

 Maker: Henri Chatelain, Paris, France
 Date: c1890
 Description and Notes:
This French made circular slide rule closely follows the design of the Boucher Calculator, which was introduced in 1876 and, presumably, was the first commercialized circular slide rule. The above picture shows the two dials of the instrument. The one on the left is fixed, and on which there are four circles represent two scales, the outermost circle is a scale of equal parts, and the inner three circles are separate sections of cubic root of a number specified by the pointer on the opposite side. The dial at right can be rotated by turning the center arbor, on which there are three different scales on four circles, the outmost circle is a scale of logarithms of sines of angles, the second one is the ordinary logarithmic scale, and the inner two circles are the square roots of the number on the logarithmic scale. There is a fixed pointer engraved on the glass cover, and the movable pointer can be rotated by pressing down the small side button while turning the center arbor. Instructions for operating this instrument can be found in The slide rule: a practical manual, pp. 98103.
 More Pictures:
Closer View of the Front Face
 Closer View of the Back Face
 Side View

