from December to May is the period of the rains, and during this time the river slowly rises until it attains a height of nearly forty feet above its low-water mark. It floods the banks, extends in great lagoons over a monstrous waste of country, and forms a huge district, called locally the Gapo, which is for the most part too marshy for foot-travel and too shallow for boating. About June the waters begin to fall, and are at their lowest at October or November. Thus our expedition was at the time of the dry season, when the great river and its tributaries were more or less in a normal condition.
The current of the river is a slight one, the drop being not greater than eight inches in a mile. No stream could be more convenient for navigation, since the prevailing wind is southeast, and sailing boats may make a continuous progress to the Peruvian frontier, dropping down again with the current. In our own case the excellent engines of the Esmeralda could disregard the sluggish flow of the stream, and we made as rapid progress as if we were navigating a stagnant lake. For three days we steamed northwestwards up a stream which even here, a thousand miles from its mouth, was still so enormous that from its center the two banks were mere shadows upon the distant skyline. On the fourth day after leaving Manaus we turned into a tributary which at its mouth was little smaller than the main stream. It narrowed rapidly, however, and after two more days’ steaming we reached an Indian village, where the Professor insisted that we should land, and that the Esmeralda should be sent back to Manaus. We should soon come upon rapids, he explained, which would make its further use impossible. He added privately that we were now approaching the door of the unknown country, and that the fewer whom we took into our confidence the better it would be. To this end also he made each of us give our word of honor that we would publish or say nothing which would give any exact clue as to the whereabouts of our travels, while the servants were all solemnly sworn to the same effect. It is for this reason that I am compelled to be vague in my narrative, and I would warn my readers that in any map or diagram