• Physician and Geologist James Hutton, 1726
• Mathematical Physicist William Thomson, 1st Baron Kelvin, 1824

James Hutton was born in Edinburgh on 3 June 1726 OS as one of five children of William Hutton, a merchant who was Edinburgh City Treasurer, but who died in 1729 when James was still young. Hutton's mother - Sarah Balfour - insisted on his education at the High School of Edinburgh where he was particularly interested in mathematics and chemistry, then when he was 14 he attended the University of Edinburgh as a "student of humanity", i.e., Classics (Latin and Greek). He was apprenticed to the lawyer George Chalmers WS when he was 17, but took more interest in chemical experiments than legal work and at the age of 18 became a physician's assistant as well as attending lectures in medicine at the University of Edinburgh. After three years he studied the subject in Paris (University of Paris), then in 1749 took the degree of Doctor of Medicine at Leyden with a thesis on blood circulation. Around 1747 he had a son by a Miss Edington, and though he gave his child James Smeaton Hutton financial assistance, he had little to do with the boy who went on to become a post office clerk in London.

After his degree Hutton returned to London, then in mid 1750 went back to Edinburgh and resumed chemical experiments with close friend, James Davie. Their work on production of sal ammoniac from soot led to their partnership in a profitable chemical works, manufacturing the crystalline salt which was used for dyeing, metalworking and as smelling salts and previously was available only from natural sources and had to be imported from Egypt. Hutton owned and rented out properties in Edinburgh, employing a factor to manage this business.

Hutton inherited from his father the Berwickshire farms of Slighhouses, a lowland farm which had been in the family since 1713, and the hill farm of Nether Monynut. In the early 1750s he moved to Slighhouses and set about making improvements, introducing farming practices from other parts of Britain and experimenting with plant and animal husbandry. He recorded his ideas and innovations in an unpublished treatise on The Elements of Agriculture.

This developed his interest in meteorology and geology. In a 1753 letter he wrote that he had "become very fond of studying the surface of the earth, and was looking with anxious curiosity into every pit or ditch or bed of a river that fell in his way”. Clearing and draining his farm provided ample opportunities. Playfair describes Hutton as having noticed that “a vast proportion of the present rocks are composed of materials afforded by the destruction of bodies, animal, vegetable and mineral, of more ancient formation”. His theoretical ideas began to come together in 1760. While his farming activities continued, in 1764 he went on a geological tour of the north of Scotland with George Maxwell - Clerk, ancestor of the famous James Clerk Maxwell.

In 1768 Hutton returned to Edinburgh, letting his farms to tenants but continuing to take an interest in farm improvements and research which included experiments carried out at Slighhouses. He developed a red dye made from the roots of the madder plant.

He had a house built in 1770 at St John’s Hill, Edinburgh, overlooking Salisbury Crags. This later became the Balfour family home and, in 1840, the birthplace of the psychiatrist James Crichton - Browne. Hutton was one of the most influential participants in the Scottish Enlightenment, and fell in with numerous first class minds in the sciences including John Playfair, philosopher David Hume and economist Adam Smith. Hutton held no position in Edinburgh University and communicated his scientific findings through the Royal Society of Edinburgh. He was particularly friendly with Joseph Black, and the two of them together with Adam Smith founded the Oyster Club for weekly meetings, with Hutton and Black finding a venue which turned out to have rather disreputable associations.

Between 1767 and 1774 Hutton had considerable close involvement with the construction of the Forth and Clyde canal, making full use of his geological knowledge, both as a shareholder and as a member of the committee of management, and attended meetings including extended site inspections of all the works. In 1777 he published a pamphlet on Considerations on the Nature, Quality and Distinctions of Coal and Culm which successfully helped to obtain relief from excise duty on carrying small coal.

Hutton hit on a variety of ideas to explain the rock formations he saw around him, but according to Playfair he "was in no haste to publish his theory; for he was one of those who are much more delighted with the contemplation of truth, than with the praise of having discovered it”. After some 25 years of work, his Theory of the Earth; or an Investigation of the Laws observable in the Composition, Dissolution, and Restoration of Land upon the Globe was read to meetings of the Royal Society of Edinburgh in two parts, the first by his friend Joseph Black on 7 March 1785, and the second by himself on 4 April 1785. Hutton subsequently read an abstract of his dissertation Concerning the System of the Earth, its Duration and Stability to Society meeting on 4 July 1785, which he had printed and circulated privately. In it, he outlined his theory as follows;

The solid parts of the present land appear in general, to have been composed of the productions of the sea, and of other materials similar to those now found upon the shores. Hence we find reason to conclude:
1st, That the land on which we rest is not simple and original, but that it is a composition, and had been formed by the operation of second causes.
2nd, That before the present land was made, there had subsisted a world composed of sea and land, in which were tides and currents, with such operations at the bottom of the sea as now take place. And,
Lastly, That while the present land was forming at the bottom of the ocean, the former land maintained plants and animals; at least the sea was than inhabited by animals, in a similar manner as it is at present.
Hence we are led to conclude, that the greater part of our land, if not the whole had been produced by operations natural to this globe; but that in order to make this land a permanent body, resisting the operations of the waters, two things had been required;
1st, The consolidation of masses formed by collections of loose or incoherent materials;
2ndly, The elevation of those consolidated masses from the bottom of the sea, the place where they were collected, to the stations in which they now remain above the level of the ocean.

