686, he hints at further revelations:
As to Mr Hevelius we heare as yet no farther from him, and I am very unwilling to let my indignation loose upon him, but will unless I see some publick notice taken elsewhere, let it sleep until after his death if I chance to outlive him, for I would not hasten his departure by exposing him and his observations as I could do and as I truly think he deserves I should.
In fact, Halley never did ‘expose’ Hevelius and his observations; there was no need, since the death of Hevelius brought an end to the controversy as there was nobody left to champion the cause, such as it was, of open sights.
Back in 1679, however, Halley seems to have been at something of a loose end after his trip to Danzig. He spent some of his time in Oxford and part of it in London, where we get glimpses of him from mentions in Hooke’s diary and other sources. We can pick up his trail properly again in December 1680, when he left for the Grand Tour through France and Italy that was part of the usual upbringing of a young gentleman. He travelled with a friend, Robert Nelson, who was the same age as him (twenty-three at the start of the trip); Nelson was the son of a wealthy merchant and a Fellow of the Royal Society, although one of the gentlemen Fellows with a dilettante interest in science, not an active scientist.
It was just before the two gentlemen set off on their tour that Halley had his first encounter with a comet. A bright comet visible to the naked eye from London was a major talking point in the city in November 1680, and naturally Halley, Hooke and others made observations of the way it moved against the background of stars as it closed in on the Sun. It was soon lost to sight in the glare of the Sun, but Halley saw it again, now moving away from the Sun, after he had crossed the English Channel (a very rough and unpleasant crossing) and was on the way to Paris. In one of his notebooks, Isaac Newton recorded that ‘Hally has told me that on his journey to Paris on Dec 8 old style he saw the tail of the comet rising perpendicularly from the horizon’, a form of words which suggests that soon after his return to England (early in 1682) Halley met Newton and discussed the comet with him. This would have been the first time they met each other.
Already, Halley’s version of the Grand Tour was deviating from that of most young men. The usual focus of attention was the art, culture and historical sights of Europe. But Halley’s tour was as much a scientific expedition as anything else, and in Paris he lost no time in visiting Giovanni Cassini,fn8 the head of the Paris Observatory, to compare notes and make more observations. The large, bright comet was as much a sensation in Paris as it had been in London, with many people seeing it as a portent of disaster, a supernatural phenomenon.
So little was known about comets at the time that there was some doubt about whether what had been seen was one comet or two, one moving towards the Sun and one moving away from it. Flamsteed was one of those who subscribed to the view that it was indeed one object, but even Flamsteed did not realise that it was following an orbit around the Sun. He envisaged that some sort of magnetic effect had first attracted the comet towards the Sun and then repelled it. On 15 January 1681 Halley wrote to Hooke that:
The general talk of the virtuosi here is about the Comet, which now appears, but the cloudy weather has permitted him to be but very seldom observed. Whatever shall be made publick about him here, I shall take care to send you, and I hope when you shall please to write to me you will do me the favour to let me know what has been observed in England.
Halley moved on from Paris to Saumur, from where he wrote to Hooke again in May, including news of the latest ideas about the comet:
Monsieur Cassini did me the favour to give me his books of ye Comett Just as I was goeing out of towne; he, besides the Observations thereof, wch. he made till the 18 of March new stile, has given a theory of its Motion wch. is, that this Comet was the same with that appeared to Tycho Anno 1577, that it performs its revolution in a great Circle including the earth …
Halley says that Hooke might find it hard to accept all this, but that it is certainly an idea worth thinking about. As it happens, Cassini was mistaken in thinking that the ‘new’ comet was a return of the one seen by Tycho Brake in 1577, but it does seem that he planted an idea in Halley’s mind.
In the same letter, Halley describes to Hooke a comparison he has made of the cities of London and Paris, which would have been of great interest to Hooke the architect and city planner. By carefully pacing out the dimensions of Paris, Halley has found that it is ‘not soe great a Continuum of houses as London’, but ‘by reason of theire living many in a house’ it has a larger population. This is born out by the evidence of burials and christenings: in 1680, he points out, 24,411 people were buried in Paris, ‘whereas at London 20000 is reckoned a high bill, and the Christenings farr exceed ours, haveing been almost 19000, when we have ordinarily 12 or 13000’. He then goes on to mention an insight, which has just come to him:
halfe as many were married as were borne; and not more; it will from hence follow, supposing it alwaies the same, that one half of man
kinde dies unmarried, and that it is necessary for each married Couple to have 4 Children one with another to keep mankind at stand. this Notion Occurred whilst I was writeing …
The notion that Halley tosses away in a letter – that populations produce more offspring than the number required to reach maturity and reproduce in their turn – was one of the key insights that two centuries later would lead Charles Darwin and Alfred Russel Wallace independently to the idea of evolution by natural selection. Halley never followed that path, but he did return to the study of population statistics, and later wrote a paper that laid the foundations of the actuarial approach to life insurance.
