Out of the Shadow of a Giant: How Newton Stood on the Shoulders of Hooke and Halley. John Gribbin
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.fn3 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.fn4
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.fn5 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.