At Glen Tilt in the Cairngorm mountains in the Scottish Highlands in 1785, Hutton found granite penetrating metamorphic schists, in a way which indicated that the granite had been molten at the time. This showed to him that granite formed from cooling of molten rock, not precipitation out of water as others at the time believed, and that the granite must be younger than the schists.

He went on to find a similar penetration of volcanic rock through sedimentary rock near the center of Edinburgh, at Salisbury Crags, adjoining Arthur's Seat: this is now known as Hutton's Section. He found other examples in Galloway in 1786, and on the Isle of Arran in 1787.

The existence of angular unconformities had been noted by Nicolas Steno and by French geologists including Horace - Bénédict de Saussure, who interpreted them in terms of Neptunism as "primary formations". Hutton wanted to examine such formations himself to see “particular marks” of the relationship between the rock layers. On the 1787 trip to the Isle of Arran he found his first example of Hutton's Unconformity to the north of Newton Point near Lochranza, but the limited view meant that the condition of the underlying strata was not clear enough for him, and he incorrectly thought that the strata were conformable at a depth below the exposed outcrop.

Later in 1787 Hutton noted what is now known as the Hutton Unconformity at Inchbonny, Jedburgh, in layers of sedimentary rock. As shown in the illustrations to the right, layers of greywacke in the lower layers of the cliff face are tilted almost vertically, and above an intervening layer of conglomerate lie horizontal layers of Old Red Sandstone. He later wrote of how he "rejoiced at my good fortune in stumbling upon an object so interesting in the natural history of the earth, and which I had been long looking for in vain." That year, he found the same sequence in Teviotdale.

In the Spring of 1788 he set off with John Playfair to the Berwickshire coast and found more examples of this sequence in the valleys of the Tour and Pease Burns near Cockburnspath. They then took a boat trip from Dunglass Burn east along the coast with the geologist Sir James Hall of Dunglass. They found the sequence in the cliff below St. Helens, then just to the east at Siccar Point found what Hutton called "a beautiful picture of this junction washed bare by the sea". Playfair later commented about the experience, "the mind seemed to grow giddy by looking so far into the abyss of time". Continuing along the coast, they made more discoveries including sections of the vertical beds showing strong ripple marks which gave Hutton "great satisfaction" as a confirmation of his supposition that these beds had been laid horizontally in water. He also found conglomerate at altitudes that demonstrated the extent of erosion of the strata, and said of this that "we never should have dreamed of meeting with what we now perceived”.

Hutton reasoned that there must have been innumerable cycles, each involving deposition on the seabed, uplift with tilting and erosion then undersea again for further layers to be deposited. On the belief that this was due to the same geological forces operating in the past as the very slow geological forces seen operating at the present day, the thicknesses of exposed rock layers implied to him enormous stretches of time.

Though Hutton circulated privately a printed version of the abstract of his Theory (Concerning the System of the Earth, its Duration, and Stability) which he read at a meeting of the Royal Society of Edinburgh on 4 July 1785; the full account of his theory as read at the 7 March 1785 and 4 April 1785 meetings did not appear in print until 1788. It was titled Theory of the Earth; or an Investigation of the Laws observable in the Composition, Dissolution, and Restoration of Land upon the Globe and appeared in Transactions of the Royal Society of Edinburgh, vol. I, Part II, pp. 209 – 304, plates I and II, published 1788. He put forward the view that "from what has actually been, we have data for concluding with regard to that which is to happen thereafter." This restated the Scottish Enlightenment concept which David Hume had put in 1777 as "all inferences from experience suppose ... that the future will resemble the past", and Charles Lyell memorably rephrased in the 1830s as "the present is the key to the past". Hutton's 1788 paper concludes; "The result, therefore, of our present enquiry is, that we find no vestige of a beginning, – no prospect of an end." His memorably phrased closing statement has long been celebrated.

Following criticism, especially the arguments from Richard Kirwan who thought Hutton's ideas were atheistic and not logical, Hutton published a two volume version of his theory in 1795, consisting of the 1788 version of his theory (with slight additions) along with a lot of material drawn from shorter papers Hutton already had to hand on various subjects such as the origin of granite. It included a review of alternative theories, such as those of Thomas Burnet and Georges - Louis Leclerc, Comte de Buffon.

The whole was entitled An Investigation of the Principles of Knowledge and of the Progress of Reason, from Sense to Science and Philosophy when the third volume was completed in 1794. Its 2,138 pages prompted Playfair to remark that “The great size of the book, and the obscurity which may justly be objected to many parts of it, have probably prevented it from being received as it deserves.”