From Saumur, Halley and Nelson followed a leisurely route to Rome, arriving in October, and planned to spend a few months touring Italy; he certainly must have intended to visit Venice. In August, Halley had observed a lunar eclipse from Avignon, providing data that Flamsteed, who had observed the eclipse from Greenwich, was able to use to calculate the longitude of Avignon relative to Greenwich. Somewhere along the way, Halley also picked up some astronomical observations made in India, which he passed on to Hooke as they could be used to determine the longitude of the observing site in Ballasore. The exiled Queen Christina of Sweden was living in Rome at the time, and was sufficiently interested in astronomy that she had offered a prize for anyone who could successfully predict the return of the comet. The prize was never won, although several astronomers, including Cassini and Hevelius (but not Halley), tried for it. The Queen would have been eager to have Halley’s first-hand account of the observations from London and Paris, and of the latest ideas about comets from the virtuosi. It seems highly likely that he met her, although there is no record of the meeting.
But then things took an unexpected turn – actually, two unexpected turns. In late November, less than a year after setting out on his travels, Halley was called back to England on family business, but it cannot have been too urgent since he travelled via Genoa, Paris and Holland, instead of by the most direct route. Nelson did not return with him; he fell in love with a widow, two years older than him, who had a twelve-year-old son by her first husband. They married in 1682; she was the second daughter of the first Earl of Berkeley, whose home at Durdans had provided a safe base for Hooke during the plague year.
Just why Halley had to cut short his Grand Tour is not clear. A plausible guess is that it had something to do with the second marriage of his father, who had been a widower since 1672. The exact date of the marriage is not known, but Halley was back in London on 24 January 1682, as noted by both Hooke and John Aubrey, who says: ‘He hath contracted an acquaintance and friendship with all the eminent mathematicians in France and Italie’. Halley then goes off the radar until, out of the blue, we find that on 20 April 1682 he too got married, to Mary Tooke, the daughter of a prominent lawyer. The brief biography in MacPike describes her as ‘an agreeable young Gentlewoman; and a Person of real merit’. We know nothing about the relationship of the couple before the wedding, and precious little afterwards. Although they were together for more than fifty years, the only surviving records concerning Mary are those of the birth of her children: Margaret, born in 1685, Katherine, born in 1688, and Edmond, born in 1698. Possibly there were other children who died in infancy. None of the three children we know about had children of their own.
These changes in Halley’s life did not take him out of the mainstream of science in England. The couple lived in Islington, then a village just outside London, where Halley set up an observatory and continued to make astronomical observations, and from where he could easily make the short journey to attend meetings of the Royal Society. The key project that he embarked upon was a series of observations of the Moon, intended to cover the entire cycle, some eighteen years long, of its apparent movement against the background stars, with a view to producing tables that mariners could use to find their local time, and hence the longitude, by observing the position of the Moon. In August and September 1682 another bright comet was visible, and was monitored by Halley, along with his other observations, but more notably by Robert Hooke. This was not as sensational as the comet of 1680–81, but played an important part in the development of Hooke’s ideas about gravity, as we discuss in Chapter Seven. It also later became significant in Halley’s own investigation of the heavens. Halley published a couple of solid but unspectacular papers on astronomical topics in 1683, and measured the orbital period of the moon Titan (itself only discovered in 1655 by Huygens) around Saturn, coming up with a figure of fifteen days, twenty-two hours, forty-one minutes, close to the modern value. He appeared to have settled into a comfortable way of life, combining both domesticity and science, when his world was turned upside down by the sudden death of his father, in dramatic and unexplained circumstances, on 5 March 1684.
