At the age of 36, at the time of the expedition with Speke, Burton was one of the great minds and most remarkable personalities of the Victorian era - an author, explorer, scientist and poet. A brilliant linguist, he mastered over 30 languages, including Arabic, Persian, Turkish, Hindi and Swahili. He wrote 43 books, translated the Arabian Nights, The Perfumed Garden and the Kama Sutra (the private publication of which - to avoid prosecution for obscenity - made him a considerable fortune in his later years), and laid the foundations for modern ethnography and anthropology.

Above all, Burton was a romantic and an Arabist, who had no time for the constricting moral code of Victorian England. His hunger for intellectual and erotic experience blazed in his burning dark eyes. The poet Wilfrid Scawen Blunt described his face as 'the most sinister I have ever seen, dark, cruel, treacherous, with eyes like a wild beast'. Burton was a mass of contradictions: a libertine, an intensely fastidious scholar; an irascible and intolerant Indian Army officer, a brilliant swordsman, a man who loved to disguise himself in Eastern clothes. There was almost too much in Richard Burton that could be restrained in a single body.

After the disastrous time with John Speke, Burton never led another African expedition. Subsequently, he held consular posts on Fernando Po (now Bioko, an island off the coast of Cameroon) and at Santos in Brazil, Damascus and Trieste, and was knighted in 1886.

After his death in 1890, Lady Burton, who had shared in much of his travelling and writing, burned her husband's journals. She lobbied hard for him to be buried in Westminster Abbey, but such was Burton's reputation as a controversialist and libertine that the Church of England would not assent to her request. Then, claiming that he had had an 11th-hour conversion to Catholicism, she had him buried in a Roman Catholic cemetery at Mortlake, south-west London (see Death of a dream).

John Hanning Speke (1827-64)
Born at Ilminster in Somerset, he entered the Indian Army at the age of 17, and later fought in the Punjab. He had a mania for shooting and would frequently go on solitary shooting expeditions in the distant Himalayas. After 10 years in the army, he was due a three-year leave, beginning in 1854. He saved his money and planned on exploring Africa, gathering specimens of rare birds and animals, with the aim of establishing a natural history museum in his father's house, Jordans.

Speke presented a stark contrast to Burton, whom he accompanied on an 1855 expedition to Somalia. Aged 30 at the time of their trek to find the source of the Nile, he was a puritanical and athletic Indian Army officer and a careful planner. And yet, as Burton was to discover, this model of muscular Victorian Christianity was almost as complicated as himself. What Burton found disturbing was Speke's habit of brooding endlessly over chance remarks and imagined slights that could easily have been smoothed over. It must be said, however, that Burton's fiery temper and acute intellect would have sorely tested the tolerance of a man far more easy-going than John Speke.

After the fractious expedition with Burton, Speke returned to Africa in 1860 with Captain James Grant, who proved to be an ideal companion. However, it was Speke alone - he had commanded Grant to go elsewhere - who found the source of the White Nile at Urondogani, about 40 miles downstream from Lake Victoria, on 21 July 1862. On his way home, he cabled the Royal Geographical Society that 'the Nile is settled'.

On his return to London, Speke's findings came under attack, not least from Burton. The latter asked how he could be sure that the two large bodies of water he had encountered in 1858 and 1862 were both part of Lake Victoria? He had not circumnavigated the lake to make sure, and he had not followed the north-flowing river downstream for any distance. Although we now know that Speke did find the source of the Nile, there is much to criticise in his methodology.

Speke was about to debate this with Burton when he shot himself while hunting. Whether this was accidental or self-inflicted is still subject to controversy.

Henry Morton Stanley (1841-1904)
Born the illegitimate John Rowlands in Denbigh, Wales, in 1859 he went as a cabin boy to New Orleans, where he was adopted by a merchant named Stanley. Eight years later, under his new name, he joined the New York Herald and, as its special correspondent, travelled to Abyssinia (Ethiopia), Spain and the Crimea. In 1869, he was instructed to 'find Livingstone' in Africa.

Stanley was well qualified for the task. The writer Alan Moorehead has described him as a 'businessman-explorer, not in the sense of his wanting to trade in Africa, but in the extremely logical, sensible and efficient way in which he went about the problem of setting up an expedition and getting it to journey's end. It may have been ruthless, but it was also expert, and one must never lose sight of the fact that he was very determined and very brave.'

In 1871, Stanley left Zanzibar for Tanganyika, and made his historic rendezvous with Livingstone at Ujiji in 10 November, doffing his cap and uttering the greeting, 'Dr Livingstone, I presume?' Three years later, he explored Lake Tanganyika and traced the Congo to the sea. It was on this expedition that he was also able to confirm Speke's theory that the source of the Nile lay in Lake Victoria (unlike Speke, he circumnavigated it). Stanley's return to Zanzibar in 1877 marked the end of the heroic era of the exploration of the White Nile.

