Plaster model of a three-dimensional surface. The printed inscription reads, "Model of the magnetic surface presented in the discussion Part II of the magnetic observations made at the Girard College Philadelphia between 1840 and 1845, by Prof. A.D. Bache. Const. and modeled by Ferdinand Engel, Washington, D.C. 1862." A handwritten description reads, "Presented by A.D. Bache." The observations which this model records are described in a 36-page pamphlet published by the Smithsonian Institution in 1863. A note with the model states that Bache intended to make two more models of the horizontal and vertical forces; one of these is now at the Smithsonian.