The Invention of Nature

14

Going in Circles

Maladie Centrifuge

ON 14 SEPTEMBER 1818, the day of his forty-ninth birthday, Humboldt boarded the stagecoach in Paris to travel once again to London – his third visit in only four years. Five days later, he arrived in the middle of the night at Wilhelm’s house in Portland Place. By now he was so famous that the London papers announced his visit in the column ‘Fashionable Arrivals’. He was still trying to organize his expedition to India, and Wilhelm’s diplomatic status in London helped to open some important doors. Wilhelm, for example, facilitated a private audience with the Prince Regent who assured Alexander of his support for the venture. Humboldt also met the British government official who oversaw the activities of the East India Company – George Canning, the president of the Board of Control, who pledged help. After these meetings Humboldt was certain that any hurdles that the East India Company could ‘place in my way’ would be removed. After more than a decade of cajoling and pleading, India finally seemed to be within his reach. Convinced that the directors would give their permission, Humboldt now turned his attention to King Friedrich Wilhelm III who had mentioned in the past that he might be willing to finance the voyage.

At the time of Humboldt’s London visit, the Prussian king was conveniently at the Congress of Aix-la-Chapelle, today’s Aachen in Germany. On 1 October 1818 the four Allied powers – Prussia, Austria, Britain and Russia – had convened in Aachen to discuss the withdrawal of their troops from France as well as a future European alliance. With Aachen only 200 miles east of Calais, travelling directly there from London would save Humboldt a dreaded visit to Berlin – a city he had not visited for eleven years – and around 1,000 miles of unnecessary travelling.

On 8 October, less than three weeks after his arrival in London, Humboldt was on his way again, but trailed by rumours. There were reports in British newspapers that Humboldt was rushing to the congress in Aachen to ‘be consulted on the affairs of South America’. The French secret police had similar suspicions, believing that Humboldt carried a detailed report about the rebelling colonies. A Spanish minister had also been dispatched to Aachen in the hope of securing European support for Spain in its battle against Simón Bolívar’s army. But by the time Humboldt arrived, it had become clear that the Allies had no interest in meddling with Spanish colonial ambitions – the balance of power in post-Napoleonic Europe was a much more pressing concern. Instead Humboldt could focus on what The Times called his ‘own affair’ – extracting money from the Prussians for his expedition to India.

In Aachen Humboldt informed the Prussian Chancellor, Karl August von Hardenberg, that the difficulties regarding his expedition had been almost entirely removed. The only hindrance for the ‘complete guarantee of my enterprise’, Humboldt claimed, was financial. Within twenty-four hours Friedrich Wilhelm III had granted Humboldt the money. Humboldt was ecstatic. After fourteen years in Europe, he would finally be able to leave. He would be able to climb the mighty Himalaya and extend his Naturgemälde across the globe.

When Humboldt returned to Paris from Aachen, he began his preparations in earnest. He bought books and instruments, corresponded with people who had travelled to Asia and worked on his exact route. He would first visit Constantinople, and then the snow-capped dormant volcano Mount Ararat near today’s border between Iran and Turkey. From there he would go south, travelling overland across the whole of Persia to Bandar Abbas on the Persian Gulf from where he would sail to India. He was taking language lessons in Persian and Arabic, and one wall of his bedroom in his small Parisian apartment was covered with a huge map of Asia. But, as always, everything took longer than Humboldt had initially thought.

He had still not published the full results from his Latin American exploration. Together all the books would eventually become the thirty-four-volume Voyage to the Equinoctial Regions of the New Continent – it included the multi-volume travel account Personal Narrative but also more specialized books on botany, zoology and astronomy. Some, such as Personal Narrative and the Political Essay on the Kingdom of New Spain, had few or no illustrations and were affordable for a wider audience while others, such as Vues des Cordillères with its stunning depictions of Latin America’s landscapes and monuments, were large volumes that cost a fortune. In its entirety Voyage to the Equinoctial Regions of the New Continent would become the most expensive work ever privately published by a scientist. Humboldt had employed mapmakers, artists, engravers and botanists for years now and the expenses were so high that they ruined him financially. He still had his income from the Prussian king and from his book sales but had to live frugally. His inheritance had been completely used up. He had spent 50,000 thalers on his expedition and about double that on his publications and life in Paris.

