Thomson's next enterprise was undertaken for the sultan of Zanzibar, who believed that the coal reported by Livingstone in 1862 as existing in the Rovuma valley might be turned to profitable account. The sultan invited Thomson to make an expert examination. This Thomson carried out in 1881. The result was a disappointment to the sultan—the ‘coal’ was only useless shale.
A very different task was that to which Thomson, under the auspices of the Royal Geographical Society, next braced himself—the opening up of a route between the seaboard of Eastern Africa and the northern shore of Victoria Nyanza. He left the coast with a caravan 140 strong on 15 March 1882, and reached Taveta, at the foot of Mount Kilimanjaro, on 5 May. On 3 May the expedition entered the territory of the dreaded Masai, to find the tribe in a state of dangerous excitement as the result of a recent conflict with a party led by Dr. Fischer, a German explorer. Forming an encampment at Taveta, Thomson proceeded with ten men to examine the Kilimanjaro mountain, and, having travelled 230 miles in five and a half marches, he ascended the mountain to a height of nearly nine thousand feet. September found the explorer at Lake Navaisha, where Fischer had been obliged to turn homeward. At El Meteita Thomson left his main body to proceed with a trading caravan to Lake Baringo, and, taking with him only thirty men, made one of those rapid detours, which were always congenial to him, for the purpose of visiting Mount Kenia. On the way he discovered the noble range, fourteen thousand feet high, which he named after Lord Aberdare, president of the Royal Geographical Society. On reaching the neighbourhood of Lake Baringo (3,300 feet above sea level) he took a much-needed rest at Njemps or Nnems (0.30 N., 36.5 E.) among the friendly Wa-Kwafi. Having (16 Nov.) once more got his caravan (reduced to about a hundred men) into marching order, he pushed steadily and patiently from Baringo eastwards to Victoria Nyanza, and on 10 Dec. he bathed in the waters of the great birth-lake of the Nile. Here he was obliged to retrace his steps owing to the treacherous hostility of the king of Uganda, which was reported to him in time. On his homeward route he turned northwards to visit Mount Elgon (14,094 feet), and was rewarded by a discovery of a wonderful series of prehistoric caves suggestive of the existence at one time of a civilisation very different from that half-barbarism which now turns them to account. On the last day of 1882 Thomson was nearly killed by a wounded buffalo, and for weeks he had to be carried in a litter. On 24 Feb. 1883 the caravan resumed its march for Lake Naivasha, but by the 27th its leader was disabled by dysentery, and further progress was impossible for eight or nine weeks. Meanwhile the expedition was in daily danger of complete annihilation from the ferocious and suspicious Masai. Towards the end of April the appearance of Jumba Kimameta, a coast trader, along with whose caravan part of the inland journey had been performed, gave a happy turn to events. On 7 May Thomson parted with this friendly caravan, and carried out his original idea of making for Mombasa via Teita. By the 24th he had reached Rabai, and celebrated the event by walking through the village—the first walk he had taken for three months.
On his return to London in broken health in the summer of 1883 he was received with the utmost cordiality. Explorer after explorer had been previously baffled in attempts to traverse the country of the Masai, one of the most warlike of all African tribes, and Thomson's record of heroic endurance and adventurous bravery, which he published under the title of ‘Through Masai Land,’ took the world by storm.
By the end of 1884 Thomson was fit to undertake new explorations, and when, in 1885, the Royal Geographical Society bestowed on him the founder's gold medal, he was already in the Western Sudan. On this occasion he was in the service of the National African Company, and his mission was to forestall the efforts of Germany to enter into direct relations with the kings of Sokoto and Gand[ua]. The chief difficulties lay in outwitting Malikè, king of N[ua]pe, who considered his interests as a middleman endangered, and in reducing a mob of undisciplined and mutinous carriers to a recognition of authority. Starting from Akassa (15 March 1885), the expedition passed up the Niger to Rabba (7 April) and thence struck inland to Sokoto (21 May), Wurnū (23 May), and Gand[ua] (7 or 8 June). By September Thomson was in England once more with a record of work brilliantly done. He had made treaties with the great potentates of the Sudan which proved of the highest service to British interests.
Thomson's health was still weak, and the remainder of 1885, with 1886 and 1887, was devoted to its restoration. He paid during this period visits to the continent and made useful contributions to questions of geographical and political interest. He strongly advocated the selection of the east coast Masai-land route for the expedition to be sent for the relief of Emin Pasha; but his rival,