His new theories placed him into opposition with the then popular Neptunist theories of Abraham Gottlob Werner, that all rocks had precipitated out of a single enormous flood. Hutton proposed that the interior of the Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands. This theory was dubbed "Plutonist" in contrast to the flood oriented theory.

As well as combating the Neptunists, he also opened up the concept of deep time for scientific purposes, in opposition to Catastrophism. Rather than accepting that the earth was no more than a few thousand years old, he maintained that the Earth must be much older, with a history extending indefinitely into the distant past. His main line of argument was that the tremendous displacements and changes he was seeing did not happen in a short period of time by means of catastrophe, but that processes still happening on the Earth in the present day had caused them. As these processes were very gradual, the Earth needed to be ancient, in order to allow time for the changes. Before long, scientific inquiries provoked by his claims had pushed back the age of the earth into the millions of years – still too short when compared with the accepted 4.6 billion year age in the 21st century, but a distinct improvement.

It has been claimed that the prose of Principles of Knowledge was so obscure that it also impeded the acceptance of Hutton's geological theories. Restatements of his geological ideas (though not his thoughts on evolution) by John Playfair in 1802 and then Charles Lyell in the 1830s popularized the concept of an infinitely repeating cycle, though Lyell tended to dismiss Hutton's views as giving too much credence to catastrophic changes.

Lyell's books had widespread influence, not least on the up and coming young geologist Charles Darwin who read them with enthusiasm during his voyage on the Beagle, and has been described as Lyell's first disciple. In a comment on the arguments of the 1830s, William Whewell coined the term uniformitarianism to describe Lyell's version of the ideas, contrasted with the catastrophism of those who supported the early 19th century concept that geological ages recorded a series of catastrophes followed by repopulation by a new range of species. Over time there was a convergence in views, but Lyell's description of the development of geological ideas led to wide belief that uniformitarianism had triumphed.

It was not merely the earth to which Hutton directed his attention. He had long studied the changes of the atmosphere. The same volume in which his Theory of the Earth appeared contained also a Theory of Rain. He contended that the amount of moisture which the air can retain in solution increases with temperature, and, therefore, that on the mixture of two masses of air of different temperatures a portion of the moisture must be condensed and appear in visible form. He investigated the available data regarding rainfall and climate in different regions of the globe, and came to the conclusion that the rainfall is regulated by the humidity of the air on the one hand, and mixing of different air currents in the higher atmosphere on the other.

Hutton also advocated uniformitarianism for living creatures too – evolution, in a sense – and even suggested natural selection as a possible mechanism affecting them:

"...if an organised body is not in the situation and circumstances best adapted to its sustenance and propagation, then, in conceiving an indefinite variety among the individuals of that species, we must be assured, that, on the one hand, those which depart most from the best adapted constitution, will be the most liable to perish, while, on the other hand, those organised bodies, which most approach to the best constitution for the present circumstances, will be best adapted to continue, in preserving themselves and multiplying the individuals of their race." – Investigation of the Principles of Knowledge, volume 2.

Hutton gave the example that where dogs survived through "swiftness of foot and quickness of sight... the most defective in respect of those necessary qualities, would be the most subject to perish, and that those who employed them in greatest perfection... would be those who would remain, to preserve themselves, and to continue the race". Equally, if an acute sense of smell were "more necessary to the sustenance of the animal... the same principle [would] change the qualities of the animal, and.. produce a race of well scented hounds, instead of those who catch their prey by swiftness". The same "principle of variation" would influence "every species of plant, whether growing in a forest or a meadow". He came to his ideas as the result of experiments in plant and animal breeding, some of which he outlined in an unpublished manuscript, the Elements of Agriculture. He distinguished between heritable variation as the result of breeding, and non - heritable variations caused by environmental differences such as soil and climate.

Though he saw his "principle of variation" as explaining the development of varieties, Hutton rejected the idea of evolution originating species as a "romantic fantasy". As a deist, he thought the mechanism allowed species to form varieties better adapted to particular conditions and was evidence of benevolent design in nature. Studies of Charles Darwin's notebooks have shown that Darwin arrived separately at the idea of natural selection which he set out in his 1859 book On the Origin of Species, but it has been speculated that he may have had some half - forgotten memory from his time as a student in Edinburgh of ideas of selection in nature as set out by Hutton, and by William Charles Wells and Patrick Matthew who had both been associated with the city before publishing their ideas on the topic early in the 19th century.

Lord Kelvin is widely known for realizing that there was a lower limit to temperature, absolute zero; absolute temperatures are stated in units of kelvin in his honor. On his ennoblement in 1892 in honor of his achievements in thermodynamics, and of his opposition to Irish Home Rule, he adopted the title Baron Kelvin of Largs and is therefore often described as Lord Kelvin. He was the first UK scientist to be elevated to the House of Lords. The title refers to the River Kelvin, which flows close by his laboratory at the University of Glasgow. His home was the imposing red sandstone mansion Netherhall, in Largs on the Firth of Clyde. Despite offers of elevated posts from several world renowned universities Lord Kelvin refused to leave Glasgow, remaining Professor of Natural Philosophy for over 50 years, until his eventual retirement from that post. The Hunterian Museum at the University of Glasgow has a permanent exhibition on the work of Lord Kelvin including many of his original papers, instruments and other artifacts.