None of this stopped Humboldt. He received loans from friends and banks, and mostly chose to ignore his financial situation, his debt growing steadily.

While he was working on his books, Humboldt continued his preparations for India. He dispatched Karl Sigismund Kunth to Switzerland, the nephew of his old childhood teacher Gottlob Johann Christian Kunth and the botanist who had taken over the botanical publications when Bonpland had slowed down too much. The plan was that Kunth was going to accompany Humboldt to India but was first to examine plants in the Alps, so that he could compare them to those on Mount Ararat and in the Himalaya. Humboldt’s old travel companion, Aimé Bonpland, was no longer available. When Joséphine Bonaparte had died in May 1814, Bonpland had stopped working in her garden at Malmaison. Bored with his life in Paris – ‘my whole existence is too predictable,’ Bonpland had written to his sister – he had been keen to embark on new adventures but had become impatient with Humboldt’s delayed travel plans.

Bonpland had always wanted to return to South America. He travelled to London to meet Simón Bolívar’s men and other revolutionaries who had come to Britain in order to rally support for their fight against Spain. Bonpland had even supplied them with books and a printing press, as well as smuggling weapons. Soon the South Americans were competing for Bonpland’s services. Francisco Antonio Zea, the botanist who would become Vice-President of Colombia under Bolívar, had asked Bonpland to continue the work of the deceased botanist José Celestino Mutis in Bogotá. At the same time the representatives from Buenos Aires hoped Bonpland would establish a botanic garden there. Bonpland’s knowledge of potentially useful plants held economic possibilities for the new nations. Just as the British had founded a botanical garden in Calcutta as a storehouse for the empire and for useful crops, so was the Argentinians’ plan. Bonpland was to help them to introduce ‘new methods of practical agriculture’ from Europe.

The revolutionaries were trying to lure European scientists to Latin America. Science was like a nation without borders, it united people and – so they hoped – would place an independent Latin America on an equal footing with Europe. When Zea was appointed as Colombia’s Plenipotentiary Minister to Britain, he received instructions not only to obtain support for their political struggle but also to promote the immigration of scientists, craftsmen and farmers. ‘The illustrious Franklin obtained more good in France for his country through the natural sciences than through all the diplomatic efforts,’ Zea was reminded by his superiors.

The prospect of Bonpland emigrating had been particularly exciting for the revolutionaries, given his extensive knowledge of Latin America. Everybody was ‘impatiently waiting for you’, one of them had told Bonpland. In spring 1815, as royalist troops were regaining much of the territory along the Río Magdalena in New Granada and the revolutionary army was decimated by desertion and disease, Bolívar himself had found time to write and offer Mutis’s position in Bogotá to Bonpland. But in the end Bonpland had been too worried about the brutal civil war that had been raging in New Granada and Venezuela. Instead he had left France at the end of 1816 for Buenos Aires.

Twelve years after he had left South America with Humboldt, Bonpland was sailing back – this time loaded with fruit tree saplings, vegetable seeds, grapes and medicinal plants to start a new life. After a couple of years in Buenos Aires, though, Bonpland had had enough of city life. He had never enjoyed the orderly work of a studious scholar. He was a field botanist who loved finding rare plants but was useless when it came to sorting them. Over the years he assembled 20,000 dried plants but his herbarium was a complete mess with specimens piled into boxes, loosely bound together and not even mounted on paper. In 1820 Bonpland settled in Santa Ana on the Paraná River in Argentina near the border with Paraguay where he collected plants and grew yerba mate – leaves that were brewed like tea and a popular drink in South America.

