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

Theory of Motion:

of Light

of Gravity

of Magneticks

of Gunpowder

of the Heavens

Improving shipping

– watches

– Opticks

– Engines for trade

– Engines for carriage

Inquiry into the figures of Bodys

– qualitys of Bodys

Hooke worked on many of these projects (and others) in parallel.

We can only pick out the highlights, and describe them consecutively, even when two or more of them overlapped chronologically. The extraordinary fact is, though, that Hooke worked on an array of subjects at the same time, while also giving his lectures and doing more experiments at the behest of the Royal Society. But let’s begin with some of his first work for the Royal, using the air pump that Boyle had given to the Society, and which only Hooke could operate effectively. With that tool, he carried out the two duties that were the key to the survival of the Royal Society, a survival that he alone ensured. First, he entertained the Fellows with dramatic demonstrations. The importance of this cannot be overemphasised. It was this kind of showy demonstration that fascinated the more dilettante Fellows and which brought in a flow of subscriptions to keep the Royal afloat, even if that flow was sometimes only a trickle. Secondly, and much more important to us, he carried out experiments that advanced scientific knowledge profoundly.

A good example of Hooke’s skill as a showman, and the way this linked up with his scientific studies, is provided by his work with hollow glass balls. He delighted his audience with demonstrations in which the balls ‘exploded’ as they cooled down after being blown from molten glass, and the way air rushed into them when they were placed under pressure in the chamber (receiver) of the vacuum apparatus and cracked open. Among other things, though, this set Hooke thinking about the strength of arches and other curved structures, so the experiments fed directly into his later work as an architect.

It also seems that Hooke was not afraid to experiment on himself. In his diary entry for 7 May 1662, John Evelyn (himself a Fellow) describes a meeting of the Royal Society attended by the King’s cousin, Prince Rupert:

I waited on Prince Rupert to our Assembly, where were tried several experiments of Mr. Boyle’s Vacuum: a man thrusting in his arme, upon exhaustion of the ayre, had his flesh immediately swelled, so as the bloud was neere breaking the vaines, & insufferable: he drawing it out, we found it all speckled.

There is little doubt that the experimental subject was Hooke himself. Some years later, he built a receiver large enough to sit in, and did so while an assistant pumped the air out. He described how this caused pain in his ears, deafness and giddiness, before he decided enough was enough and the air was let back in. But a discussion of Hooke’s most important work with the vacuum pump can wait until we discuss his great book, Micrographia.

Although he was not afraid to experiment upon himself, Hooke was far more reluctant than most of his contemporaries to experiment on other animals, at least when it clearly caused them pain. At the beginning of the 1660s, nobody knew exactly what the importance of breathing was in sustaining life. One school of thought held that although the circulation of the blood was clearly important, the role of breathing was simply to act as a pumping mechanism, by which the in and out motion of the thorax stirred up the blood and kept it flowing. The idea that something from the air mixed with blood in the lungs and was essential for life was a minority view. In one indecisive experiment at the beginning of 1663, Hooke placed a live chick and a burning lamp in a sealed chamber to see which one lasted longer. The lamp went out, but the chick survived. This, however, neither proved nor disproved the hypothesis. It was not until November 1664 that Hooke, possibly at Boyle’s suggestion, conceived of an experiment on a living dog, which could be dissected ‘displaying his whole thorax, too see how long, by blowing air into his lungs, life might be preserved, and whether anything could be discovered concerning the mixture of the air with the blood in the lungs.’

The gruesome experiment was carried out on 7 November. With the dog cut open and all its organs exposed, unable to breathe of its own volition, air was pumped into the lungs of the dog by a pair of bellows through a hollow cane stuck into a hole in the dog’s windpipe. The experiment was a success, in that the dog lived during it. As Hooke wrote to Boyle on 10 November 1664:

at any time, if the bellows were suffered to rest . . the animal would presently begin to die, the lungs falling flaccid, and the convulsive motions immediately seizing the heart and all the other parts of the body; but upon renewing the reciprocal motions of the lungs, the heart would beat again as regularly as before, and the convulsive motions of the limbs would cease.

But in the same letter, Hooke confessed that although the experiment suggested several other lines of investigation:

I shall hardly be induced to make any further trials of this kind, because of the torture of the creature: but certainly the enquiry would be very noble, if we could any way find a way so as to stupefy the creature, as that it might not be sensible [conscious].

Three years later, Hooke was asked to repeat the demonstration, but initially refused. Two doctors, who were less squeamish about such matters, tried to replace him, but made such a mess of the operation that Hooke, by then an employee of the Royal, was ordered to do it and repeated his earlier success.

