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30 foot slide rule.TIF

117 30-foot slide rule, in 20 sections (made by Edwin Thatcher)

This gigantic slide rule can be read to four decimal places. It was patented by Edwin Thatcher, C.E., on November 1, 1881. Manufactured by Keuffel & Esser. A similar instrument was used to do the calculations that led to the discovery of the planet Pluto.