Hooke's role at the Royal Society was to demonstrate experiments from his own methods or at the suggestion of members. Among his earliest demonstrations were discussions of the nature of air and the implosion of glass bubbles that had been sealed with enclosed hot air. He also demonstrated that a dog could be kept alive with its
thorax opened, provided air was pumped in and out of its lungs. He noted the difference between
venous blood and
arterial blood, and thus demonstrated that the ("food of life") and [flames] were the same thing. There were also experiments on gravity, the falling of objects, the weighing of bodies, the measurement of
barometric pressure at different heights, and the movement of
pendulums up to long. His biographer described him as England's first
meteorologist, in her description of his essay
Method for making a history of the weather. (Hooke specifies that a thermometer, a
hygrometer, a wind gauge, and a record sheet be used for proper weather records.)
Astronomy In May 1664, using a
refracting telescope, Hooke observed the
Great Red Spot of
Jupiter for two hours as it moved across the planet's face. In March 1665, he published his findings and from them, the Italian astronomer
Giovanni Cassini calculated the
rotation period of Jupiter to be nine hours and fifty-five minutes. One of the most-challenging problems Hooke investigated was the measurement of the distance from Earth to a star other than the Sun. Hooke selected the star
Gamma Draconis and chose the method of
parallax determination. In 1669, after several months of observing, Hooke believed the desired result had been achieved. It is now known his equipment was far too imprecise to obtain an accurate measurement. Hooke's
Micrographia contains illustrations of the
Pleiades star cluster and
lunar craters. He conducted experiments to investigate the formation of these craters and concluded their existence meant the Moon must have its own gravity, a radical departure from the contemporaneous
Aristotelian celestial model. He also was an early observer of the
rings of Saturn, and discovered one of the first-observed
double stars,
Gamma Arietis (a
visual binary), in 1664. To achieve these discoveries, Hooke needed better instruments than those that were available at the time. Accordingly, he invented three new mechanisms: the
Hooke joint, a sophisticated
universal joint that allowed his instruments to smoothly follow the apparent motion of the observed body; the first
clockwork drive to automate the process; and a
micrometer screw that allowed him to achieve a precision of ten
seconds of arc. Hooke was dissatisfied with
refracting telescopes so he built the first practical
Gregorian telescope that used a silvered glass mirror.
Mechanics In 1660, Hooke discovered
the law of
elasticity that bears his name and describes the linear variation of
tension with extension in an
elastic spring. Hooke first described this discovery in an anagram "ceiiinosssttuv", whose solution he published in 1678 as ("As the extension, so the force"). His work on elasticity culminated in his development of the
balance spring or hairspring, which for the first time enabled a portable timepiecea watchto keep time with reasonable accuracy. A bitter dispute between Hooke and
Christiaan Huygens on the priority of this invention was to continue for centuries after the death of both but a note dated 23 June 1670 in the journals of the Royal Society, describing a demonstration of a balance-controlled watch before the Royal Society, may support Hooke's claim to priority for the idea. Nevertheless, it is Huygens who is credited with building the first watch to use a balance spring. Hooke's announcement of his law of elasticity using an
anagram was a method scientists, such as Hooke, Huygens, and
Galileo, sometimes used to establish priority for a discovery without revealing details. Hooke used mechanical analogues to understand fundamental processes such as the motion of a spherical pendulum and of a ball in a hollow cone, to demonstrate central force due to gravity, and a hanging chain net with point loads to provide the optimum shape for a dome with heavy cross on top. Despite continuing reports to the contrary, Hooke did not influence
Thomas Newcomen's invention of the
steam engine; this myth, which originated in an article in the third edition of
Encyclopædia Britannica, has been found to be mistaken.
