Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley. John Gribbin
probably because he genuinely just wanted to be left alone to get on with his life in peace. Katherine helped him to find a retreat from the turmoil of the times. Their father had left to Robert a small estate in Dorset – not much compared with the large estates in Ireland once intended for the older brothers, but enough for the youngest son, and by chance one of the few possessions the Earl had left at his death. Thanks to Katherine’s connections, the estate was not confiscated by Parliament, and Robert was allowed to live there from 1645 onwards, setting up his own laboratory where he carried out chemical experiments. On visits to London, he stayed with Lady Ranelagh, and like-minded experimental philosophers used to gather to meet with him at her house. Boyle referred to this as an ‘invisible college’; we don’t know who was involved, but there must have been considerable overlap with the group we mentioned earlier, including Wilkins.
Boyle’s fortunes improved in the 1650s, after the Civil Wars ended. One of his surviving brothers, now Lord Broghill, was in favour with Parliament for his part in crushing the Irish. This rubbed off on the rest of the Boyle family, and Robert was able to visit Ireland to pick up some of the threads of their former life. Horrified by the terrible conditions of the Irish people, after some soul-searching he got the estates running as a benevolent landlord (by the standards of the time) who used much of the income for charitable ends. This still left him enough to live on comfortably and continue in the role of gentleman scientist back in Dorset. But John Wilkins, who had met Boyle in London and knew his abilities, invited Robert to move to Oxford, where he could not just carry out his own experiments but be in the company of other people with similar interests. After mulling the offer over, Boyle made the move in 1655. He was never formally part of the university (as he put it himself, ‘never a Professor of Philosophy, nor a Gown-man’), but he had his own laboratory, and he also (as Wilkins had probably hoped) helped to finance the work of some of his fellow experimental philosophers. Boyle lived and worked in a house known as Deep Hall, on the High (convenient for the coffee shops!), and it was here, in 1656 (the year, incidentally, that Edmond Halley was born), that Robert Hooke came to live and work as Boyle’s paid assistant, although possibly they had already met.
Back in September 1653, when Wilkins was already trying to persuade Boyle to move to Oxford, he had sent a letter to him by messenger. It read, in part:
This bearer is the young man I recommended to you. I am apt to believe, that upon trial you will approve of him. But if it should happen otherwise, it is my desire he be returned, it not being so much to prefer him, as to serve you.fn5
Lisa Jardine has suggested that the young man in question was Hooke, and that he was sent as part of the attempt to entice Boyle to Oxford, by showing that a skilled assistant would be available there:
If it be not, Sir, prejudicial to your other affairs, I should exceedingly rejoice in your being stayed in England this winter, and the advantage of your conversation at Oxford, where you will be a means to quicken and direct us in our enquiries … shall be most ready to provide the best accommodation for you, that this place will afford.
The immediate plan to persuade Boyle to Oxford that winter was aborted because he had to travel to Ireland to deal with the urgent business concerning the family estates we have already mentioned. The young man, presumed to be Hooke, returned to Oxford. But when Boyle did make the move some two years later, it seems that he was already aware of the abilities of the man who did indeed become his assistant.
The greatest achievement of the Boyle–Hooke collaboration was an improved air pump, which made it possible for them to carry out experiments both at greatly reduced air pressure and at pressures greater than ordinary atmospheric pressure. That simple sentence, though, needs unpacking in order to put Hooke’s achievements, in particular, into perspective.
First, although Hooke was a paid assistant to Boyle, this was a genuine collaboration. Hooke was more than a ‘mere’ technician who did things at Boyle’s direction. This was a very unusual – indeed, possibly unique – working relationship for the time, but it is made clear in Boyle’s published works, where Hooke is regularly mentioned by name as a co-experimenter. Other assistants are not so acknowledged. Secondly, an air pump might not sound like a dramatic invention today. But in the middle of the seventeenth century it was the highest of high-tech scientific equipment, equivalent, in terms of the insights it gave, to CERN’s Large Hadron Collider, or the Hubble Space Telescope, today. It was cutting-edge technology, leading to breakthrough science. And the man who made the air pump, and made it work, was Robert Hooke, still in his early twenties. If there had been Nobel Prizes in the seventeenth century, Hooke would have walked away with one, for this achievement alone.
It all started with an experiment carried out by the Italian Evangelista Torricelli (one of Galileo’s pupils) in 1644. This seemed to shed light on a puzzle that had vexed philosophers for centuries: was it possible for a vacuum, nothing at all, to exist? One school of thought held that matter must be continuous; a rival hypothesis described matter in terms of tiny particles (atoms) moving through the void (vacuum, or empty space). Torricelli took a glass tube, closed at one end, and filled it with mercury. He then put a finger over the open end, and submerged that end below the surface of a dish of mercury before taking his finger away and raising the closed end of the tube into the vertical. Instead of all the mercury flowing out of the tube, the level dropped only until there was a column nearly thirty inches high standing above the level of the liquid in the dish, with nothing at all in the space above the column. This seemed to be the definitive proof of the reality of the vacuum, and along the way the height of the mercury in the tube was explained as a result of the pressure of the weight of the air pushing down on the surface of the mercury in the dish. Torricelli had invented the barometer, for measuring atmospheric pressure, and similar instruments were soon tested by being carried up mountains, where the lower air pressure meant that the column of mercury was shorter than at sea level. Which suggested that if the air continued to thin out, then above the atmosphere there must be empty space.
Instead of carrying the equipment up a mountain, Boyle wanted to try it out inside a vessel where air could be pumped out to lower the pressure. If he could make a vacuum inside the vessel, the level of mercury in the column would fall as the air was removed, until it would not be supported in a column at all. But first, he needed a way to make a vacuum in the laboratory. This is where Hooke came in. Otto von Guericke, in Saxony, had already made a reasonably efficient air pump, which he had used to suck air out of two large copper hemispheres that were placed together rim to rim to make a sphere, but with no mechanical fastenings at the join. With air pressure inside the sphere reduced, the pressure of the atmosphere outside squeezed the hemispheres together so tightly that in a famous demonstration made to Emperor Ferdinand III in 1654 thirty horses could not pull them apart.
Von Guericke’s pump was large and cumbersome, needing two men to operate, and, of course, there was no way to see inside his copper sphere. Boyle needed something that could be operated by one man, with a chamber made of glass through which experiments could be observed. He first approached the greatest scientific instrument maker of the time, Ralph Greatorex, in London. But his forte was making precision instruments, and his attempt at the heavier machinery required for the pump was not up to Boyle’s needs. So it was Hooke, at the end of the 1650s, who designed and built the breakthrough instrument, using funds supplied by Boyle. He went to London to oversee the manufacture of the heavy components in the workshops there (we don’t know if he worked on these himself), then had them taken to Oxford, where he put the pump together and made it work.
The vacuum chamber consisted of a glass sphere fifteen inches in diameter, known as the ‘receiver’, with a brass lid four inches in diameter, which could be opened to place apparatus inside the sphere. A tapering hole in the base of the sphere stood on top of a tight-fitting brass cylinder, sealed with a leather collar. The brass lid had a small tight-fitting stopper, sealed with oil (referred to by Hooke as ‘sallad oil’), that could be turned to tug a string attached to the stopper in order to set off an experiment inside the globe. The cylinder below the globe connected to a brass pump fitted with an ingenious rack-and-pinion system, which allowed air easily to be pumped out of or into the globe. Hooke’s pump sucked air from the cylinder using a piston that was connected to a rod cut with teeth which engaged with a gear wheel that could be wound with a handle to push the piston