"I will endeavor to explain it," replied Dr. Bronson, "and in doing so will call your attention to the fertile regions of the Amazon, Orinoco, and La Plata, on the eastern side of the Andes, in contrast with the arid desert on the west. The tropical winds from the Atlantic Ocean are laden with moisture; they blow with great regularity from east to west, and thus sweep over the country drained by the rivers I have mentioned. Rain is frequent and copious all through that region; it varies with the seasons of the year, but is always sufficient to keep the channels of the streams well filled. "The rains continue up to the foot of the Andes and along their eastern slopes. The mountains condense the moisture from the warm winds, and up to the very crest of the dividing ridge there is an abundance of rain. But by the time the winds have crossed the Andes all the water they carried has been wrung from them, and when they reach the Pacific slope they have no more to give out. Thus it happens that the eastern slopes of the Andes and the great plains intervening to the Atlantic have an abundance of water, while there is little or none at all for the west. [Illustration: A WOLF EMIGRATING.] "There is a part of Peru and Bolivia where rain never falls," continued the Doctor. "It is known as the '_Despoblado_' or 'The Uninhabited,' in consequence of the severity of its climate, and the great difficulty of existing there. In the language of a once-famous statesman of America, it is 'so poor that a wolf couldn't make a decent living there.'" "Does this condition of dryness extend all along the western coast to the end of the continent?" one of the youths inquired. "No," was the reply. "As we go south through Chili we encounter more moisture in the climate, and on reaching Patagonia we find the western slopes of the Andes drenched by frequent rains, and the tops of the mountains almost constantly covered with clouds. This condition is due to the trade-winds, which blow from the south Pacific Ocean to the land; the plains east of the Andes in Patagonia are comparatively dry, and swept by cold winds from the snow-tipped summits of the mountains. Remember, we are south of the equator, and the farther south we go the more cold do we find." In conversations like this, and in the examination of books relating to Peru and other parts of South America, the time passed during the voyage from Paita to Callao. Frank was busy with Prescott's "Conquest of Peru," while Fred carefully conned the pages of "Peru, or Travel and Exploration in the Land of the Incas," by Hon. E. G. Squier. Frank declared that the work of Prescott "read like a romance," while Fred was equally enthusiastic over the book which claimed his attention. It is quite likely that they will rely upon these volumes for much of their information concerning the antiquities of Peru, and the story of its occupation by the Spanish conquerors.