2018-08-302022-02-022018-08-302022-02-02https://hdl.handle.net/11124/172490Date of manufacture: early 20th century.Invented by Edwin Thacher, an American bridge engineer. Patented in 1881, production of this 20-scale cylindrical slide rule was assumed by Keuffel and Esser Co. of New York by 1897. This unit dates to approximately 1920, serial #3499. The 20 scales provide an effective length of 30 feet with an accuracy of 4 to 5 digits. Scales for square roots are also included.born digitaldigital photographsComputersImage