William Thomson's father, James Thomson, was a teacher of mathematics and engineering at Royal Belfast Academical Institution and the son of a farmer. James Thomson married Margaret Gardner in 1817 and, of their children, four boys and two girls survived infancy. Margaret Thomson died in 1830 when William was six years old.

William and his elder brother James were tutored at home by their father while the younger boys were tutored by their elder sisters. James was intended to benefit from the major share of his father's encouragement, affection and financial support and was prepared for a career in engineering.

In 1832, his father was appointed professor of mathematics at Glasgow and the family moved there in October 1833. The Thomson children were introduced to a broader cosmopolitan experience than their father's rural upbringing, spending mid 1839 in London and, the boys, being tutored in French in Paris. Mid 1840 was spent in Germany and the Netherlands. Language study was given a high priority.

Thomson had heart problems and nearly died when he was 9 years old. He attended the Royal Belfast Academical Institution, where his father was a professor in the university department, before beginning study at Glasgow University in 1834 at the age of 10, not out of any precociousness; the University provided many of the facilities of an elementary school for able pupils, and this was a typical starting age.

In school, Thomson showed a keen interest in the classics along with his natural interest in the sciences. At the age of 12 he won a prize for translating Lucian of Samosata's Dialogues of the Gods from Latin to English.

In the academic year 1839 - 1840, Thomson won the class prize in astronomy for his Essay on the figure of the Earth which showed an early facility for mathematical analysis and creativity. Throughout his life, he would work on the problems raised in the essay as a coping strategy during times of personal stress. On the title page of this essay Thomson wrote the following lines from Alexander Pope's Essay on Man. These lines inspired Thomson to understand the natural world using the power and method of science:

Go, wondrous creature! mount where Science guides;
Go measure earth, weigh air, and state the tides;
Instruct the planets in what orbs to run,
Correct old Time, and regulate the sun;

Thomson became intrigued with Fourier's Théorie analytique de la chaleur and committed himself to study the "Continental" mathematics resisted by a British establishment still working in the shadow of Sir Isaac Newton. Unsurprisingly, Fourier's work had been attacked by domestic mathematicians, Philip Kelland authoring a critical book. The book motivated Thomson to write his first published scientific paper under the pseudonym P.Q.R., defending Fourier, and submitted to the Cambridge Mathematical Journal by his father. A second P.Q.R paper followed almost immediately.

While holidaying with his family in Lamlash in 1841, he wrote a third, more substantial, P.Q.R. paper On the uniform motion of heat in homogeneous solid bodies, and its connection with the mathematical theory of electricity. In the paper he made remarkable connections between the mathematical theories of heat conduction and electrostatics, an analogy that James Clerk Maxwell was ultimately to describe as one of the most valuable science - forming ideas.

William's father was able to make a generous provision for his favorite son's education and, in 1841, installed him, with extensive letters of introduction and ample accommodation, at Peterhouse, Cambridge. In 1845 Thomson graduated as Second Wrangler. He also won a Smith's Prize, which, unlike the tripos, is a test of original research. Robert Leslie Ellis, one of the examiners, is said to have declared to another examiner You and I are just about fit to mend his pens.

While at Cambridge, Thomson was active in sports, athletics and sculling, winning the Colquhoun Sculls in 1843. He also took a lively interest in the classics, music and literature; but the real love of his intellectual life was the pursuit of science. The study of mathematics, physics, and in particular, of electricity, had captivated his imagination.

In 1845 he gave the first mathematical development of Faraday's idea that electric induction takes place through an intervening medium, or "dielectric", and not by some incomprehensible "action at a distance". He also devised a hypothesis of electrical images, which became a powerful agent in solving problems of electrostatics, or the science which deals with the forces of electricity at rest. It was partly in response to his encouragement that Faraday undertook the research in September 1845 that led to the discovery of the Faraday effect, which established that light and magnetic (and thus electric) phenomena were related.

He was elected a fellow of St. Peter's (as Peterhouse was often called at the time) in June 1845. On gaining the fellowship, he spent some time in the laboratory of the celebrated Henri Victor Regnault, at Paris; but in 1846 he was appointed to the chair of natural philosophy in the University of Glasgow. At twenty - two he found himself wearing the gown of a learned professor in one of the oldest Universities in the country, and lecturing to the class of which he was a freshman but a few years before.

By 1847, Thomson had already gained a reputation as a precocious and maverick scientist when he attended the British Association for the Advancement of Science annual meeting in Oxford. At that meeting, he heard James Prescott Joule making yet another of his, so far, ineffective attempts to discredit the caloric theory of heat and the theory of the heat engine built upon it by Sadi Carnot and Émile Clapeyron. Joule argued for the mutual convertibility of heat and mechanical work and for their mechanical equivalence.

