Humboldt's Map of Isothermal Lines: A Milestone in Thematic Cartography

This year is the sesquicentennial anniversary of the presentation of Alexander von Humboldt's paper on isothermal lines to the Paris Academy of Science. This employment of the isarithm for spatial distribution by an outstanding natural scientist contributed largely to the widespread adoption of this technique in thematic cartography. The complexities of the original evidence are unravelled, and its influence traced on many subsequent publications, particularly atlases, in the nineteenth century.