"We crawled slowly upward on our eastward way and found it exciting enough; what shall I say of the return ride, when we had the downward grade to take us along, and the only use of the steam in the locomotive was to hold us back? The brakes were screwed tightly down, and so great is the pressure upon them that their shoes must be renewed at the end of every second round trip from Callao and back again. In four hours from the terminus we were on the shores of the Pacific, and at the end of a journey we shall long remember." Two weeks from the time our friends landed at Callao they embarked on the southern-bound steamer from that port, having taken their tickets for Mollendo. [Illustration: GUANO ISLANDS.] The first landing was at Pisco, about one hundred miles south of Callao, and connected by a short line of railway with the cotton regions of Iça. As they approached the port they passed the Chincha Islands, which have become famous as the place whence millions of tons of guano have been brought to Europe and America. Frank and Fred wished to know something about the guano trade, and the Doctor kindly informed them. [Illustration: SEA-BIRDS AT HOME.] "The guano was deposited here," said Dr. Bronson, "by the sea-birds, and the accumulations have been going on for thousands of years. No rain falls here, and consequently there was no water to wash the substance away. Mixed with the deposits of the birds were their decomposed bodies and eggs, and the bodies of seals; the seals climb to the highest places on the rocks when they are about to die, and as they were very abundant here, it is safe to say that millions of them have died on the Chincha Islands. Guano is of great value as a manure; the ancient Peruvians were well aware of its qualities, and by the laws of the Incas everybody was forbidden, under pain of death, to land on the islands during the breeding season, and the same penalty was affixed to killing the birds at any time. "The guano deposits were first made known to Europe in 1804," the Doctor continued, "through a description by Baron von Humboldt. He said the islands were covered to a depth of fifty or sixty feet with pure guano; the long ages that had been consumed in the accumulation may be understood when he says that during the three centuries since the coming of the Spaniards the growth had been only a small fraction of an inch!" "Was it brought to Europe in Humboldt's time?" one of the youths inquired. "No," was the reply; "the first shipment was made in 1840, and consisted of twenty barrels, which were taken to Liverpool. It was tried on a farm near that city, and resulted so favorably that large orders were immediately sent for more. In the following year several cargoes were sent from the islands, and from that time the trade increased rapidly. Farmers in Europe and America learned the value of guano in making a wonderful increase of the producing power of their fields, and the demand for it became general.