Thomson was intrigued but skeptical. Though he felt that Joule's results demanded theoretical explanation, he retreated into an even deeper commitment to the Carnot – Clapeyron school. He predicted that the melting point of ice must fall with pressure, otherwise its expansion on freezing could be exploited in a perpetuum mobile. Experimental confirmation in his laboratory did much to bolster his beliefs.

In 1848, he extended the Carnot – Clapeyron theory still further through his dissatisfaction that the gas thermometer provided only an operational definition of temperature. He proposed an absolute temperature scale in which a unit of heat descending from a body A at the temperature T° of this scale, to a body B at the temperature (T−1)°, would give out the same mechanical effect [work], whatever be the number T. Such a scale would be quite independent of the physical properties of any specific substance. By employing such a "waterfall", Thomson postulated that a point would be reached at which no further heat (caloric) could be transferred, the point of absolute zero about which Guillaume Amontons had speculated in 1702. Thomson used data published by Regnault to calibrate his scale against established measurements.

In his publication, Thomson wrote:

... The conversion of heat (or caloric) into mechanical effect is probably impossible, certainly undiscovered

But a footnote signaled his first doubts about the caloric theory, referring to Joule's very remarkable discoveries. Surprisingly, Thomson did not send Joule a copy of his paper, but when Joule eventually read it he wrote to Thomson on 6 October, claiming that his studies had demonstrated conversion of heat into work but that he was planning further experiments. Thomson replied on 27 October, revealing that he was planning his own experiments and hoping for a reconciliation of their two views.

Thomson returned to critique Carnot's original publication and read his analysis to the Royal Society of Edinburgh in January 1849, still convinced that the theory was fundamentally sound. However, though Thomson conducted no new experiments, over the next two years he became increasingly dissatisfied with Carnot's theory and convinced of Joule's. In February 1851 he sat down to articulate his new thinking. However, he was uncertain of how to frame his theory and the paper went through several drafts before he settled on an attempt to reconcile Carnot and Joule. During his rewriting, he seems to have considered ideas that would subsequently give rise to the second law of thermodynamics. In Carnot's theory, lost heat was absolutely lost but Thomson contended that it was "lost to man irrecoverably; but not lost in the material world". Moreover, his theological beliefs led to speculation about the heat death of the universe.

I believe the tendency in the material world is for motion to become diffused, and that as a whole the reverse of concentration is gradually going on – I believe that no physical action can ever restore the heat emitted from the Sun, and that this source is not inexhaustible; also that the motions of the Earth and other planets are losing vis viva which is converted into heat; and that although some vis viva may be restored for instance to the earth by heat received from the sun, or by other means, that the loss cannot be precisely compensated and I think it probable that it is under compensated.

Compensation would require a creative act or an act possessing similar power.

In final publication, Thomson retreated from a radical departure and declared "the whole theory of the motive power of heat is founded on ... two ... propositions, due respectively to Joule, and to Carnot and Clausius." Thomson went on to state a form of the second law:

It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects.

In the paper, Thomson supported the theory that heat was a form of motion but admitted that he had been influenced only by the thought of Sir Humphry Davy and the experiments of Joule and Julius Robert von Mayer, maintaining that experimental demonstration of the conversion of heat into work was still outstanding.

As soon as Joule read the paper he wrote to Thomson with his comments and questions. Thus began a fruitful, though largely epistolary, collaboration between the two men, Joule conducting experiments, Thomson analyzing the results and suggesting further experiments. The collaboration lasted from 1852 to 1856, its discoveries including the Joule – Thomson effect, sometimes called the Kelvin – Joule effect, and the published results did much to bring about general acceptance of Joule's work and the kinetic theory.

Thomson published more than 650 scientific papers and applied for 70 patents (not all were issued). Regarding science, Thomson wrote the following.

In physical science a first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about and express it in numbers you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.

Though now eminent in the academic field, Thomson was obscure to the general public. In September 1852, he married childhood sweetheart Margaret Crum, daughter of Walter Crum; but her health broke down on their honeymoon and, over the next seventeen years, Thomson was distracted by her suffering. On 16 October 1854, George Gabriel Stokes wrote to Thomson to try to re-interest him in work by asking his opinion on some experiments of Michael Faraday on the proposed transatlantic telegraph cable.

Faraday had demonstrated how the construction of a cable would limit the rate at which messages could be sent – in modern terms, the bandwidth. Thomson jumped at the problem and published his response that month. He expressed his results in terms of the data rate that could be achieved and the economic consequences in terms of the potential revenue of the transatlantic undertaking. In a further 1855 analysis, Thomson stressed the impact that the design of the cable would have on its profitability.