This was just two months after Halley, Hooke and Wren had debated the nature of gravity, and the puzzle of proving that it obeyed an inverse square law of attraction, after a meeting of the Royal Society. When family business related to the death of his father meant that Halley had to visit Alconbury, near Huntingdon, later in 1684, he took the opportunity to visit Newton in Cambridge to discuss these ideas with him, with far-reaching results. His father’s death, and Newton’s response to that visit, changed Halley’s life dramatically; we shall pick up the threads in Chapter Eight, after giving Hooke’s perspective on the story of gravity.
CHAPTER SIX
OF SPRING AND SECRETARYSHIP
We left Hooke in 1677, after the hatchet-burying exchange of letters with Newton concerning their priority dispute about light. As Newton retreated once again from the scene, things soon got better still for Hooke with the death of his nemesis, Oldenburg. Among other things, this gave Hooke the opportunity to publish the one piece of work for which he always received credit and was never overshadowed, without having to worry about Oldenburg’s ‘snares’.
As we have seen, in the early 1670s Hooke was occupied with many projects, and the Royal Society was not top of his list of priorities. The Royal itself had declined from its early enthusiasm, and become more of a kind of gentlemen’s debating club, with less real science being done, partly because Hooke was not so active. But he was still involved in the dispute with Hevelius, the Gregorian telescope, experiments with magnetic ‘lodestones’, and in particular watchmaking, alongside his surveying and architectural work. The watchmaking is particularly important to our story, because it was closely linked with his study of springs. Although Hooke never got a patent for his ideas, in the mid-1670s and later he was working closely with the master watchmaker Thomas Tompion, and later also with John Bennett, who produced a succession of spring-driven timepieces to Hooke’s designs, which Hooke sold. The watches became widely used in Hooke’s circle. It was through this work that he discovered his law of springs. We don’t know when he discovered the law, but we do know when he first revealed it. On 2 September 1675 he wrote in his diary that he had told Tompion that ‘all springs at liberty bending equal spaces by equal increases of weight’. A day later, he made a spring scale ‘to shew the King’, in which a vertical coiled spring was stretched by adding weights to the bottom, moving a pointer down a scale to indicate the weight. Here again is an example of the overlapping of Hooke’s many interests; such a scale clearly had practical uses in, for example, building work. But with Oldenburg still Secretary of the Royal, it was not yet time for Hooke to present formally all his ideas about springs.
One of the ideas which Hooke described to Boyle (in April 1677) but never formally presented to the semi-moribund Royal Society was a ‘wheelhorse’, which seems to have been a proposal for a two-wheeled vehicle propelled by the feet – the kind of pedal-less bicycle, or velocipede, that was popular in the early nineteenth century. Such a vehicle would have appealed to Hooke, who hated horse-riding, but it seems he never built one. At the end of April, he observed a comet, which encouraged him to prepare a Cutler lecture on the subject of comets, which was later published.fn1 This seems to ha
ve revived his interest in the way planets move around the Sun, which he discussed with Wren.
They knew, of course, about Johannes Kepler’s laws of planetary motion. These empirical laws, based on the observations made by Tycho Brahe, encapsulate three facts. First, planets move in elliptical orbits, with the Sun at one focus of the ellipse. Second, planets move faster when closer to the Sun, and slower when further away, in such a way that an imaginary line linking the planet and the Sun traces out equal areas in equal times, at any part of the orbit. Third, the square of each planet’s ‘year’ is proportional to the cube of its average distance from the Sun. It is actually possible to work out the inverse square law of gravity from Kepler’s laws, as Newton later showed.fn2 But Hooke and Wren did not do this. What Hooke did realise was that these laws imply that the force of the Sun’s attraction acts more powerfully when the planet is closer to the Sun, and is weaker when the planet is further away.
But at that time this was still almost an aside from his architectural work. It was in March 1677 that Hooke presented his design for what became the Pepysian Library at Magdalene College, Cambridge, although he was not involved in the construction. And on a personal note, it was in August that year that Grace departed for her fateful visit to the Isle of Wight, a month after Hooke had taken on another assistant (and lodger), Thomas Crawley. The month after Grace departed, two deaths shook up Hooke’s world. On 12 September, Tom Giles died of smallpox, shaking up Hooke’s domestic life. But on 5 September, Oldenburg had died, probably of malaria, shaking up Hooke’s scientific life and leading to a revival of the Royal Society.
