In 1802 he left Callao for Acapulco, still accompanied by Bonpland, who attended to botanical researches. The letters of Minister Urquizo procured him in New Spain as elsewhere the attention and cooperation of the highest officials, and he was enabled to make investigations connected not alone with the abstract and concrete sciences, which formed his chief aim, but with the political and economic condition of the country hitherto so jealously veiled. During a stay from March, 1803, till the same month in the following year, he made trips in different directions, embracing the mining districts of Real del Monte, Guanajuato, and Zacatecas, the agricultural regions of Michoacan, and back past the snow-crowned volcanoes Popocatepetl and Iztaccihuatl to the aboriginal center of Cholula with it famous pyramid, and thence to the miasmatic gulf-shore, directing his penetrating observations on ancient and modern society, on mouldering ruins and unfolding elements of a higher culture, on supernal phenomena and subterranean forms, all the more interesting since in connection with his southern explorations, they formed the base for several branches of science, to which he lays claim as founder, notably in meteorology and physical geography. He returned to Europe in 1804, and began soon after at Paris, with aid from different quarters, to prepare for publication the fruit of his remarkable tour, a task occupying a great part of his attention for over 20 years, and intimately connected with other works of encyclopædic range which flowed from his ever busy pen, and procured him a fame almost unrivalled in scientific annals.
The results of his American journey were given to the world in Voyage aux régions équinoxiales du nouveau continent, fait dans les années 1799 à 1804, divided into six parts in 30 folio and quarto volumes, and embracing such sections as Relation Historique, Vues des Cordillères, Atlas, Examen critique de l'histoire de la géographie du nouveau continent, spoken of in connection with my Summary of Geographical Knowledge and Discovery, Hist. Cent. Am., i., and Essai Politique sur le Royaume de la Nouvelle Espagne, Paris, 1811, 2 vols., the rest relating especially to natural history, geology, meteorology^ accompanied by maps and drawings. Most of these and other works have been translated into different languages, and issued in many editions, both complete and abbreviated. My remarks must for the present be confined to the Essai Politique, as the only section which relates particularly to New Spain. It is dedicated to Carlos IV., as his patron in this instance, and has a valuable atlas. After an introductory explanation of the maps, it opens with physical geography and its bearing on agriculture and other industries, and proceeds to treat of population, the causes which affect births and deaths, notably epidemic diseases, and characteristics and conditions of the races and castes. Then follows a description of the different intendencias and provinces, their extent, resources, and leading towns; the second volume with the three last directions is devoted respectively to agriculture and mines, commerce and manufactures, revenue and defences, followed by two supplements of notes and additions. Like many other parts of Humboldt's work, this was first issued in sections from 1808-11, as noticed in Edinburgh Rev., April 1810, and November 1811, the first complete edition being the folio before me of 1811. I have also the first German issue of 1809-14 in 5 vols., 8vo, and English and Spanish editions, those of later date containing additions.