At the end of 1662 in another series of experiments, he demonstrated how a hollow glass ball that would float on top of cold water gradually sank to the bottom when the water was warmed, or could be made to ‘hover’ partway up the vessel if the temperature conditions were just right. He correctly suggested that the heat ‘loosened’ the water (that is, reduced its density), which was another step towards an understanding of matter as made up of atoms and molecules. He also invented (at least in principle; we are not sure if he made it) an efficient water heater in which a heated piece of copper at the bottom of a tub of water would heat the whole vessel as the warm, loosened water rose to the top and was replaced by descending cooler water. He had ‘discovered’ convection – but he went too far when he speculated that this might make it possible to manufacture a perpetual motion machine in which the water circulated endlessly through a system of pipes without any further heating once it had been started. More practically, he pointed out that because the cold sea at high latitudes could support heavier ships than the ‘loosened’ water closer to the equator, ships setting out from polar latitudes to the tropics should not be fully laden. Much later, starting in the late nineteenth century, merchant ships were marked with ‘Plimsoll lines’ showing exactly how far they could be safely loaded, depending on the waters they were visiting.

Hooke’s investigations of pressure, density and convection fed directly into another lifelong interest of his: the weather, and the possibility of forecasting the weather. This became a major thread of his work in September 1663, when Wilkins, on behalf of the Royal, asked Hooke to collect daily records of the weather, in the hope that these might reveal patterns that could be used in prediction. Wilkins probably had in mind a simple note of whether it was sunny or cloudy, rainy or dry, and so on. But Hooke never did anything by halves, and he began by setting out a systematic schedule of everything scientific weather observers should take note of (wind speed and direction, temperature, humidity, air pressure, the appearance of the sky, and so on) before he put those principles into practice. He said that the weather observer should also note what illnesses (human and animal) were rife at the time, what diseases and pests were affecting the crops, and many other items. All of this was to be recorded in a standard format, so that the data for each month could be scanned at a glance. Among these details, Hooke was the first person to establish a standard list of terms to describe different kinds of cloud cover.

The project soon developed far beyond the simple record keeping envisaged by Wilkins. You can’t keep reliable records unless you have reliable instruments to measure with, and a reliable scale against which to calibrate those measurements. It was Hooke who defined the freezing point of distilled water as the zero of temperature, marked on sealed glass thermometers, an idea enshrined in later temperature scales with the boiling point of water set as the second fixed number, though by then nobody remembered it had been Hooke’s idea. He realised that thermometers were affected by the expansion and contraction of the glass as it warmed and cooled, and studied the effect. To measure humidity, he observed the way the ears of the wild oat and wild geranium bent more or less as the humidity changed, and adapted this for use in a hygroscope.

But he made perhaps his most significant weather discovery in September 1664, just after he first moved into rooms at Gresham College.

This harked back to his work with Boyle on ‘Mr. Townly’s hypothesis’. It used a portable barometer shaped like a letter J, as in that work, but this time with the long end of the tube closed and the bottom (the short limb of the J) open to the air. Mercury in the U-bend of the J would be pushed down more when the atmospheric pressure was higher, forcing the mercury on the other side further up the long arm of the tube. Similarly, when the pressure fell, the mercury in the long arm fell. By the end of 1663, Hooke had converted this into a ‘wheel’ barometer, with a pointer that moved around a dial like the face of a clock to show how the pressure was changing. He did this by twisting a thread around the axle of the pointer, with the other end of the thread attached to a weight floating in the mercury in the open end of the tube, and a counterweight on the other side of the axle hanging free in the air. As the mercury moved up and down, the thread tugged the pointer round the dial one way or the other. And if the friction of the axle made it stick, all you had to do was to tap the barometer to get it to unstick and move to the appropriate position.

On 6 October 1664, Hooke wrote to Boyle to tell him of a great discovery he had made using one of these barometers:

I have also, since my settling at Gresham college, which has been now full five weeks, constantly observed the baroscopical index … and have found it most certainly to predict rainy and cloudy weather, when it falls very low; and dry and clear weather, when it riseth very high, which if it continues to do, as I have hitherto observed it, I hope it will help us one step towards the raising a theoretical pillar, or pyramid, from the top of which, when raised and ascended, we may be able to see the mutations of the weather at some distance before they approach us, and thereby being able to predict, and forewarn, many dangers may be prevented, and the good of mankind very much promoted.

