Maxwell’s thermodynamic surface is an 1874 sculpture made by Scottish physicist James Clerk Maxwell showing the various states of a fictitious water-like substance on a three-dimensional volume-energy-entropy plot, based on the American scientist Willard Gibbs’ graphical thermodynamics papers of the early 1870s. Maxwell drew lines of equal pressure (isopiestics) and of equal temperature (isothermals) on his plaster cast by placing it in the sunlight, and "tracing the curve when the rays just grazed the surface." One of his letters reports sending sketches of these lines to Thomas Andrews.