Tsetse, an insect like a common fly, destroyed horses and oxen, so that many traders lost literally every ox in their team.  As for the Lake, it lay somewhat beyond the outskirts of his new district, and was reported terrible for fever.  He heard that Mr. Moffat intended to visit it, but he was somewhat alarmed lest his friend should suffer.  It was not Moffat, but Livingstone, however, that first braved the risks of that fever swamp.

A subject of special scientific interest to the missionary during this period was—­the desiccation of Africa.  On this topic he addressed a long letter to Dr. Buckland in 1843, of which, considerably to his regret, no public notice appears to have been taken, and perhaps the letter never reached him.  The substance of this paper may, however, be gathered from a communication subsequently made to the Royal Geographical Society[20] after his first impression had been confirmed by enlarged observation and discovery.  Around, and north of Kuruman, he had found many indications of a much larger supply of water in a former age.  He ascribed the desiccation to the gradual elevation of the western part of the country.  He found traces of a very large ancient river which flowed nearly north and south to a large lake, including the bed of the present Orange River; in fact, he believed that the whole country south of Lake ’Ngami presented in ancient times very much the same appearance as the basin north of that lake does now, and that the southern lake disappeared when a fissure was made in the ridge through which the Orange River now proceeds to the sea.  He could even indicate the spot where the river and the lake met, for some hills there had caused an eddy in which was found a mound of calcareous tufa and travertine, full of fossil bones.  These fossils he was most eager to examine, in order to determine the time of the change; but on his first visit he had no time, and when he returned, he was suddenly called away to visit a missionary’s child, a hundred miles off.  It happened that he was never in the same locality again, and had therefore no opportunity to complete his investigation.

[Footnote 20:  See Journal, vol. xxvii. p. 356.]

Dr. Livingstone’s mind had that wonderful power which belongs to some men of the highest gifts, of passing with the utmost rapidity, not only from subject to subject, but from one mood or key to another entirely different.  In a letter to his family, written about this time, we have a characteristic instance.  On one side of the sheet is a prolonged outburst of tender Christian love and lamentation over a young attendant who had died of fever suddenly; on the other side, he gives a map of the Bakhatla country with its rivers and mountains, and is quite at home in the geographical details, crowning his description with some sentimental and half-ludicrous lines of poetry.  No reasonable man will fancy that in the wailings of his heart there was any levity or want