river. So far as climate goes, it was immaterial what time we chose for our expedition, as the temperature ranges from seventy-five to ninety degrees both summer and winter, with no appreciable difference in heat. In moisture, however, it is otherwise; 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 south-east, 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 north-westwards up a stream which even here, a thousand miles from its mouth, was still so enormous that from its centre the two banks
