Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley. John Gribbin
Jeremiah Horrocks had speculated along the same lines – that comets follow closed orbits around the Sun – three decades earlier. It happens that Hooke was wrong about this particular comet: it was not the same one that was seen in 1618, and it did not return in 1711. But the improving telescopic technology of the time was starting to show astronomers that comets did not move in straight lines, but followed curved paths through space; this was the beginning of the idea that led Halley, before too long, to make the prediction of the return of the comet that now bears his name. The significance for Hooke’s story is that it seems that, by the mid-1660s at the latest, he was already thinking about the possibility that comets (and therefore the planets) were under the influence of some kind of force, reaching out to them across space from the Sun itself. He realised that comets are part of the Sun’s family, not something weird or magical. This was among the insights that led him to carry out several experiments to investigate the nature of gravity, which we describe later. It is worth getting slightly ahead of our story, however, to highlight one of Hooke’s most important insights (perhaps the most important), which (like so many of his ideas) has been misattributed for hundreds of years.
Going back into the mists of time, it had been assumed by natural philosophers that the ‘natural’ motion of objects such as planets unaffected by friction or other forces was circular. This had to be so, they reasoned, because circles are perfect, and only perfection could be at work in the heavens. They interpreted the seemingly irregular motion of planets in terms of epicycles, where the planets were constrained to move in small perfect circles around points which themselves moved in perfect circles around the Earth, or the Sun. When, only a short time before Hooke was born, Galileo carried out experiments involving balls rolling down inclined planes, he found that the balls rolled off the end of the ramp horizontally – literally, towards the horizon – and he realised that if there were no friction they would keep rolling for ever. But he knew that the Earth was round, so to him ‘horizontal’ motion meant always moving towards an always receding horizon, in a circle around the Earth. It was Hooke who realised, partly from his studies of comets, that any object that is not acted upon by an external force will keep moving in a straight line. Does that sound familiar? It should. It is something we all learn in school, where it is called ‘Newton’s First Law’ of motion. But it was Hooke who came up with it, and who (as we shall see) explained it to Newton.
On 21 March 1666, when nobody outside Cambridge and few people inside Cambridge had heard of Isaac Newton, Hooke gave a lecture to the Royal about gravity, where he presented some of these ideas. He described several experiments involving his study of gravity, which he stated was ‘one of the most universal active principles in the world’ and set out his ambition to determine:
whether this gravitating or attractive power be inherent in the parts of the earth [and] whether it be magnetical, electrical, or of some other nature distant from either
as well as ‘to what distance the gravitating power of the earth acts’.
On 23 May that year he presented his big idea to another meeting of the Royal, and in a paper entitled ‘Inflexion of a Direct Motion into a Curve by a Supervening Attractive Principle’. In that lecture (and many times afterward) Hooke used a long pendulum, with the bob moving in a circle, or (crucially, in terms of understanding the motion of the planets) an ellipse, not just to and fro; this demonstrated the nature of orbital motion, which, he pointed out, required a force (in this case, supplied via the string of the pendulum) to keep the bob ‘in orbit’. By attaching a secondary, shorter string, with its own bob, partway down the pendulum he could also demonstrate the motion of a ‘moon’ around a ‘planet’. The idea he presented to the Fellows (which really was ‘a very new opinion’) was that the natural motion of a planet is in a straight line – a tangent to its orbit – and that it is deflected from this tangential path by a force of attraction stemming from the centre of the planetary system – that is, a force emanating from the Sun. As he explained to the Fellows:
I have often wondered why the planets should move about the Sun according to Copernicus’s supposition, being not included inn any solid orbsfn6 … nor tied to it, as their centre, by any visible strings.
He stressed that ‘all bodies, that have but one single impulse’ ought to move in straight lines, and inferred that there must be another ‘impulse’ acting on the planets. If that impulse were a force of attraction from the Sun then:
all the phenomena of the planets seem possible to be explained by the common principle of mechanic motions [and] the phenomena of the comets as well as of the planets may be solved.
These two ideas, ‘Newton’s’ first law and the force of attraction between the Sun and planets (an inward, or centripetal, force), are the keys to the ‘Newtonian’ revolution in science that took place two decades later. It might have happened sooner, and had a different name, if Hooke’s attention had not been diverted by dramatic developments in England in 1665 and 1666. Conveniently for us, however, he had summed up what he described as his ‘first endeavours’ in a book published just before those changes took place.
Micrographia, Hooke’s great book, was written and published on the instructions of the Royal Society as a deliberate attempt to promote the Society and its aims. Hooke has been described as a ‘reluctant author’,fn7 and almost all of his published work resulted from his contractual obligations, primarily to the Royal Society and to a slightly lesser extent to John Cutler and in connection with his role as a Gresham Professor. But the background to Micrographia predates Hooke’s appointment as Curator of Experiments.
At the beginning of the 1660s, Christopher Wren was supposed to be preparing a book of microscopical observations for presentation to the King, who had seen some of his drawings of microscopic objects and been impressed by them, but the newly appointed Savilian Professor of Astronomy found that he had too much on his plate, and passed this task on to Hooke, who took over the work in September 1661. The design and manufacture of optical instruments – telescopes and microscopes – was improving dramatically at this time, and although Hooke was involved in developing some of the ideas that went into these instruments, he relied on expert craftsmen, notably Richard Reeve, for the tools of his trade. As he put it in his book: ‘all my ambition is that I may serve to the great Philosophers of this Age, as the makers and grinders of my Glasses did to me’.
By the end of 1662, Hooke was presenting some of his microscopic studies to the Royal. The first of these observations, presented in December that year, dealt with the patterns of ice crystals seen in ‘frozen urine, frozen water, and snow’. The Fellows were sufficiently impressed that at the Council meeting of 25 March 1663 Hooke was ‘solicited to prosecute his microscopical observations, in order to publish them’. In the months that followed, Hooke made many specific observations at the behest of individual Fellows, as well as following up his own interests. The Council kept a keen eye on the progress of the work, with the book intended to provide an example of the experimental method, which was at the heart of their philosophy, and which they explicitly took from Bacon. In the book, Hooke emphasises the need ‘to begin to build anew upon a sure Foundation of Experiments’, and explicitly cites the ‘Noble and Learned’ Bacon as an inspiration. The book was partially intended as propaganda for the Society itself and for the new way of studying the world. It succeeded dramatically on both counts, thanks to Hooke’s known genius as a scientist and his perhaps unexpected skill as a writer. But it only got into print after some heart-searching by the Council, which has been detailed by John Harwood.fn8
Hooke had more or less enough material for his book by March 1664, a year after he had formally been instructed to carry out the work. By then, the Royal had chosen a printer and discussed such details as the official Royal Society imprimatur to go in the front of the book. This emphasised in the clearest way that it was a Royal Society book, stating that:
By the Council of the Royal Society of London for Improving of Natural knowledge.
Ordered, That the Book, written by Robert Hooke, M.A. Fellow of this Society, Entitled, Micrographia, or some Physiological Descriptions of Minute Bodies, made by Magnifying Glasses, with Observations and Inquiries thereupon, Be printed by John Martyn and James Allestry, Printers to the said Society
Novem.