Two years later, he led a third expedition, during which he helped found the Congo Free State, and in 1887-9 undertook a rescue mission to come to the aid of the German-born explorer and doctor Emin Pasha in the Sudan. Stanley became a Member of Parliament in 1895 and was knighted in 1899, five years before his death.

Stanley did not go to Africa to reform its people, colonise it or make scientific discoveries. He went there to make a name for himself, and this he did. It is ironic, therefore, that his successful expeditions opened up central Africa for missionaries and scientists.

David Livingstone (1813-73)
The legendary missionary and traveller was born in Strathclyde, worked in a cotton mill between the ages of 10 and 24, then trained as a physician in Glasgow and was ordained under the London Missionary Society in 1840. He worked for several years in Bechuanaland (now Botswana) and then, from 1852 to 1863, explored central Africa, discovering Lake Ngami and the Victoria Falls and, on an expedition to the Zambesi, Lakes Shirwa and Nyasa.

Livingstone, who had attended Speke's funeral, sided with Burton, writing: 'Poor Speke has turned his back on the real sources of the Nile.' In 1866, at the invitation of the Royal Geographical Society, he returned to Africa to establish the matter once and for all, but the river he encountered turned out to be the Congo.

Livingstone was never at his best when travelling with other white men. Unlike other explorers, whose views would now attract accusations of racism, Livingstone made the life of the Africans his own. He understood them and the hardships they had to endure, particularly slavery, in a way that his colleagues did not. He wrote: 'The strangest disease I have seen in this country seems really to have been broken-heartedness, and it attacks free men who have been captured and made slaves.'

Broken in health and sickened by the depredations of the slave trade, Livingstone was 'found' by Henry Morton Stanley at Ujiji in November 1871. The doctor later wrote: 'When my spirits were at their lowest ebb, the good Samaritan was close at hand, for one morning Susi [an African servant] came running at the top of his speed and gasped out, "An Englishman! I see him!" and off he darted to meet him. The American flag at the head of the caravan told of the nationality of the stranger. Bales of goods, baths of tin, huge kettles, cooking pots, tents, etc. made me think, "This must be a luxurious traveller, and not one at his wits' end like me."'

When Livingstone died in Africa on 1 May 1873, it took 11 months for his remains to be carried by his African friends to the coast and thence to Zanzibar. On their arrival in England in April 1874, they were housed in the map room of the Royal Geographical Society before their interment at Westminster Abbey. Henry Morton Stanley was among the pallbearers.

The plaque on Livingstone's tomb reads: 'For 30 years his life was spent in an unwearied effort to evangelise the native races, to explore the undiscovered secrets, to abolish the desolating slave trade, of Central Africa, where with his last words he wrote: "All I can add in my solitude is may Heaven's rich blessing come down on everyone, American, English or Turk, who will help to heal this open sore of the world ..."'

Samuel White Baker (1821-93)
The son of a wealthy British shipowner and banker, Samuel Baker had a passion for travelling and big game hunting and shot elephants in Sri Lanka, tigers in India and bears in the Balkans.

In 1860, he travelled to Africa accompanied by his beautiful Hungarian-born second wife. Baker's aim was to navigate the Nile up to the Sudan and learn Arabic on the journey. His expedition was lavishly equipped: food from Fortnum & Mason and guns he had designed himself made by the finest gunsmiths in London.

One year after leaving Cairo, the couple arrived in Khartoum, where Baker received an assignment from the Royal Geographical Society to search for Speke and Grant. He set off up the Nile and, after a journey that lasted over a year and covered 1,000 miles, arrived at Gondoroko in the Sudan. Two weeks later, Speke and Grant turned up there, too.

While the other two explorers returned to Cairo, Baker decided to push on up the Nile. He later wrote that he considered 'my mission as terminated, but ... Speke and Grant with characteristic candour and generosity gave me the map of their route, showing that they had been unable to complete the actual exploration of the Nile and that a most important portion remained to be demonstrated.'

A year later, Baker and his wife reached Lake Luta Nzige, which he renamed Lake Albert. Baker wrote, 'I rushed into the lake, and thirsty with heat and fatigue, with a heart full of gratitude, I drank deeply from the source of the Nile' (it wasn't). Thus, until Stanley finally solved the mystery of the Nile, the Bakers, for all their courage and resourcefulness, only muddied the waters of the debate. Nevertheless, Baker was awarded the gold medal of the Royal Geographical Society and a knighthood soon followed.