On 25 November 1821, exactly five years after Bonpland had left France for Argentina, Humboldt wrote to him, sending some money but also complaining that he hadn’t heard from his ‘old companion-in-fortune’. Bonpland never received the letter. On 8 December 1821, two weeks after Humboldt had posted his letter, 400 Paraguayan soldiers crossed the border into Argentina and stormed Bonpland’s farm in Santa Ana. On the orders of José Gaspar Rodríguez de Francia, the dictator of Paraguay, the men killed Bonpland’s workers and put him in chains. Francia accused Bonpland of agricultural espionage and feared that his flourishing plantation would be in competition with the Paraguayan yerba mate trade. Bonpland was dragged into Paraguay where he was imprisoned.

Old friends tried to help. Bolívar, who was by then in Lima attempting to purge the Spanish from Peru, wrote to Francia, requesting Bonpland’s freedom as well as threatening to march to Paraguay to rescue him. Francia could count on him as an ally, Bolívar said, but only if the ‘innocent whom I love, will not become the victim of injustice’. Humboldt also did what he could through his European contacts. He dispatched letters to Paraguay signed by famous scientists and asked his old London acquaintance George Canning (who by now was the Foreign Secretary) to involve the British consul in Buenos Aires – but Francia refused to release Bonpland.

Meanwhile Humboldt’s own travel plans had come to a standstill. Despite the support of the Prince Regent and of George Canning, the East India Company continued to refuse Humboldt entrance to India. It felt as if he had been going round in circles in the past few years. Whereas his years in Latin America, and those just after, had been marked by breathless activity and a constant forward trajectory, Humboldt now felt choked by stagnation. He was no longer the dashing, heroic young explorer who was celebrated for his adventures but a distinguished and respected scientist in his fifties. Most of his middle-aged contemporaries would have been glad to be admired and courted for their knowledge, but Humboldt was not ready to settle. There was still so much to do. He was so fretful that one friend called his restlessness a ‘maladie centrifuge’ – Humboldt’s centrifugal illness.

Frustrated, annoyed and upset, Humboldt felt cheated and unappreciated. He now announced that he would turn his back on Europe. He would move to Mexico where he planned to establish an institute for the sciences. In Mexico, he would surround himself with scholarly men, he told his brother in October 1822, and enjoy the ‘liberty of thought’. At least there, he was ‘greatly respected’. He was absolutely certain that he would spend the rest of his life outside Europe. A few years later, Humboldt told Bolívar that he still planned to move to Latin America. No one really knew what Humboldt wanted or where he intended to go. Wilhelm summed it up when he said: ‘Alexander always envisages things as being huge, and then not even half of it happens.’

The East India Company might have been uncooperative but it seemed that everybody else in Britain was enthusiastic about Humboldt. Many of the British scientists whom he had met in London now visited him in Paris. The famous chemist Humphry Davy came again, as did John Herschel, the son of astronomer William Herschel, and Charles Babbage, the mathematician hailed today as the father of the computer. Humboldt ‘derived pleasure from assisting’, Babbage said, no matter how famous or unknown the caller. Oxford geologist William Buckland was equally excited to meet Humboldt in Paris. Never had he heard a man talk faster or with more brilliance, Buckland wrote to a friend. As always, Humboldt was generous with his knowledge and collections, opening his cabinet and notebooks to Buckland.

One of the most significant scientific encounters was with Charles Lyell, the British geologist whose work would help Charles Darwin shape his ideas about evolution. Fascinated by the formation of the earth, Lyell had travelled across Europe in the early 1820s to investigate mountains, volcanoes and other geological formations for his revolutionary work, Principles of Geology. Then, in the summer of 1823, around the same time as news of Bonpland’s imprisonment had reached Bolívar, an enthusiastic twenty-five-year-old Lyell went to Paris with his bags full of introductory letters to Humboldt.