Hooke’s vision was not immediately fulfilled: too many other elements, not least rapid communication systems to enable the collation of data from widespread observers, would be required before the vision became reality. It would be two centuries before Admiral Robert FitzRoy ‘invented’ the weather forecast, but when he did so the kind of links between atmospheric pressure and weather that Hooke had discovered were a key ingredient. And, as FitzRoy’s rank highlights, among the ‘many dangers’ Hooke referred to were the hazards of storms at sea.

Although this particular development was of no immediate benefit to mariners, as we mentioned in connection with Hooke’s work on timekeepers, maritime matters were of vital importance to England in the second half of the seventeenth century, and therefore they were of vital importance to the Royal Society as a means of proving its worth to the King. Naval wars with the Dutch involved fleets as far away as America, the Caribbean, West Africa and even the East Indies. It was during a lull in these activities (under the Treaty of Breda, also known as the Peace of 1667) that England formally gained the former Dutch colony of Nieuw Amsterdam, which they had captured in 1664, and promptly renamed it New York. Hooke invented several devices for studying the sea, or working under the waves. One was a depth sounder, which worked by dropping a hollow ball attached to a heavy weight into the sea. When the weight hit bottom, it released the ball, which floated to the surface. By timing how long it took before the ball surfaced, the depth could be calculated. At least, it could in a flat calm with good seeing conditions. In practice, under less than ideal conditions, from the small ships of the seventeenth century the balls could not be spotted as soon as they surfaced (if at all) so the technique was impractical. In the nineteenth century, however, the same idea was dreamed up, independently, by an American, J. M. Brooke, and was used to measure the depth of the sea bed when the first transatlantic telegraph cables were laid in the middle of that century.

Another of Hooke’s devices was more immediately successful. This was a bucket on a long line, with hinged lids that allowed it to fill with water at depth, but closed when it was pulled to the surface. This was effective in bringing back samples, which could be studied to measure such things as the saltiness and (with luck) the creatures that lived at depth.

In February 1664 (still before he was being paid by the Royal), Hooke served on a committee that investigated the practical possibilities of diving. He devised a system where a diver working on the bed of a river, or in shallow water at sea, could be supplied with a succession of air-filled lead boxes lowered from the surface, from which he could breathe through a tube. This was reasonably successful during trials in a large tub set up outside the Royal and in the Thames. These and other ideas, including diving goggles, a life jacket, and plans for a submarine, were summed up in an account Hooke published in 1691, but they are only tangentially of interest to our story of Hooke the scientist, as another example of his versatility and capacity for hard work.

But another aspect of Hooke’s maritime work ties in more closely with the main thread of our story. This was his interest in the use of astronomy for navigation, which led him to design and manufacture more accurate instruments for measuring the height of the Sun and stars above the horizon – a key to determining latitude, but also a key to measuring the positions of the stars relative to one another more accurately for other astronomical purposes. This involved better sights (in effect, little telescopes), and instruments calibrated and marked to exquisite precision. One of Hooke’s instruments (a quadrant), presented to the Royal in February 1665 (while in the middle of the hassles concerning his appointments as Cutlerian Lecturer and Gresham Professor), was just seventeen inches across, but could measure angular distances as small as ten seconds of arc. Since there are 60 seconds in a minute of arc, 60 minutes in a degree, and 360 degrees in a circle, this means that the instrument could measure precisely angles that are only 1/360th of a degree, or 0.0000077 of a circle. The unprecedented accuracy of Hooke’s instruments led to an argument with the much older astronomer Johannes Hevelius of Danzig, who could not believe the superiority of Hooke’s designs; the controversy, detailed later, also brought in Edmond Halley, in one of his first missions as a Fellow of the Royal Society.

In much of his astronomical work, especially in the first half of the 1660s, Hooke collaborated with his friend Christopher Wren, who was based in Oxford but still in communication with the Royal. Astronomers of the time were lucky enough to see several comets, and in December 1664 the Royal asked Hooke and Wren to make observations and report on a new comet that had become visible.

Hooke observed from London, Wren from Oxford, and their results plus measurements from other observers were combined and reported by Hooke. Pepys attended a lecture at Gresham College on 1 March 1665 and tells us that on that day (a couple of weeks after he had demonstrated his quadrant), Hooke talked about:

the late Comett, among other things proving very probably that this is the very same Comett that appeared before in the year 1618, and that in such time probably it will appear again – which is a very new opinion.

New to Pepys, and to Hooke, although we now know that the English clergyman and astronomer 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 orbs

… 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.