About the time the slide rule was becoming the pocket calculator of the scientist, a simple calculating machine was described by Wilhelm Schickard, and in the late 1600s, Gottfried Leibniz introduced a cylindrical geared computing devise that could add, subtract, multiply and extract roots and looks remarkably similar to the mechanical calculating devises of the early to mid-1900s. These devises remained more objects of curiosity than practical tools until 1820 when Charles Thomas invented an arithometer with levers and a geared drum. In 1895, O. Steiger patented a machine that could multiply with one pull of the handle. An arithmetical machine called the Curta consists of a small 2.5 inch diameter, 3.5 inch tall cylinder with a handle on top and vertical adjustable scales along the sides of the cylinder. It is named after its inventor and was perhaps the highest quality small calculator invented.