Gravitation While many of Hooke's contemporaries, such as Isaac Newton, believed in
aether as a medium for transmitting attraction and repulsion between separated celestial bodies, Hooke argued for an attracting principle of gravitation in
Micrographia (1665). In a communication to the Royal Society in 1666, he wrote: Hooke's 1674 Gresham lecture,
An Attempt to Prove the Motion of the Earth by Observations (published 1679), said gravitation applies to "all celestial bodies" and restated these three propositions. Hooke's statements up to 1674 make no mention, however, that an inverse square law applies or might apply to these attractions. His model of gravitation was also not yet universal, though it approached universality more closely than previous hypotheses. Hooke did not provide accompanying evidence or mathematical demonstration; he stated in 1674: "Now what these several degrees [of gravitational attraction] are I have not yet experimentally verified", indicating he did not yet know what law the gravitation might follow; and about his whole proposal, he said: "This I only hint at present ... having my self many other things in hand which I would first , and therefore cannot so well attend it" (i.e. "prosecuting this Inquiry"). In November 1679, Hooke initiated a notable exchange of letters with Newton that was published in 1960. Hooke's ostensible purpose was to tell Newton he (Hooke) had been appointed to manage the Royal Society's correspondence; Hooke therefore wanted to hear from members about their research or their views about the research of others. Hooke asked Newton's opinions about various matters. Among other items, Hooke mentioned "compounding the celestial motions of the planets of a direct motion by the tangent and an attractive motion towards the central body"; his "hypothesis of the or causes of springinesse"; a new hypothesis from Paris about planetary motions, which he described at length; efforts to carry out or improve national surveys; and the difference of latitude between London and Cambridge. Newton's reply offered "a of my own" about a terrestrial experiment rather than a proposal about celestial motions that might detect the Earth's motion; the experiment would use a body suspended in air and then dropped. Hooke wanted to discern how Newton thought the falling body could experimentally reveal the Earth's motion by its direction of deviation from the vertical but Hooke went on hypothetically to consider how its motion could continue if the solid Earth had not been in the way, on a spiral path to the centre. Hooke disagreed with Newton's idea of the body's continuing motion. A further short correspondence developed; towards the end of it, writing on 6 January 1680 to Newton, Hooke communicated his "supposition ... that the Attraction always is in a duplicate proportion to the Distance from the , and Consequently that the Velocity will be in a subduplicate proportion to the Attraction and Consequently as Kepler Supposes to the Distance". (Hooke's inference about the velocity is incorrect.) In 1686, when the first book of Newton's
Principia was presented to the Royal Society, Hooke said he had given Newton the "notion" of "the rule of the decrease of Gravity, being reciprocally as the squares of the distances from the ". At the same time, according to
Edmond Halley's contemporaneous report, Hooke agreed "the Demonstration of the Curves generated thereby" was wholly Newton's. According to a 2002 assessment of the early history of the inverse square law: "by the late 1660s, the assumption of an 'inverse proportion between gravity and the square of distance' was rather common and had been advanced by a number of different people for different reasons". In the 1660s, Newton had shown for planetary motion under a circular assumption, force in the radial direction had an inverse-square relation with distance from the centre. Newton, who in May 1686 was presented with Hooke's claim to priority on the inverse square law, denied he was to be credited as author of the idea, giving reasons including the citation of prior work by others. Newton also said that, even if he had first heard of the inverse square proportion from Hooke (which Newton said he had not), he would still have some rights to it because of his mathematical developments and demonstrations. These, he said, enabled observations to be relied upon as evidence of its accuracy while according to Newton, Hooke, without mathematical demonstrations and evidence in favour of the supposition, could only guess it was approximately valid "at great distances from the centre". Newton accepted and acknowledged, in all editions of his
Principia, that Hooke and others had separately appreciated the inverse square law in the solar system. Newton acknowledged Wren, Hooke, and Halley in this connection in his "Scholium to Proposition 4" in Book1. In a letter to Halley, Newton also acknowledged his correspondence with Hooke in 1679–1680 had reawakened his dormant interest in astronomical matters but that did not mean, according to Newton, Hooke had told Newton anything new or original. Newton wrote: Whilst Newton was primarily a pioneer in mathematical analysis and its applications, and optical experimentation, Hooke was a creative experimenter of such great range who left some of his ideas, such as those about gravitation, undeveloped. In 1759, decades after the deaths of both Newton and Hooke,
Alexis Clairaut, mathematical astronomer eminent in his own right in the field of gravitational studies, reviewed Hooke's published work on gravitation. According to
Stephen Peter Rigaud, Clairaut wrote: "The example of Hooke and that of Kepler [serves] to show what a distance there is between a truth that is glimpsed and a truth that is demonstrated".
I. Bernard Cohen said: "Hooke's claim to the inverse-square law has masked Newton's far more fundamental debt to him, the analysis of curvilinear orbital motion. In asking for too much credit, Hooke effectively denied to himself the credit due him for a seminal idea".
Horology Hooke made important contributions to the science of timekeeping and was intimately involved in the advances of his time; these included refinement of the pendulum as a better regulator for clocks, increased precision of clock mechanisms and the use of the
balance spring to improve the timekeeping of watches.
