But that which demonstrates the humidity of the climate most strikingly is the number of earthen sponges or oozes met with. In going to Bangweolo from Kizinga, I crossed twenty-nine of these reservoirs in thirty miles of latitude, on a south-east course: this may give about one sponge for every two miles. The word "Bog" conveys much of the idea of these earthen sponges; but it is inseparably connected in our minds with peat, and these contain not a particle of peat, they consist of black porous earth, covered with a hard wiry grass, and a few other damp-loving plants. In many places the sponges hold large quantities of the oxide of iron, from the big patches of brown haematite that crop out everywhere, and streams of this oxide, as thick as treacle, are seen moving slowly along in the sponge-like small red glaciers. When one treads on the black earth of the sponge, though little or no water appears on the surface, it is frequently squirted up the limbs, and gives the idea of a sponge. In the paths that cross them, the earth readily becomes soft mud, but sinks rapidly to the bottom again, as if of great specific gravity: the water in them is always circulating and oozing. The places where the sponges are met with are slightly depressed valleys without trees or bushes, in a forest country where the grass being only a foot or fifteen inches high, and thickly planted, often looks like a beautiful glade in a gentleman's park in England. They are from a quarter of a mile to a mile broad, and from two to ten or more miles long. The water of the heavy rains soaks into the level forest lands: one never sees runnels leading it off, unless occasionally a footpath is turned to that use. The water, descending about eight feet, comes to a stratum of yellow sand, beneath which there is another stratum of fine white sand, which at its bottom cakes, so as to hold the water from sinking further.
It is exactly the same as we found in the Kalahari Desert, in digging sucking places for water for our oxen. The water, both here and there, is guided by the fine sand stratum into the nearest valley, and here it oozes forth on all sides through the thick mantle of black porous earth, which forms the sponge. There, in the desert, it appears to damp the surface sands in certain valleys, and the Bushmen, by a peculiar process, suck out a supply. When we had dug down to the caked sand there years ago, the people begged us not to dig further, as the water would all run away; and we desisted, because we saw that the fluid poured in from the fine sand all round the well, but none came from the bottom or cake. Two stupid Englishmen afterwards broke through the cake in spite of the entreaties of the natives, and the well and the whole valley dried up hopelessly. Here the water, oozing forth from the surface of the sponge mantle, collects in the centre of the slightly depressed valley which it occupies, and near the head of the depression forms a sluggish stream; but further down, as it meets with more slope, it works out for itself a deeper channel, with perpendicular banks, with, say, a hundred or more yards of sponge on each side, constantly oozing forth fresh supplies to augment its size. When it reaches rocky ground it is a perennial burn, with many aquatic plants growing in its bottom. One peculiarity would strike anyone: the water never becomes discoloured or muddy. I have seen only one stream muddied in flood, the Choma, flowing through an alluvial plain in Lopéré. Another peculiarity is very remarkable; it is, that after the rains have entirely ceased, these burns have their largest flow, and cause inundations. It looks as if towards the end of the rainy season the sponges were lifted up by the water off their beds, and the pores and holes, being enlarged, are all employed to give off fluid. The waters of inundation run away. When the sponges are lifted up by superabundance of water, all the pores therein are opened: as the earthen mantle subsides again, the pores act like natural valves, and are partially closed, and by the weight of earth above them, the water is thus prevented from running away altogether; time also being required to wet all the sand through which the rains soak, the great supply may only find its way to the sponge a month or so after the great rains have fallen.
I travelled in Lunda, when the sponges were all supersaturated. The grassy sward was so lifted up that it was separated into patches or tufts, and if the foot missed the row of tufts of this wiry grass which formed the native path, down one plumped up to the thigh in slush. At that time we could cross the sponge only by the native paths, and the central burn only where they had placed bridges: elsewhere they were impassable, as they poured off the waters of inundation: our oxen were generally bogged—all four legs went down up to the body at once. When they saw the clear sandy bottom of the central burn they readily went in, but usually plunged right over head, leaving their tail up in the air to show the nervous shock they had sustained.
These sponges are a serious matter in travelling. I crossed the twenty-nine already mentioned at the end of the fourth month of the dry season, and the central burns seemed then to have suffered no diminution: they were then from calf to waist deep, and required from fifteen to forty minutes in crossing; they had many deep holes in the paths, and when one plumps therein every muscle in the frame receives a painful jerk. When past the stream, and apparently on partially dry ground, one may jog in a foot or more, and receive a squirt of black mud up the thighs: it is only when you reach the trees and are off the sour land that you feel secure from mud and leeches. As one has to strip the lower part of the person in order to ford them, I found that often four were as many as we could cross in a day. Looking up these sponges a bird's-eye view would closely resemble the lichen-like vegetation of frost on window panes; or that vegetation in Canada-balsam which mad philosophical instrument makers will put between the lenses of the object-glasses of our telescopes. The flat, or nearly flat, tops of the subtending and transverse ridges of this central country give rise to a great many: I crossed twenty-nine, a few of the feeders of Bangweolo, in thirty miles of latitude in one direction. Burns are literally innumerable: rising on the ridges, or as I formerly termed them mounds, they are undoubtedly the primary or ultimate sources of the Zambezi, Congo, and Nile: by their union are formed streams of from thirty to eighty or 100 yards broad, and always deep enough to require either canoes or bridges. These I propose to call the secondary sources, and as in the case of the Nile they are drawn off by three lines of drainage, they become the head waters (the caput Nili) of the river of Egypt.
Thanks to that all-embracing Providence, which has watched over and enabled me to discover what I have done. There is still much to do, and if health and protection be granted I shall make a complete thing of it.
[Then he adds in a note a little further on:—]
But few of the sponges on the watershed ever dry; elsewhere many do; the cracks in their surface are from 15 to 18 inches deep, with lips from 2 to 3 inches apart. Crabs and other animals in clearing out their runs reveal what I verified by actually digging wells at Kizinga and in Kabuiré, and also observed in the ditches 15 feet deep dug by the natives round many of their stockades, that the sponge rests on a stratum of fine white washed sand. These cracks afford a good idea of the effect of the rains: the partial thunder-showers of October, November, December, and even January, produce no effect on them; it is only when the sun begins to return from his greatest southern declination that the cracks close their large lips. The whole sponge is borne up, and covers an enormous mass of water, oozing forth in March and April forming the inundations. These floods in the Congo, Zambesi, and Nile require different times to reach the sea. The bulk of the Zambesi is further augmented by the greater rains finding many pools in the beds of its feeders filled in February, as soon as the sun comes north.
Mem.—In apparent contradiction of the foregoing, so far as touches the sources of the Zambesi, Syde bin Habib informed me a few days ago that he visited the sources of the Liambai and of the Lufira. Each comes out of a fountain; the Lufira one is called Changozi, and is small, and in a wood of large trees S.W. of Katanga; the fountain of the Liambai is so large that one cannot call to a person on the other side, and he appears also very small there—the two fountains are just five hours distant from each other. He is well acquainted with the Liambai (Leeambye), where I first met him. Lunga, another river, comes out of nearly the same spot which goes into the Leuñge, Kafué (?). Lufira is less than Kalongosi up there; that is less than 80 or 200 yards, and it has deep waterfalls in it. The Koné range comes down north, nearly to Mpméto's. Mkana is the chief of the stone houses in the Baloba, and he may be reached by three days of hard travelling from Mpwéto's; Lufira is then one long day west. As Muabo refuses to show me his "mita," "miengelo," or "mpamankanana" as they are called, I must try and get to those of the Baloba of Mkana.
Senegal swallows pair in the beginning of December.
Note.—Inundation.