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PERTURBATIOM" 198 PERU PERTTTRBATION, any disturbance or irregularity in the movement of a planet in its orbit. Every heavenly body, by the law of gravitation, possesses an at- tractive power over every other one. When, therefore, the orbits of any tAvo approach, each causes a perturbation in the movement of the other. Magnetic perturbation, irregular declination of the magnetic needle. This may be produced by earthquakes, by volcanic eruption, by the aurora borealis, etc. PEBU, a maritime republic of South America, bounded on the N. by Ecuador, on the W. by the Pacific, on the S. and S. E. by Bolivia and Chile, and on the E. by Brazil; area, 695,720 square miles; pop. (1910) 4,500,000; (1920) 5,000,000; capital, Lima. Topography. — The general outline re- sembles a triangle, the base of which is formed by the boundary line between Peru and Ecuador on the N. On the E. side of the Andes, and between the Ama- zon and the Purus, there is a wide and unexplored expanse of country, upon which both Peru and Brazil have claims, though the boundary is now generally regarded as marked by the Rio Javary. The country is 1,100 miles in length, 780 miles in extreme width along the N. boundary, but it is little more than 50 miles wide in the extreme S. The is- lands on the Peruvian coast, though valu- able, are extremely few in number, and small in extent. In the N. are the Lobos (i. e., "Seal") Islands, forming a group of three, and so called from the seals which frequent them. The largest of them, Lobos de Tierra, is 5 miles long by two miles wide, and the others, lying 30 miles S. W., are much smaller. On their E. sides they are covered by guano, and the quantity on the whole group, when it began to be exported from them, was stated to be 4,000,000 tons. The islands of Macabi and Guanoi)e, near the Lobos, were originally supposed to contain 2,- 280,000 tons of guano; but the guano exported has very greatly exceeded that amount, and it has been calculated that there are still 750,000 tons of guano on the former and 500,000 tons on the latter. Physical Features. — The surface of Peru is divided into three distinct and well defined tracts or belts, the climates of which are of every variety, from tor- rid heat to Arctic cold, and the produc- tions of which range from the stunted herbage of the high mountain slopes to the oranges and citrons, the sugar canes and cottons of the luxuriant tropical val- leys. These three regions are the Coast, the Sierra, and the Montana. The Sierra embraces all the mountainous re- gion between the W. base of the maritime Cordillera and the E. base of the Andes, or the East Cordillera. These ranges are, in this country, about 100 miles apart on an average, and have been esti- mated to cover an area of 200,000 square miles. Transverse branches connect the one range with the other, and high pla- teaus, fertile plains, and deep tropical valleys lie between the lofty outer bar- riers. The mountain chains which girdle the plain of Titicaca trend toward the N. W., and form what is called the Knot of Cuzco; the Knot comprises six minor mountain chains, and has an area thrice larger than that of Switzerland. Here the valleys enjoy an Indian climate, and are rich in tropical productions; to the N. and E. of the Knot extend luxuriant forests, while the numberless mountain slopes are covered with waving crops of wheat, barley, and other cereals, and with potatoes ; and higher up extend rich pasture lands, where huge herds of vicunas and pacas feed. The valley of the Apurimac is 30 miles in average width, and extends N. W. for about 300 miles. This valley is the most popu- lous region of Peru. From Cuzco pro- ceed two chains toward the N. W.; they unite again in the Knot of Pasco. This Knot contains the table-land of Bombon, 12,300 feet above the sea-level, as well as other table-lands at a height of 14,000 feet, the highest in the Andes ; otherwise, however, the physical features of the country resemble those of the vicinity of Cuzco. The valley of the river Maranon, which is upward of 300 miles in length, is narrow, deep, and nearer the equator than any other valley of the Sierra, and consequently it is the hottest portion of this region, and its vegetation is thor- oughly tropical in character. The con- formation of the surface of the Sierra is of the most wonderful description. After the table-lands of Tibet, those of the Peruvian Andes are the highest in the world; but, unlike those of Tibet, the table-lands of Peru are the seat of a comparatively high civilization, and are studded over with towns and villages, perched on heights exceeding in eleva- tion the summits of the Jungfrau and the Matterhorn. ^ Nor are such towns the mere eyries of miners who are tempted to ascend thus high in search of the preci- ous metals; for, even at this elevation, the climate is pleasant, and wheat, maize, barley, rye, and potatoes thrive well. The city of Cuzco, situated in a region of rare beauty, and enjoying a temperate climate, is 11,380 feet above sea-level, or 2,000 feet higher than the Great St. Ber- nard. The climate of the Sierra, how- ever, is not always so delightful. In general terms it may be described as mild and variable, with moderate rains. In