any of the scales could be easily compared across the rule face. The "Mannheim rule" became the standard slide rule arrangement for the later 19th century and remained a common standard throughout the slide-rule era. The growth of the engineering profession during the later 19th century drove widespread slide-rule use, beginning in Europe and eventually taking hold in the United States as well. The duplex rule was invented by William Cox in 1891 and was produced by Keuffel and Esser Co. of New York. In 1881, the American inventor Edwin Thacher introduced his cylindrical rule, which had a much longer scale than standard linear rules and thus could calculate to higher precision, about four to five significant digits. However, the Thacher rule was quite expensive, as well as being non-portable, so it was used in far more limited numbers than conventional slide rules. Astronomical work also required precise computations, and, in 19th-century Germany, a steel slide rule about two meters long was used at one observatory. It had a microscope attached, giving it accuracy to six decimal places. In the 1920s, the novelist and engineer Nevil Shute Norway (he called his autobiography Slide Rule) was Chief Calculator on the design of the British R100 airship for Vickers Ltd. from 1924. The stress calculations for each transverse frame required computations by a pair of calculators (people) using Fuller's cylindrical slide rules for two or three months. The simultaneous equation contained up to seven unknown quantities, took about a week to solve, and had to be repeated with a different selection of slack wires if the guess on which of the eight radial wires were slack was wrong and one of the wires guessed to be slack was not slack. After months of labour filling perhaps fifty foolscap sheets with calculations "the truth stood revealed' (and) produced a satisfaction almost amounting to a religious experience". Throughout the 1950s and 1960s, the slide rule was the symbol of the engineer's profession in the same way the stethoscope is that of the medical profession. German rocket scientist Wernher von Braun bought two Nestler slide rules in the 1930s. Ten years later he brought them with him when he moved to the US after World War II to work on the American space effort. Throughout his life he never used any other slide rule. He used his two Nestlers while heading the NASA program that