Darwin had to stay at Cambridge until June 1831. He studied Paley's "Natural Theology or Evidences of the Existence and Attributes of the Deity" (first published in 1802), which made an argument for divine design in nature, explaining adaptation as God acting through laws of nature. He read John Herschel's new book, "Preliminary Discourse on the Study of Natural Philosophy" (1831), which described the highest aim of natural philosophy as understanding such laws through inductive reasoning based on observation, and Alexander von Humboldt's "Personal Narrative" of scientific travels in 1799–1804. Inspired with "a burning zeal" to contribute, Darwin planned to visit Tenerife with some classmates after graduation to study natural history in the tropics. In preparation, he joined Adam Sedgwick's geology course, then on 4 August travelled with him to spend a fortnight mapping strata in Wales.