Galileo had observed the regularity of a pendulum and Huygens first incorporated it in a clock; in 1668, Hooke demonstrated his new device to keep a pendulum swinging regularly in unsteady conditions. His invention of a tooth-cutting machine enabled a substantial improvement in the accuracy and precision of timepieces. Waller reported the invention was, by Hooke's death, in constant use among clock makers. Hooke announced he conceived a way to build a
marine chronometer to determine longitude. and with the help of Boyle and others, he attempted to patent it. In the process, Hooke demonstrated a pocket-watch of his own devising that was fitted with a
coil spring attached to the arbour of the balance. Hooke's refusal to accept an
escape clause in the proposed exclusive contract for the use of this idea resulted in its abandonment. Hooke developed the principle of the balance spring independently of Huygens and at least five years beforehand. Huygens published his own work in
Journal de Scavans in February 1675 and built the first functioning watch to use a balance spring.
Microscopy In 1663 and 1664, Hooke made his microscopic, and some astronomic, observations, which he collated in
Micrographia in 1665. His book, which describes observations with microscopes and telescopes, as well as original work in biology, contains the earliest-recorded observation of a microorganism, the microfungus
Mucor. Hooke coined the term "
cell", suggesting a resemblance between plant structures and
honeycomb cells.The hand-crafted, leather-and-gold-tooled microscope he designed and used to make the observations for
Micrographia, which
Christopher Cock made for him in London, is on display at the
National Museum of Health and Medicine in
Maryland. Hooke's work developed from that of
Henry Power, who published his microscopy work in
Experimental Philosophy (1663); in turn, the Dutch scientist
Antonie van Leeuwenhoek went on to develop increased magnification and so reveal
protozoa,
blood cells, and
spermatozoa.
Micrographia also contains Hooke's, or perhaps Boyle's and Hooke's, ideas on combustion. Hooke's experiments led him to conclude combustion involves a component of air, a statement with which modern scientists would agree but that was not understood widely, if at all, in the seventeenth century. He also concluded respiration and combustion involve a specific and limited component of air. According to Partington, if "Hooke had continued his experiments on combustion, it is probable that he would have discovered oxygen".
Samuel Pepys wrote of the book in
his diary on 21 January 16: "Before I went to bed I sat up till two o’clock in my chamber reading of Mr. Hooke's Observations, the most ingenious book that ever I read in my life".
Palaeontology and geology One of the observations in
Micrographia is of
fossil wood, the microscopic structure of which Hooke compared to that of ordinary wood. This led him to conclude that fossilised objects like petrified wood and fossil shells such as
ammonites were the remains of living things that had been soaked in mineral-laden petrifying water. He believed that such fossils provided reliable clues about the history of life on Earth and, despite the objections of contemporary naturalists like
John Raywho found the concept of
extinction theologically unacceptablethat in some cases they might represent species that had become extinct through some geological disaster. In a series of lectures in 1668, Hooke proposed the then-heretical idea the Earth's surface had been formed by volcanoes and earthquakes, and that the latter were responsible for shell fossils being found far above sea level. In 1835,
Charles Lyell, the Scottish geologist and associate of
Charles Darwin, wrote of Hooke in
Principles of Geology: "His treatise ... is the most philosophical production of that age, in regard to the causes of former changes in the organic and inorganic kingdoms of nature".
Memory Hooke's scientific model of human
memory was one of the first of its kind. In a 1682 lecture to the Royal Society, Hooke proposed a mechanical analogue model of human memory that bore little resemblance to the mainly philosophical models of earlier writers. This model addressed the components of encoding, memory capacity, repetition, retrieval, and forgetting – some with surprisingly modern accuracy. According to psychology professor Douglas Hintzman, Hooke's model's most-interesting points are that it allows for attention and other top-down influences on encoding; it uses resonance to implement parallel, cue-dependent retrieval; it explains memory for recency; it offers a single-system account of repetition and priming; and the power law of forgetting can be derived from the model's assumption in a straightforward way.
Other On 8 July 1680, Hooke observed the
nodal patterns associated with the
modes of vibration of glass plates. He ran a
bow along the edge of a flour-covered glass plate and saw the nodal patterns emerge. In acoustics, in 1681, Hooke showed the Royal Society that musical tones can be generated using spinning brass cogs cut with teeth in particular proportions. == Architecture ==