Since his return from Latin America, one of Humboldt’s projects had been to collect and compare data on rock strata across the globe. After almost two decades he had finally published the results in his Geognostical Essay on the Superposition of Rocks, just a few months before Lyell reached Paris. This was exactly the kind of information Lyell needed for his own research. The Geognostical Essay, Lyell wrote, was ‘a famous lesson to me’. It would have placed Humboldt in the highest ranks of the science world, he believed, even if he published nothing else. During the next two months, the two men spent many afternoons together, talking about geology, Humboldt’s observations at Mount Vesuvius and mutual friends in Britain. Humboldt’s English was excellent, Lyell noted. ‘Hoombowl’, Lyell wrote to his father – the way Humboldt’s French servant pronounced his name – gave him plenty of material and useful data.

They also discussed Humboldt’s invention of isotherms, the lines that we see on weather maps today and which connect different geographical points around the globe that are experiencing the same temperatures.¹ Humboldt had come up with the design for his essay On the Isothermal Lines and the Distribution of Heat on the Earth (1817) in order to visualize global climate patterns. The essay would help Lyell to form his own theories, and also marked the beginning of a new understanding of climate – one on which all subsequent studies about the distribution of heat were based.

Until Humboldt’s isotherms, meteorological data had been collected in long tables of temperatures – endless lists of different geographical places and their climatic conditions which gave precise temperatures but were difficult to compare. Humboldt’s graphic visualization of the same data was as innovative as it was simple. Instead of confusing tables, one look at his isotherm map revealed a new world of patterns that hugged the earth in wavy belts. Humboldt believed that this was the foundation of what he called ‘vergleichende Klimatologie’ – comparative climatology. He was right, for today’s scientists still use them to understand and depict climate change and global warming. Isotherms enabled Humboldt, and those who followed, to look at patterns globally. Lyell utilized the concept to investigate geological changes in relation to climatic changes.

Map showing isotherms (Illustration Credit 14.1)

The central argument of Lyell’s Principles of Geology was that the earth had been shaped gradually by minute changes rather than by sudden catastrophic occurrences such as earthquakes or floods as other scientists thought. Lyell came to believe that these slow forces were still active in the present day which meant that he had to look at the current conditions in order to learn about the past. To argue his case for the influence of gradual forces, and to move scientific thinking away from the more apocalyptical theories of the earth’s beginning, Lyell had to explain how the surface of the planet had cooled gradually. He ‘read up’ on Humboldt, Lyell later told a friend, while working on his own theory.

Humboldt’s detailed analysis came to the surprising conclusion that temperatures were not the same along the same latitude as had been previously assumed. Altitude, landmass, proximity to oceans and winds also influenced heat distribution. Temperatures were higher on land than on sea, but also lower at higher elevations. This meant, Lyell concluded, that where geological forces had elevated the land, temperatures dropped accordingly. In the long term, he argued, this upward drift brought a cooling effect to the world climate – as the earth changed geologically, so did the climate. Years later, when pressed by a reviewer of Principles of Geology to define the moment of ‘a beginning’ of his theories, Lyell said it had been the reading of Humboldt’s essay on isotherms – ‘give Humboldt due credit for his beautiful essay’. In his own work, Lyell said, he had only given Humboldt’s climate theories a ‘geological application’.

Humboldt helped young scientists whenever he could, intellectually but also financially, no matter how difficult his own situation. So much so that his sister-in-law, Caroline, worried that his so-called friends exploited his kindness – ‘he eats dry bread, so that they can eat meat.’ But Humboldt didn’t seem to care. He was the hub of a spinning wheel, forever moving and connecting.

He wrote to Simón Bolívar to recommend a young French scientist who planned to travel through South America, as well as equipping the scientist with his own instruments. Similarly, Humboldt introduced a Portuguese botanist who intended to emigrate to the United States to Thomas Jefferson. The German chemist Justus von Liebig, who would later become famous for his discovery of the importance of nitrogen as a plant nutrient, recounted how meeting Humboldt in Paris had ‘laid the foundation of my future career’. Even Albert Gallatin, the former US Secretary of the Treasury, who had first met Humboldt in Washington and then again in London and Paris, found himself so inspired by Humboldt’s enthusiasm for indigenous people that he threw himself into studies of Native Americans in the United States. Today Gallatin is regarded as the founder of American ethnology; the reason for his interest, Gallatin wrote, was ‘the request of a distinguished friend, Baron Alexander von Humboldt’.

