The astronomers wondered why this little fort had been placed on the top of the mountain. Mokoum, who had visited the country as Livingstone's guide, explained that formerly the neighbourhood of Lake Ngami was frequented by traders in ivory and ebony. The ivory was furnished by the elephants and rhinoceroses; but the ebony trade was but too often another name for that traffic in human beings which is still carried on by the slave-traders in the region of the Zambesi. A great number of prisoners are made in the wars and pillages in the interior of the country, and these prisoners are sold as slaves. Mount Scorzef had been a centre of encampment for the ivory-traders, and it was there that they had been accustomed to rest before descending the Zambesi. They had fortified their position, to protect themselves and their slaves from depredations, since it was not an uncommon occurrence for the prisoners to be recaptured for fresh sale by the very men who had recently sold them. The route of the traders was now changed; they no longer passed the shores of the lake, and the little fort was falling into ruins. All that remained was an enclosure in the form of the sector of a circle, from the centre of which rose a small casemated redoubt, pierced with loop-holes, and surmounted by a small wooden turret. But notwithstanding the condition of ruin into which it had fallen, the fortress offered the Europeans a welcome retreat. Behind the thick sandstone walls, and armed with their rapidly-loading guns, they were confident that they could keep back an army of Makololos, and, unless their provisions and ammunition failed, they would be able to complete their observations. At present they had plenty of ammunition; the coffer in which it was contained had been placed on the same waggon which carried the steamboat, and had therefore escaped the rapacity of the natives. The great difficulty would be the possible failure of provisions. The Colonel and Strux made a careful inspection of the store, and found that there was only enough to last the eighteen men for two days. After a short breakfast, the astronomers and the bushman, leaving the sailors still to keep watch round the walls, assembled in the redoubt to discuss their situation. "I cannot understand," said Mokoum, "why you are so uneasy. You say that we have only provisions for two days; but why stay here? Let us leave to-morrow, or even to-day. The Makololos need not hinder us; they could not cross the lake, and in the steamboat we may reach the northern shore in a few hours." The astronomers looked at each other; the idea, natural as it was, had not struck them before. Sir John was the first to speak. "But we have not yet completed the measurement of our meridian." "Will the Makololos have any regard for your meridian?" asked the hunter. "Very likely not," answered Sir John; "but _we_ have a regard for it, and will not leave our undertaking incomplete. I am sure my colleagues agree with me."