“The movement of the waters, which affects so powerfully the distribution of the fishes, forms in itself a very curious phenomenon. There is, as it were, a rhythmical correspondence in the rise and fall of the affluents on either shore of the Amazons, causing the great body of the water, in its semiannual tides, to sway alternately more to the north or to the south. On the southern side of the valley, the rains begin in the months of September and October. They pour down from the table-lands of Brazil and the mountains of Bolivia with cumulative force, gathering strength as the rainy season progresses, swelling the head-waters of the Purus, Madeira, Tapajoz, and other southern tributaries, and gradually descending to the main stream. The process is a slow one, however, and the full force of the new flood is not felt in the Amazons until February and March. During the month of March, in the region below the confluence of the Madeira, for instance, the rise of the Amazons averages a foot in twenty-four hours, so great is the quantity of water poured into it. At about the same period with the southern rains, or a little earlier, say in the months of August and September, the snows in the Andes begin to melt and flow down towards the plain. This contribution from the Cordilleras of Peru and Equador, coinciding with that from the highlands of Brazil and Bolivia, swells the Amazons in its centre and on its southern side to such an extent that the bulk of the water pushes northward, crowding upon its northern shore, and flowing even into the tributaries which open on that side of the river, and are now at their lowest ebb. Presently, however, the rains on the table-lands of Guiana, and on the northern spurs of the Andes, where the rainy season prevails chiefly in February and March, repeat the same process in their turn. During April and May the northern tributaries are rising, and they reach their maximum in June. Thus, at the end of June, when the southern rivers have already fallen considerably, the northern rivers are at their flood-tide. The Rio Negro, for instance, rises at Manaos to about forty-five feet above its lowest level. This mass of water from the north now presses against that in the centre, and bears it southward again. The rainy season along the course of the Amazons is from December till March, corresponding very nearly, in the time of the year and in duration, with our winter. It must be remembered that the valley of the Amazons is not a valley in the ordinary sense, bordered by walls or banks enclosing the waters which flow between. It is, on the contrary, a plain some seven or eight hundred miles wide and between two and three thousand miles long, with a slope so slight that it hardly averages more than a foot in ten miles. Between Obydos and the sea-shore, a distance of about eight hundred miles, the fall is only forty-five feet; between Tabatinga and the sea-shore, a distance of more than two thousand miles in a straight