The Thacher's Calculating Instrument shown above is at Washington and Lee University in Lexington, Virginia. It has a cylinder 4 in. in diameter and 18 in. long that rotates and slides in and out between the array of twenty equally-spaced parallel bars. The bars have the usual logarithmic slide rule scale spread out over their entire length, while the cylinder also has a logarithmic scale engraved lengthwise on it. The net effect is two very long scales, and calculations using four to five significant figures can be made. I do not know when Washington and Lee bought their example, but in 1934 this instrument was sold by Keuffel and Esser for $80.00. Currently a Thatcher will bring several hundred dollars at auction.
The Thacher instrument below is in the collection of historical instruments at Dartmouth College.
The Thacher in the picture below was on display at the physics department of the University of Arkansas in 2001.
The Thacher below is at Franklin and Marshall College in Lancaster, Pennsylvania.
The 24 in. long slide rule (below) at Denison University in Granville, Ohio was manufactured by the Engineers Slide Rule Company of Philadelphia. The scales were patented July 2, 1901 by Edwin Thacher (clearly the designer of the first instruments described on this page), and it was designed by E. M. Scofield of Philadelphia.
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