The continent of America presents us, on the largest scale, with two regions, placed in the same conditions as to temperature, but in which we successively encounter the circumstances which are most favorable to the formation and fall of rain in one case, and to its absence in the other. Setting out from Panama, and proceeding towards the south, we encounter the bay of Cupica, the provinces of San Bonaventura, Choco, and Esmeraldas; in this country, covered with thick forests, and intersected with a multitude of streams, the rains are almost incessant; in the interior of Choco, scarcely a day passes without rain. Beyond Tumbez, towards Payta, an order of things entirely different commences: the forests have entirely disappeared, the soil is sandy, agriculture scarcely exists, and here rain is almost unknown. When I was at Payta, the inhabitants informed me that it had not rained for seventeen years! The same want of rain is common in the whole of the country which surrounds the desert of Sechura, and extends to Lima; in these countries, rain is as rare as trees are. In Choco, where the soil is thickly covered with trees, it rains almost continually; and on the coasts of Peru, where the soil is sandy, without trees, and devoid of verdure, it never rains; and this, as I have said, under a climate which enjoys the same temperature, and whose general features and distance from the mountains are nearly the same. Piura is not more remote from the Andes of Assuay than are the moist plains of Choco from the Western Cordillera. The facts which have now been laid before the reader seem to authorize me to infer 1st. That extensive destruction of forests lessens the quantity of running water in a country. 2d. That it is impossible to say precisely whether this diminution is due to a less mean annual quantity of rain, or to more active evaporation, or to these two effects combined. 3d. That the quantity of running water does not appear to have suffered any diminution or change in countries which have known nothing of agricultural improvement. 4th. That independently of preserving running streams, by opposing an obstacle to evaporation, forests economize and regulate their flow. 5th. That agriculture established in a dry country, not covered with forests, dissipates an additional portion of its running water. 6th. That clearings of forest land of limited extent may cause the disappearance of particular springs, without our being therefore authorized to conclude that the mean annual quantity of rain has been diminished. 7th, and lastly. That in assuming the meteorological data collected in intertropical countries, it may be presumed that clearing off the forests does actually diminish the mean annual quantity of rain which falls. |