As Humboldt helped friends and fellow scientists to advance their careers and travels, his own chances of being allowed to enter India had dwindled to nothing. He fed his wanderlust with trips through Europe – Switzerland, France, Italy and Austria – but it wasn’t the same. He was unhappy. It was also becoming increasingly difficult to justify his decision to live in Paris to the Prussian king. Since Humboldt’s return from Latin America two decades earlier, Friedrich Wilhelm III had repeatedly pressed him to return to Berlin. For twenty years the king had paid him an annual stipend with no strings attached. Humboldt had always argued that he needed Paris’s scientific environment to write his books but the climate in the city and France had changed.

After Napoleon’s removal and imprisonment on the remote island of St Helena in 1815, the Bourbon monarchy had been reinstated with the crowning of Louis XVIII² – the brother of Louis XVI who had been guillotined during the French Revolution. Though absolutism had not returned to France, the country that had held the torch of liberty and equality had become a constitutional monarchy. Only one per cent of the French population was eligible to elect the lower house of parliament. Though Louis XVIII respected some liberal views, he had arrived in France from exile with a train of ultra-royalist émigrés who wanted to return to the old ways of the pre-revolutionary Ancien Régime. Humboldt had watched them coming back and had seen how they burned with hate and a desire for revenge. ‘Their tendency to absolute monarchy is irresistible,’ Charles Lyell had written to his father from Paris.

Then in 1820 the king’s nephew, the Duc de Berry – third in line to the throne – was murdered by a Bonapartist. After that there was no holding back the royalist tide any more. Censorship became harsher, people could be held without trial and the wealthiest people received a double vote. In 1823 the ultra-royalists gained the majority in the lower house of parliament. Humboldt was deeply upset, telling one American visitor that all it took was one look at the Journal des Débâts – a newspaper founded in 1789, during the French Revolution – to see how the freedom of the press had become curtailed. Humboldt was also beginning to feel uncomfortable at the way that religion, with all its constraints on scientific thinking, was reasserting its grip on French society. With the return of the ultra-royalists, the power of the Catholic Church rose. By the mid-1820s new church spires were rising across the Paris skyscape.

Paris was ‘less disposed than ever’ to be a centre for the sciences, Humboldt wrote to a friend in Geneva, as the funds for laboratories, research and teaching were slashed. The spirit of enquiry was stifled as scientists found themselves having to curry favours from the new king. The savants had become ‘pliant tools’ in the hands of politicians and princes, Humboldt told Charles Lyell in 1823, and even the great George Cuvier had sacrificed his genius as a naturalist for a new quest for ‘ribbons, crosses, titles and Court favours’. There was so much political wrangling in Paris that governmental positions seemed to change as quickly as in a game of musical chairs. Every man he met now, Humboldt said, was either a minister or an ex-minister. ‘They are scattered thick as the leaves in autumn,’ he told Lyell, ‘and before one set have time to rot away, they are covered by another and another.’

French scientists feared that Paris was going to lose its status as a centre for innovative scientific thinking. At the Académie des Sciences, Humboldt said, the savants did little and what little they did often ended in quarrels. Even worse, the scholars had formed a secret committee to sanitize the library there – removing books that propounded liberal ideas like those written by Enlightenment thinkers such as Jean-Jacques Rousseau and Voltaire. When the childless Louis XVIII died in September 1824 his brother Charles X, the leader of the ultra-royalists, became king. All those who believed in liberty and in the values of the revolution knew that the intellectual climate could only become more repressive.