Thomson contended that the speed of a signal through a given core was inversely proportional to the square of the length of the core. Thomson's results were disputed at a meeting of the British Association in 1856 by Wildman Whitehouse, the electrician of the Atlantic Telegraph Company. Whitehouse had possibly misinterpreted the results of his own experiments but was doubtless feeling financial pressure as plans for the cable were already well underway. He believed that Thomson's calculations implied that the cable must be "abandoned as being practically and commercially impossible."

Thomson attacked Whitehouse's contention in a letter to the popular Athenaeum magazine, pitching himself into the public eye. Thomson recommended a larger conductor with a larger cross section of insulation. However, he thought Whitehouse no fool and suspected that he might have the practical skill to make the existing design work. Thomson's work had, however, caught the eye of the project's undertakers and in December 1856, he was elected to the board of directors of the Atlantic Telegraph Company.

Thomson became scientific adviser to a team with Whitehouse as chief electrician and Sir Charles Tilston Bright as chief engineer but Whitehouse had his way with the specification, supported by Faraday and Samuel F. B. Morse.

Thomson sailed on board the cable laying ship HMS Agamemnon in August 1857, with Whitehouse confined to land owing to illness, but the voyage ended after 380 miles (610 km) when the cable parted. Thomson contributed to the effort by publishing in the Engineer the whole theory of the stresses involved in the laying of a submarine cable, and showed that when the line is running out of the ship, at a constant speed, in a uniform depth of water, it sinks in a slant or straight incline from the point where it enters the water to that where it touches the bottom.

Thomson developed a complete system for operating a submarine telegraph that was capable of sending a character every 3.5 seconds. He patented the key elements of his system, the mirror galvanometer and the siphon recorder, in 1858.

Whitehouse still felt able to ignore Thomson's many suggestions and proposals. It was not until Thomson convinced the board that using purer copper for replacing the lost section of cable would improve data capacity, that he first made a difference to the execution of the project.

The board insisted that Thomson join the 1858 cable laying expedition, without any financial compensation, and take an active part in the project. In return, Thomson secured a trial for his mirror galvanometer, about which the board had been unenthusiastic, alongside Whitehouse's equipment. However, Thomson found the access he was given unsatisfactory and the Agamemnon had to return home following the disastrous storm of June 1858. Back in London, the board was on the point of abandoning the project and mitigating their losses by selling the cable. Thomson, Cyrus West Field and Curtis M. Lampson argued for another attempt and prevailed, Thomson insisting that the technical problems were tractable. Though employed in an advisory capacity, Thomson had, during the voyages, developed real engineer's instincts and skill at practical problem solving under pressure, often taking the lead in dealing with emergencies and being unafraid to lend a hand in manual work. A cable was finally completed on 5 August.

Thomson's fears were realized when Whitehouse's apparatus proved insufficiently sensitive and had to be replaced by Thomson's mirror galvanometer. Whitehouse continued to maintain that it was his equipment that was providing the service and started to engage in desperate measures to remedy some of the problems. He succeeded only in fatally damaging the cable by applying 2,000 V. When the cable failed completely Whitehouse was dismissed, though Thomson objected and was reprimanded by the board for his interference. Thomson subsequently regretted that he had acquiesced too readily to many of Whitehouse's proposals and had not challenged him with sufficient energy.

A joint committee of inquiry was established by the Board of Trade and the Atlantic Telegraph Company. Most of the blame for the cable's failure was found to rest with Whitehouse. The committee found that, though underwater cables were notorious in their lack of reliability, most of the problems arose from known and avoidable causes. Thomson was appointed one of a five member committee to recommend a specification for a new cable. The committee reported in October 1863.

In July 1865 Thomson sailed on the cable laying expedition of the SS Great Eastern but the voyage was again dogged with technical problems. The cable was lost after 1,200 miles (1,900 km) had been laid and the expedition had to be abandoned. A further expedition in 1866 managed to lay a new cable in two weeks and then go on to recover and complete the 1865 cable. The enterprise was now feted as a triumph by the public and Thomson enjoyed a large share of the adulation. Thomson, along with the other principals of the project, was knighted on 10 November 1866.

To exploit his inventions for signalling on long submarine cables, Thomson now entered into a partnership with C.F. Varley and Fleeming Jenkin. In conjunction with the latter, he also devised an automatic curb sender, a kind of telegraph key for sending messages on a cable.

Thomson took part in the laying of the French Atlantic submarine communications cable of 1869, and with Jenkin was engineer of the Western and Brazilian and Platino - Brazilian cables, assisted by vacation student James Alfred Ewing. He was present at the laying of the Pará to Pernambuco section of the Brazilian coast cables in 1873.

Thomson's wife had died on 17 June 1870 and he resolved to make changes in his life. Already addicted to seafaring, in September he purchased a 126 ton schooner, the Lalla Rookh and used it as a base for entertaining friends and scientific colleagues. His maritime interests continued in 1871 when he was appointed to the board of inquiry into the sinking of the HMS Captain.

