"And another thing," said the Doctor, "that should be mentioned, is the annual rise and fall. There is a succession of freshets in the tributaries of the Amazon, so that the main stream can never run low. Most of its affluents are in the southern hemisphere, and consequently the river has its greatest flood when the sun is south of the equator. The rise is gradual, beginning in September or October, and increasing not more than one foot daily, and often less than that. The difference between the highest and lowest levels is about forty-five feet, and at the time of the flood vast areas of land are covered with water. Once in every six years the flood is greater than usual." "The Amazon is too large to be content with one name," said Frank. "From its mouth to the junction with the Negro it is called the Amazon, or the Amazons; from the Negro to the Peruvian frontier it is the Solimoens; and the part in Peru is the Marañon. But these distinctions are passing away since the river was opened to universal navigation; the Solimoens is now generally called the Middle Amazon and the Marañon the Upper Amazon. Probably another twenty years will see the old names disappear altogether." Manaos is on the Rio Negro, ten miles above the junction of the latter stream with the Amazon. Frank and Fred observed with interest the change from one river to the other, which was as marked as that from the Mississippi to the Missouri, near Alton, Illinois. The Amazon is yellow, while the Negro, as its name indicates, is black. For miles the line between the two waters is sharply defined; they hold apart from each other, as if unwilling to mingle, but the greater river at length absorbs the smaller, and henceforth, to the sea, the yellow color is retained. The youths dipped some water from the two rivers and placed it in glasses side by side. That of the Amazon was like milk, as sometimes seen in boarding-houses or cheap restaurants, while the water of the Negro was clear, with a tinge of red. The difference in the banks of the rivers was as marked as that of their waters, those of the Amazon being low and broken, as on the Mississippi. The banks of the Negro gave no indication of alluvial washings, but presented many sandy beaches, backed by low hills covered with dark forests, in which few palms or similar trees were visible.