Humboldt himself had changed too. Now in his mid-fifties, his brown hair had turned silver-grey and his right arm was almost paralysed by rheumatism – the long-term effect, he explained to friends, of sleeping on wet ground in the rainforest at the Orinoco. His clothes were old-fashioned, tailored in the style of the years just after the French Revolution: fitted striped breeches, a yellow waistcoat, a blue tailcoat, a white cravat, tall boots and a shabby black hat. No one in Paris, a friend remarked, dressed like that any more. Humboldt’s reasons were as political as they were parsimonious. With his inheritance long gone, he lived in a small plain apartment overlooking the Seine, consisting only of a sparsely furnished bedroom and a study. Humboldt had neither the money nor the taste for luxuries, elegant clothes or opulent furniture.

Then, in autumn 1826, after more than two decades, Friedrich Wilhelm III finally ran out of patience. He wrote to Humboldt that ‘you must already have completed the publication of the works, which you believed could only be accomplished satisfactorily in Paris.’ The king could no longer extend permission for him to stay in France – a country that, in any event, ‘ought to be an object of hatred to every true Prussian’. As Humboldt read that the king was now awaiting his ‘speedy return’, there could be no doubt that this was an order.

Humboldt desperately needed the money from his annual stipend because the cost of his publications had left him, he admitted, ‘poor as a church mouse’. He had to live on what he earned but he was useless when it came to his finances. ‘The only thing in heaven or earth that M. Humboldt does not understand,’ his English translator had remarked, ‘is business.’

Paris had been his home for more than twenty years and his closest friends lived there. It was a painful decision but in the end Humboldt agreed to move to Berlin – but only under the condition that he was allowed to travel to Paris regularly for several months at a time to continue his research. It was not easy, he wrote to the German mathematician Carl Friedrich Gauß in February 1827, to give up his freedom and scientific life. Having only recently accused George Cuvier of betraying the revolutionary spirit, Humboldt now became a courtier himself, entering a world in which he would have to negotiate a fine balance between his liberal political beliefs and his royal duties. It would be almost impossible, he feared, to find ‘the middle ground between the oscillating opinions’.

On 14 April 1827 Humboldt left Paris for Berlin but not without one of his usual detours. He travelled via London, in what may have been a last desperate effort to convince the East India Company to grant him permission to explore India. Nine years had passed since his last visit in 1818, when he had stayed with his brother Wilhelm. Since then Wilhelm had been recalled from his diplomatic posting in Britain and now lived in Berlin,³ but Humboldt quickly reconnected with his old British acquaintances. He tried to make the most of his three-week visit.

Humboldt was passed on from one person to another – politicians, scientists and a ‘force of noblemen’. At the Royal Society, Humboldt met his old friends John Herschel and Charles Babbage, and attended a meeting during which one of the fellows presented ten maps that were part of a new atlas of India which had been commissioned by the East India Company – a painful reminder of what Humboldt was missing. He had dinner with Mary Somerville,⁴ one of the few female scientists in Europe, and visited the botanist Robert Brown at the botanic garden at Kew just to the west of London. Brown had explored Australia as one of Joseph Banks’s plant collectors, and Humboldt was keen to learn about Antipodean flora.

Humboldt was also invited to an elegant party at the Royal Academy and dined with his old acquaintance George Canning, who just two weeks previously had become the British Prime Minister. At Canning’s dinner, Humboldt was delighted to meet his old friend from Washington, DC, Albert Gallatin, who was now the American Minister in London. Only the attention of the British aristocracy annoyed Humboldt. Paris was a sleepy town compared to ‘my torments here’, he wrote to a friend, because everybody seemed to want a piece of him. In London ‘every sentence begins’, he complained, with ‘you will not leave without having seen my country-house: it is only 40 miles from London.’

Humboldt’s most exciting day, however, was spent not with scientists or politicians but with a young engineer, Isambard Kingdom Brunel, who had invited Humboldt to observe the construction of the first tunnel under the Thames. The idea of building a tunnel under a river was as daring as it was dangerous, and no one had ever succeeded in doing such a thing.