In June 1873, Thomson and Jenkin were on board the Hooper, bound for Lisbon with 2,500 miles (4,020 km) of cable when the cable developed a fault. An unscheduled 16 day stop over in Madeira followed and Thomson became good friends with Charles R. Blandy and his three daughters. On 2 May 1874 he set sail for Madeira on the Lalla Rookh. As he approached the harbor, he signaled to the Blandy residence "Will you marry me?" and Fanny signaled back "Yes". Thomson married Fanny, 13 years his junior, on 24 June 1874.

Over the period 1855 to 1867, Thomson collaborated with Peter Guthrie Tait on a text book that unified the various branches of physical science under the common principle of energy. Published in 1867, the Treatise on Natural Philosophy did much to define the modern discipline of physics.

Between 1870 and 1890 a theory purporting that an atom was a vortex in the ether was immensely popular among British physicists and mathematicians. About 60 scientific papers were written by around 25 scientists. Following the lead of Thomson and Tait, they developed a mathematical theory of knots which lives on to this day. The "Vortex Theory" was killed by the Michelson - Morley experiment and is of interest today mainly to historians of science.

Thomson was an enthusiastic yachtsman, his interest in all things relating to the sea perhaps arising, or at any rate fostered, from his experiences on the Agamemnon and the Great Eastern.

Thomson introduced a method of deep sea sounding, in which a steel piano wire replaces the ordinary land line. The wire glides so easily to the bottom that "flying soundings" can be taken while the ship is going at full speed. A pressure gauge to register the depth of the sinker was added by Thomson.

About the same time he revived the Sumner method of finding a ship's place at sea, and calculated a set of tables for its ready application. He also developed a tide predicting machine.

During the 1880s, Thomson worked to perfect the adjustable compass in order to correct errors arising from magnetic deviation owing to the increasing use of iron in naval architecture. Thomson's design was a great improvement on the older instruments, being steadier and less hampered by friction, the deviation due to the ship's own magnetism being corrected by movable masses of iron at the binnacle. Thomson's innovations involved much detailed work to develop principles already identified by George Biddell Airy and others but contributed little in terms of novel physical thinking. Thomson's energetic lobbying and networking proved effective in gaining acceptance of his instrument by The Admiralty.

Scientific biographers of Thomson, if they have paid any attention at all to his compass innovations, have generally taken the matter to be a sorry saga of dim - witted naval administrators resisting marvelous innovations from a superlative scientific mind. Writers sympathetic to the Navy, on the other hand, portray Thomson as a man of undoubted talent and enthusiasm, with some genuine knowledge of the sea, who managed to parlay a handful of modest ideas in compass design into a commercial monopoly for his own manufacturing concern, using his reputation as a bludgeon in the law courts to beat down even small claims of originality from others, and persuading the Admiralty and the law to overlook both the deficiencies of his own design and the virtues of his competitors'.
The truth, inevitably, seems to lie somewhere between the two extremes.

Charles Babbage had been among the first to suggest that a lighthouse might be made to signal a distinctive number by occultations of its light but Thomson pointed out the merits of the Morse code for the purpose, and urged that the signals should consist of short and long flashes of the light to represent the dots and dashes.

Thomson did more than any other electrician up to his time in introducing accurate methods and apparatus for measuring electricity. As early as 1845 he pointed out that the experimental results of William Snow Harris were in accordance with the laws of Coulomb. In the Memoirs of the Roman Academy of Sciences for 1857 he published a description of his new divided ring electrometer, based on the old electroscope of Johann Gottlieb Friedrich von Bohnenberger and he introduced a chain or series of effective instruments, including the quadrant electrometer, which cover the entire field of electrostatic measurement. He invented the current balance, also known as the Kelvin balance or Ampere balance (SiC), for the precise specification of the ampere, the standard unit of electric current.

In 1893, Thomson headed an international commission to decide on the design of the Niagara Falls power station. Despite his previous belief in the superiority of direct current electric power transmission, he was convinced by Nikola Tesla's demonstration of three phase alternating current power transmission at the Chicago World's Fair of that year and agreed to use Tesla's system. In 1896, Thomson said "Tesla has contributed more to electrical science than any man up to his time."

Thomson remained a devout believer in Christianity throughout his life; attendance at chapel was part of his daily routine. He saw his Christian faith as supporting and informing his scientific work, as is evident from his address to the annual meeting of the Christian Evidence Society, 23 May 1889.

One of the clearest instances of this interaction is in his estimate of the age of the Earth. Given his youthful work on the figure of the Earth and his interest in heat conduction, it is no surprise that he chose to investigate the Earth's cooling and to make historical inferences of the Earth's age from his calculations. Thomson was a creationist in a broad sense, but he was not a 'flood geologist'. He contended that the laws of thermodynamics operated from the birth of the universe and envisaged a dynamic process that saw the organization and evolution of the solar system and other structures, followed by a gradual "heat death". He developed the view that the Earth had once been too hot to support life and contrasted this view with that of uniformitarianism, that conditions had remained constant since the indefinite past. He contended that "This earth, certainly a moderate number of millions of years ago, was a red-hot globe ... ."

