Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley. John Gribbin
A great part of that Doctrine & hypothesis in my Lectures in Gresham Colledge Some time before Mr Steno had published his Booke.
There is no reason to doubt Hooke’s version of affairs, and there is no doubt at all that his work preceded that of Steno, whether or not Steno got word of it via Oldenburg. Steno, by the way, never gave a clue one way or the other: he disappeared from the scientific scene after writing his book. He became a Catholic priest in 1675, and was ordained as a bishop in 1677, inflicting on his body such a harsh regime of fasting and self-denial that he died in 1686, at the age of forty-eight.
Hooke’s more extensive ideas about earthquakes, Earth history and geology will be covered in Chapter Nine. Now, we still have a fourth great insight from Micrographia to discuss, although here we diverge from ‘Espinasse’s assessment of which of the ideas Hooke presented there were most significant. She picks out his discovery of the structures he named cells (after the rooms occurred by monks in a monastery) in thin slices of cork (Observation 18). As Hooke puts it, no ‘Writer or Person’ had ‘made any mention of them before this’. But although the name was taken up and used by later biologists, it was in a slightly different context. The ‘pores’, as he also called them, that Hooke had found are not living cells, but non-living structures left over from the growth of the plant. The first person to see and study live cells under a microscope was Hooke’s Dutch contemporary Antoni van Leeuwenhoek. In 1674 he described an algae, Spirogyra, and other organisms that moved of their own volition; he named them animalcules (‘little animals’). In this area, Hooke’s work was important, but not as important as the work of van Leeuwenhoek and others. In our estimation, his astronomical Observations were of far greater importance.
Hooke was a serious and highly respected astronomer. On 9 May 1664, using a twelve-foot-long refracting telescope, he had discovered the Great Red Spot of Jupiter, and used it to measure the rotation of the giant planet. Contemporary (and now more famous) astronomers such as the Italian Giovanni Cassini picked up on the discovery, and referred to the phenomenon as ‘Hooke’s Spot’. But it was observations of something much closer to home that led Hooke to important insights that appeared in Observation 60: Of the Moon. This is a short contribution that to a casual glance looks like a mere filler. That couldn’t be more wrong.
Observation 60 provides a nice example of the scientific mind – Hooke’s scientific mind – at work: making observations, devising hypotheses, testing them by experiment and further observation, and drawing general conclusions from specific cases. Remember that this was less than sixty years after Galileo, with the aid of one of the first astronomical telescopes, discovered that the Moon is not a perfect sphere but pockmarked with craters and scarred by mountain ranges. Hooke was intrigued by the nature of these craters, and puzzled over their origin. He described them as ‘almost like a dish, some bigger, some less, some shallower, some deeper, that is, they seem to be a hollow Hemisphere, incompassed with a round rising bank, as if the substance in the middle had been digg’d up, and thrown on either side’. Which establishes, as if we did not already know, that he was a good and accurate observer.
How could such craters be formed? Hooke came up with two hypotheses and set out to test them. The first was that the craters were caused by impacts. To test this, Hooke made a mixture of water and pipe-clay, ‘into which, if I let fall any heavy body, as a Bullet, it would throw up the mixture round the place, which for a while would make a representation, not unlike these of the Moon.’ So incoming objects (bodies) would do the trick. But Hooke found it ‘difficult to imagine whence those bodies should come’, so he turned to his other idea. In this experiment he heated a pot of alabaster to the boiling point, and then, while it was still bubbling, took it off the fire and allowed it to set. Then ‘the whole surface, especially that where some of the last Bubbles have risen, will appear all over covered with small pits, exactly shaped like these of the Moon, and by holding a lighted Candle in a large dark Room, in divers positions to this surface, you may exactly represent all the Phenomena of these pits in the Moon, according as they are more or less enlightened by the Sun’.
So Hooke plumped for volcanic activity as an explanation of lunar cratering, rather than impacts. This was a perfectly reasonable conclusion to draw at the time, and for the next four hundred years volcanic activity remained a viable explanation for lunar cratering. The idea was only finally laid to rest, in favour of the impact hypothesis, when astronauts visited the Moon and its geology could be studied first hand. We now know that the craters were indeed made by impacts, in which ‘the substance in the middle had been digg’d up, and thrown on either side’. But if it was thrown up, either by impacts or by volcanic activity, something must have pulled it back down on to the surface of the Moon to make the circular ramparts surrounding the craters. That something, Hooke reasoned, must have been gravity – the Moon’s own gravitational pull.
Developing his idea, Hooke said that it ‘is not improbable, but that the substance of the Moon may be very much like that of the Earth’ (which would have amounted to heresy a few decades earlier). And then he goes beyond Galileo, who noticed the imperfection of the Moon, to draw attention to the remarkable roundness of the Moon in spite of the small irregularities we see on its surface. The Moon, he points out:
we may perceive very plainly by the Telescope, to be (bating the small inequality of the Hills and Vales in it, which are all of them likewise shaped, or levelled, as it were, to answer to the center of the Moons body) perfectly of a Spherical figure, that is, all the parts are so rang’d (bating the comparatively small ruggedness of the Hills and Dales) that the outmost bounds of them are equally distant from the Center of the Moon, and consequently, it is exceedingly probable also, that they are equidistant from the Center of gravitation; and indeed, the figure of the superficial parts of the Moon are so exactly shap’d, according as they should bye, supposing it had a gravitating principle as the Earth has.
This is mind-blowing stuff. At a time when other people talked about vortices and whirlpools being responsible for the shape of the planets and their orbits, and Isaac Newton was an unknown student who would soon be eagerly devouring Hooke’s book,fn12 Hooke is suggesting the universal principle of gravitation (he can hardly have failed to notice that Jupiter and the other planets are also round!), that all objects possess this property, which makes the moons and planets round and (although he discusses this elsewhere) holds them in their orbits around the Sun. The very last paragraph of Micrographia begins with the words: ‘To conclude, therefore, it being very probable, that the Moon has a principle of gravitation … whereby it is not only shap’d round, but does firmly contain and hold all its parts united, though many of them seem as loose as the sand on the Earth’.
We emphasise that the idea of universal gravity is of key importance. This is the beginning of an understanding that the laws of physics which operate in the Universe at large – in the Heavens – are the same as the laws that apply here on Earth. That idea is often traced back to Newton; it should be traced back to Hooke. It’s a long way from the study of the point of a needle!
Any of these four ideas, or indeed his ideas about planetary orbits and gravity, which we have already discussed, should have ensured Hooke’s status as one of the greatest scientists of all time. And remember that there were dozens of lesser ‘observations’ in Micrographia, including some of the first observations of the tiny creatures that live in water and other liquids – the Fellows were particularly intrigued by the discovery of the creatures we call nematodes, but were referred to then as ‘eels’, living in vinegar (Observation 57). Perhaps Hooke would have been suitably recognised by posterity if he had been able to develop his ideas more fully, which he clearly intended to do. In his book, he says (especially in reference to his ideas about combustion, but undoubtedly with broader relevance):
In this place I have only time to hint at an Hypothesis, which, if God permit me life and opportunity, I may elsewhere prosecute, improve and publish.
But Hooke’s opportunities to ‘prosecute, improve and publish’ his revolutionary ideas were almost immediately restricted by plague, the Great Fire of London, and a change of career that occurred in the aftermath of the fire. While he was otherwise engaged, at least some of those ideas were taken up and developed by Isaac Newton, who had an early copy of Micrographia which he read and