This mechanical model, shown here in a photograph from 1926, was used to visualize the development of population density in Germany. The appropriate figures could be compared by moving the levers.
"Wie dicht haben in Deutschland die Menschen gelebt?" ("How densely populated have the Germans lived?"), model demonstrating the development of population density in Germany during the centuries (from the collection "Technik im Bild", Mappe 2), black-and-white photograph, 1926, unknown photographer; source: Deutsches Museum (copyright) 1926, Idnr.: CD 58996.