At Callao, the main port for Peru, Humboldt observed the transit of Mercury on 9 November and studied the fertilizing properties of guano, rich in nitrogen, the subsequent introduction of which into Europe was due mainly to his writings.
En Callao, observa o tránsito de Mercurio o 9 de novembro, e estuda as propiedades fertilizantes do guano, cuxa introdución en Europa moito se debeu ás súas obras.