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Thacher 4012 Slide Rule

Keuffel and Esser
$30.00 in 1910
1891, Ours is from the period of 1910-1917
Online Manual
Smithsonian Keuffel & Esser 1741 Thacher Cylindrical Slide Rule
1917 Thacher's Calculating Instrument by K & E
The Thacher is a high precision slide rule patented in 1891

The preface to the manual describes it well.

"The ordinary Slide Rule, which is a development of Gunter's scale invented many years ago, has been found of great utility in the solution and checking of a wide variety of problems. The usual Mannheim and Duplex types are made in lengths of from 5 to 20 inches, giving results from two to four places of figures, depending upon the scale and the portion of it which is read. With the Thacher Calculating Instrument, however the scales are 30 feet long, so they will give results correctly to four and usual to five places of figures, sufficient to satisfy nearly every requirement of the professional and business man."