This is a Thacher's Calculating Instrument in exceptionally fine overall condition, complete with case and instruction manual.

In 1881, Edwin Thacher, an American bridge engineer patented this cylindrical slide rule, 4 inches in diameter and 18 inches long. In 1882, scales were divided for Thacher's instrument by W. F. Stanley of London.

The cylindrical rule can be both rotated and moved horizontally within an open network of 20 pairs of scales, mounted on metal bars, the open network itself also capable of rotating as a unit.

The 20 scales afford the instrument an effective length of 30 feet, and an accuracy of 4-5 digits. Scales for square roots were also included.

Mounted permenantly on the mahogany base is a brief set of instructions.

Thacher's Slide Rule was produced by the famous Keuffel and Esser Company of New York (Model No. 4012). This example is in unusually clean condition, showing little, if any, regular use. The scales are all in fine condition, completely legible, having no gaps or tears, and showing only a minimum of spots or "foxing". The base-mounted instructions are all clearly legible. The brass retains all of its original lacquer, in near perfect condition, and the maple handles show almost no sign of use. The mahogany case is also in excellent condition. The manual is perfectly legible, though it does show evidence of aging.

This instrument is Serial Number 2552, dating it to approximately 1917. Overall, an extremely attractive instrument with a wonderful, mellow, nearly perfect patina. It is quite rare to find a Thacher's instrument complete, and in such excellent condition.