Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley. John Gribbin
leaving a vacuum in the tube. The piston could be pumped up and down repeatedly, sucking more and more air out of the glass vessel. This apparatus became known as ‘Boyle’s air pump’, which it was in the sense that he paid for it and owned it (just as Dolly Parton’s hair is her own). But as Boyle acknowledged, it was made by Hooke, and Hooke was the experimenter who operated it during the many investigations that followed. In the fragment of autobiography quoted by Waller, Hooke said:
In 1658, or 9, I contriv’d and perfected the Air-pump for Mr Boyle, having first seen a Contrivance for that purpose made for the same honourable Person by Mr Gratorix, which was too gross to perform any great matter.
Some idea of the significance of the pump is that, even by the end of the 1660s, there were only half a dozen comparable air pumps in Europe, and three of them had been made by Hooke.
Boyle and Hooke carried out many experiments with their pump and vacuum chamber – Boyle later described forty-three of them in his book New Experiments Physico-Mechanical Touching the Spring of the Air, published in 1660. These included burning (or attempting to burn) substances such as candles, coal, charcoal and gunpowder in a vacuum, with results that convinced them that fire was not one of the ‘four elements’ (fire, earth, air and water) as the Ancient Greeks had taught, but involved a chemical process. Candles, for example, went out when air was removed from the globe, and burning coals died away, but, crucially, reignited when air was let back in. One of the other experiments showed that water boils at a lower temperature when the air pressure is reduced. But one of their most important discoveries is hinted at in the title of Boyle’s book. Every stroke of the handle of Hooke’s air pump demonstrated the ‘spring’ of the air, just like the springiness felt when using a bicycle pump today, and Hooke set out to measure this springiness – what we now call air pressure.
Around this time, at the end of the 1650s, the Englishman Richard Towneley was carrying out experiments with a Torricelli barometer on Pendle Hill, in Lancashire. He was following the example of continental experimenters, notably Florin Périer. Like them, he found that the pressure of the air measured by the barometer is lower at higher altitude, and he surmised (without carrying out experiments to test the idea) that the pressure is less because the air is thinner – that is, less dense – at higher altitude. He mentioned this idea to Boyle, who asked Hooke to devise a way to test it.
Hooke did this in 1660 or 1661, using a long glass tube shaped like the letter J, with the top open and the short arm of the J at the bottom sealed. He poured a little mercury into the top of the tube so that it partly filled the U-bend at the bottom but left some air trapped in the closed end. With the level of mercury the same on both sides of the U-bend, the trapped air was at atmospheric pressure. But Hooke could increase the pressure on the trapped air by pouring more mercury in, forcing some of it round the bend and squeezing the trapped air into a smaller volume. Boyle was short-sighted and bad at arithmetic, so we know for sure that it was Hooke who not only designed the experiment but also made the careful observations and records that showed that the volume of the trapped air was inversely proportional to the pressure applied. Double the pressure, and the volume halves; triple the pressure and the volume is reduced to one-third, and so on. These results were published in the second edition of Boyle’s book, in 1662, and became known as ‘Boyle’s Law’, although he did not use that name himself. Hooke’s own account appeared in his book Micrographia, published in 1665:
Having lately heard of Mr. Townly’s Hypothesis, I shaped my course in such sort, as would be most convenient for the examination of that Hypothesis.
After describing the experiment (Hooke tells us that the long arm of the J-tube was about fifty inches long), he concludes:
and by making several other tryals, in several other degrees of condensation [compression] of the Air, I found them exactly answer the former Hypothesis.
The discovery itself was significant. The measurements of the springiness of the air fed into the development of theoretical ideas about the nature of matter, leading up to the idea of atoms and molecules flying about in the vacuum and colliding with one another. It also had practical implications, because the idea of making vacuums using pistons, and using the weight of air (atmospheric pressure) to compress pistons, found applications in steam engines. But from our point of view the most important thing about these experiments is the way they were carried out and reported. For the first time, experimental philosophers described their experiments in great detail, along with the way they overcame difficulties and how they interpreted their results. They not only gave a table showing the actual measurements of pressure made in the course of the investigation, but also included alongside these the numbers corresponding to ‘What the pressure should be according to the Hypothesis’. The match was not perfect; of course there were experimental errors. But they (or rather Hooke) had found that the accuracy of the hypothesis was confirmed within the limits of experimental error. And everything was laid out carefully so that other experimenters could repeat the whole process and see if their results agreed. It was only later, when many other experiments had indeed confirmed this, that the hypothesis was elevated to the status of a law, albeit with the wrong name attached to it.
While in Oxford, Hooke also developed his interests in astronomy and timekeeping, which we have already mentioned. Some of his other activities can wait until we discuss the contents of Micrographia. But there was one interest in particular that Hooke at first eagerly investigated in Oxford but then (for sound scientific reasons) abandoned – flying. This change of heart is described in the autobiography:
I contriv’d and made many trials about the Art of flying in the Air, and moving very swift on the Land and Water, of which I shew’d several Designs to Dr. Wilkins then Warden of Wadham College, and at the same time made a Module [model], which, by the help of Springs and Wings, rais’d and sustain’d itself in the Air; but finding by my own trials, and afterwards by Calculation, that the Muscles of a Mans Body were not sufficient to do anything considerable of that kind, I apply’d my Mind to contrive a way to make artificial Muscles; divers designs wherefore I shew’d also at the same time to Dr. Wilkins, but was in many of my Trials frustrated of my expectations.
The details surrounding one other project which Hooke worked on in the late 1650s and early 1660s are less clear, because for commercial reasons (in the hope, never realised, of making a fortune from his invention) for a long time Hooke kept details of his work on clocks and watches secret, and when he did report them he was inclined to exaggerate his achievement to strengthen his case. Nevertheless, it is quite clear that by about 1658 he was deeply interested in the possibility of designing an accurate timepiece – a chronometer – that would solve the problem of finding longitude at sea. This was of vital importance to an emerging maritime power such as England or their Dutch rivals.
Finding the latitude of a ship at sea was a relatively simple matter of measuring the height of the Sun above the horizon at local noon. But determining longitude was a much more difficult problem. It was clear that the person who solved that problem would certainly become rich as a result – even before the establishment of the famous prize of £20,000 offered for the solution by the British government in 1714. Hooke was always concerned about his financial security, and looked into two ways to tackle the problem. The first was based on the idea of astronomical observations, in particular observations of the moons of Jupiter. The four largest moons (discovered by Galileo in 1610) follow regular, predictable orbits around the giant planet, changing their positions relative to one another like the hands of a heavenly clock. These orbits could be predicted from past observations, even before the discovery of the inverse square law of gravity, so by studying tables of predicted patterns (in particular, eclipses of the moons by Jupiter) and comparing them with observations, a mariner could determine the time at the place where the tables were drawn up (such as the home port, or London) and compare that with the local time. Because of the rotation of the Earth, which takes twenty-four hours to complete a 360-degree rotation, local noon is one hour later for each fifteen degrees west of the home base, and one hour earlier for each fifteen degrees east (360/24 = 15); even Oxford time, by the Sun, is five minutes behind the time at Greenwich, in London. So the difference would tell them how far east or west of home the ship was. This was one of the reasons, in addition to his interest in astronomy