With the members of the Congo Expedition, I was somewhat startled by the contrast between the apparently shrunken volume of waters and the vast breadth of the lower river; hence Professor Smith's theory of underground caverns and communications, in fact of a subterraneous river, a favourite hobby in those days. But there is not a trace of limestone formation around, nor is there the hollow echo which inevitably would result from such a tunnel. Evidently the difference is to be accounted for by the rapidity of the torrent, the effect of abnormal slope deceiving the eye. At the Mosî-wa-tunya Falls the gigantic Zambeze, from a breadth of a thousand yards suddenly plunges into a trough only forty- five to sixty feet wide: the same is the case with the Brazilian São Francisco, which, a mile wide above the Cachoeira de Paulo Affonso, is choked to a minimum breadth of fifty-one feet. At the Pongo (narrows) de Manseriche also, the Amazonas, "already a noble river, is contracted at its narrowest part to a width of only twenty-five toises, bounded on each margin by lofty perpendicular cliffs, at the end of which the Andes are fairly passed, and the river emerges on the great plain."[FN#31] Thus the Yellala belongs to the class of obstructed rapids like those of the Nile, compared with the unobstructed, of which a fine specimen is the St. Lawrence. It reminded me strongly of the Búsa (Boussa) described by Richard Lander, where the breadth of the Niger is reduced to a stone-throw, and the stream is broken by black rugged rocks arising from mid-channel. It is probably a less marked feature than the Congo, for in June, after the "Malka" or fourteen days of incessant rain, the author speaks of whirlpools, not of a regular break. I thus make the distance of the Yellala from the mouth between 116 and 117 miles and the total fall 390 feet, of which about one half (195) occurs in the sixty-four miles between Boma and the Yellala: of this figure again 100 feet belong to the section of five miles between the Vivi and the Great Rapids. The Zambeze, according to Dr. Livingstone ("First Expedition," p. 284), has a steeper declivity than some other great rivers, reaching even 7 inches per mile. With 3 to 4 inches, the Ganges, the Amazonas, and the Mississippi flow at the rate of three knots an hour in the lowest season and five or six during the flood: what, then, may be expected from the Nzadi?