"Now," he continued, "you have in that tea-saucer, bottom upward, a fairly good picture of the interior of Africa. Let the table where it is lying represent the ocean surrounding the continent. The rim on which it rests when right-side up is the mountainous ridge enclosing the central plateau, and the space in the centre is the plateau, or table-land. The slope from the ridge to the edge of the saucer is the strip of land around the coast. It varies greatly, as it is very narrow in some parts of the continent and quite broad in others. If you break a few notches at irregular intervals along the ridge you will indicate the depressions where the rivers pass from the equatorial basin to the great ocean." Frank was about to make the notches suggested by the Doctor, and thus complete the model of the "Dark Continent;" but he was checked by Fred, who suggested that they were a long way from their base of supplies, and tea-saucers could not be easily replaced. The practical illustration was consequently deferred indefinitely. "It is in the central basin," Doctor Bronson farther explained, "that we find the great lakes which form the sources of the Nile, the Livingstone, and the Zambesi." "How about the Niger?" Frank asked. "Does it come from the same basin, or does it have another origin?" "The source of the Niger," replied the Doctor, "is far to the north of the great basin where the three rivers I have named have their origin. Thus far no European has seen the source of the Niger; it has been most nearly reached by Winwood Reade, who visited the stream at a point where it was not more than a hundred yards in width, and probably forty or fifty miles from where it has its beginning. [Illustration: SUNSET ON LAKE TCHAD.] "The Niger was partially explored by the brothers Lander, Richard and John, in 1830 and 1831; they visited it again in 1832 and 1834, and endeavored to establish trade with the natives along the lower part of the river, and also on its tributary, the Benoowe. The latter stream is sometimes called the Chadda, or Tchadda, as it is supposed to rise in Lake Tchad; but whether it does so or not is not fully established. The Niger is properly formed by the junction of the Chadda and the Joliba, the latter being the more western, and pronounced by those who have seen it to be longer and larger than the Benoowe." One of the boys asked if any other Europeans than the Landers and Mr. Reade had explored the valley of the Niger.