The conditions at the Thames could not have been worse because the riverbed and the ground beneath consisted of sand and soft clay. Brunel’s father, Marc, had invented an ingenious method of building the tunnel: a cast-iron shield in the height and width of the tunnel tube. Inspired by a shipworm that bored through the toughest timber planks by protecting its head with a shell, Marc Brunel had designed a huge contraption that allowed the excavation of the tunnel while at the same time propping up the ceiling and keeping the soft clay in place. As the workers moved the metal shield in front of them under the riverbed, they built up the tunnel’s brick shell behind them. Inch by inch, and foot by foot, the length of the tunnel slowly grew. Work had begun two years previously and by the time Humboldt came to London Brunel’s men had reached about the halfway point of the 1,200-foot-long tunnel.

The work was treacherous and Marc Brunel’s diary was filled with thoughts of worry and concern: ‘anxiety increasing daily’, ‘things are getting worse every day’, or ‘every morning I say, Another day of danger over.’ His son Isambard, who had been made ‘resident engineer’ in January 1827 at the age of twenty, brought his boundless energy and confidence to the project. But the work was challenging. In early April, shortly before Humboldt arrived, more and more water seeped into the tunnel and Isambard had forty men pumping to keep the influx of water under control. There was only ‘clayey silt above their heads’, Marc Brunel worried, fearing that the tunnel could collapse at any moment. Isambard wanted to inspect the construction from the outside and asked Humboldt to join him. It would be dangerous but Humboldt didn’t care – this was too exciting to miss. He also hoped to measure the air pressure at the bottom of the river to compare it to his observations in the Andes.

The diving bell in which Humboldt descended with Brunel to the bottom of the Thames to see the construction of the tunnel (Illustration Credit 14.2)

On 26 April a huge metal diving bell that weighed almost two tons was lowered by a crane from a ship. Boats filled with curious onlookers crowded the surface of the river as the diving bell with Brunel and Humboldt inside was dropped to a depth of thirty-six feet. Air was supplied through a leather hose that was inserted at the top of the bell, and two thick glass windows offered views into the murky river water. As they descended, Humboldt found the pressure in his ears almost unbearable but he got used to it after a few minutes. They wore thick coats and looked like ‘Eskimos’, Humboldt wrote to François Arago in Paris. Down on the riverbed with the tunnel below them and only water above, it was eerily dark except for their lanterns’ weak glimmer. They spent forty minutes underwater but as they ascended the changing water pressure ruptured blood vessels in Humboldt’s nose and throat. For the next twenty-four hours he spat and sneezed blood, just as he had when climbing Chimborazo. Brunel didn’t bleed, Humboldt noted, and joked that it was seemingly ‘a privilege of Prussians’.

Two days later parts of the tunnel fell in, and then in mid-May the riverbed above the tunnel collapsed completely, creating a huge hole through which water came gushing in. Amazingly no lives were lost and after repairs were made, the work continued. By then Humboldt had left London and had arrived in Berlin.

He was now the most famous scientist in Europe and admired by colleagues, poets and thinkers alike. One man, though, had yet to read his work. That man was eighteen-year-old Charles Darwin who, at the very moment that Humboldt was being fêted in London, had given up his medical studies at the University of Edinburgh. Robert Darwin, Charles’s father, was furious. ‘You care for nothing but shooting, dogs, and rat-catching,’ he wrote to his son, ‘and you will be a disgrace to yourself and all your family.’

¹ Or in the case of isobars, the lines represent air pressure.

² During Napoleon’s reign Louis XVIII had lived in exile in Prussia, Russia and Britain.

³ Wilhelm had left London in 1818. He had then briefly held a ministerial position in Berlin but had grown frustrated with Prussia’s reactionary politics. At the end of 1819, Wilhelm had retired from his political career and moved to the family estate at Tegel, which he had inherited.

⁴ Forty-six-year-old Mary Somerville was a celebrated mathematician and polymath. In 1827 she was working on the translation of Laplace’s book The Mechanism of the Heavens into English. Her writing was so clear that the book became a bestseller in Britain. She was the only woman, Laplace said, ‘who could understand and correct his works’. Others called her the ‘queen of science’. She would later publish a book called Physical Geography which bore many similarities to Humboldt’s approach to science and the natural world.