After the publication of Charles Darwin's On the Origin of Species in 1859, Thomson saw evidence of the relatively short habitable age of the Earth as tending to contradict Darwin's gradualist explanation of slow natural selection bringing about biological diversity. Thomson's own views favored a version of theistic evolution sped up by divine guidance. His calculations showed that the sun could not have possibly existed long enough to allow the slow incremental development by evolution – unless some energy source beyond what he or any other Victorian era person knew of was found. He was soon drawn into public disagreement with geologists, and with Darwin's supporters John Tyndall and T.H. Huxley. In his response to Huxley’s address to the Geological Society of London (1868) he presented his address "Of Geological Dynamics" (1869), which, among his other writings, challenged the geologists' acceptance that the earth must be of indefinite age.

Thomson’s initial 1864 estimate of the Earth’s age was from 20 to 400 million years old. These wide limits were due to his uncertainty about the melting temperature of rock, to which he equated the earth’s interior temperature. Over the years he refined his arguments and reduced the upper bound by a factor of ten, and in 1897 Thomson, now Lord Kelvin, ultimately settled on an estimate that the Earth was 20 – 40 million years old. His exploration of this estimate can be found in his 1897 address to the Victoria Institute, given at the request of the Institute's president George Stokes, as recorded in that Institute's journal Transactions. Although his former assistant John Perry published a paper in 1895 challenging Kelvin's assumption of low thermal conductivity inside the Earth, and thus showing a much greater age, this had little immediate impact. The discovery in 1903 that radioactive decay releases heat led to Kelvin's estimate being challenged, and Ernest Rutherford famously made the argument in a lecture attended by Kelvin that this provided the unknown energy source Kelvin had suggested, but the estimate was not overturned until the development in 1907 of radiometric dating of rocks.

It was widely believed that the discovery of radioactivity had invalidated Thomson's estimate of the age of the Earth. Thomson himself never publicly acknowledged this because he had a much stronger argument restricting the age of the Sun to no more than 20 million years. Without sunlight, there could be no explanation for the sediment record on the Earth's surface. At the time, the only known source for the solar power output was gravitational collapse. It was only when thermonuclear fusion was recognized in the 1930s that Thomson's age paradox was truly resolved.

In the winter of 1860 – 1861 Kelvin slipped on some ice and fractured his leg, causing him to limp thereafter. He remained something of a celebrity on both sides of the Atlantic until his death.

Lord Kelvin was an Elder of St Columba's Parish Church (Church of Scotland) in Largs for many years. It was to that church that his remains were taken after his death in 1907. Following the funeral service there, the body was taken to Bute Hall in his beloved University of Glasgow for a service of remembrance before the body was taken to London for interment at Westminster Abbey, close by the final resting place of Sir Isaac Newton.

In 1884, Thomson delivered a series of lectures at Johns Hopkins University in the United States in which he attempted to formulate a physical model for the aether, a medium that would support the electromagnetic waves that were becoming increasingly important to the explanation of radiative phenomena. Imaginative as they were, the "Baltimore lectures" had little enduring value owing to the imminent demise of the mechanical world view.

In 1900, he gave a lecture titled Nineteenth - Century Clouds over the Dynamical Theory of Heat and Light. The two "dark clouds" he was alluding to were the unsatisfactory explanations that the physics of the time could give for two phenomena: the Michelson – Morley experiment and black body radiation. Two major physical theories were developed during the twentieth century starting from these issues: for the former, the Theory of relativity; for the second, quantum mechanics. Albert Einstein, in 1905, published the so-called "Annus Mirabilis Papers", one of which explained the photoelectric effect and was a foundation paper of quantum mechanics, another of which described special relativity.

Like many scientists, he did make some mistakes in predicting the future of technology.

Circa 1896, Lord Kelvin was initially skeptical of X-rays, and regarded their announcement as a hoax. However, this was before he saw Röntgen's evidence, after which he accepted the idea, and even had his own hand X-rayed in May 1896.

His forecast for practical aviation was negative. In 1896 he refused an invitation to join the Aeronautical Society, writing that "I have not the smallest molecule of faith in aerial navigation other than ballooning or of expectation of good results from any of the trials we hear of." And in a 1902 newspaper interview he predicted that "No balloon and no aeroplane will ever be practically successful."

The statement "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement" is given in a number of sources, but without citation. It is reputed to be Kelvin's remark made in an address to the British Association for the Advancement of Science (1900). It is often found quoted without any footnote giving the source. However, another author reports in a footnote that his search to document the quote failed to find any direct evidence supporting it. Very similar statements have been attributed to other physicists contemporary to Kelvin.

In 1898, Kelvin predicted that only 400 years of oxygen supply remained on the planet, due to the rate of burning combustibles. In his calculation, Kelvin assumed that photosynthesis was the only source of free oxygen; he did not know all of the components of the oxygen cycle. He could not even have known all of the sources of photosynthesis: for example the cyanobacterium Prochlorococcus — which accounts for more than half of marine photosynthesis — was not discovered until 1986.