Circular slide rule, brass with steel movable scale, in maroon leather case, made by Kilpatrick & Co., London, circa 1870, and used at Melbourne Observatory.

Physical Description

Circular rule with two fixed scales of brass and one movable steel scale. Outer fixed scale is for simple multiplication and division, inner scale is related to the outer by the factor pi, to allow quick calculation of circular area, cylinder and spherical volume. The movable scale was originally manipulated with a key, fitting the rule's central hole, and a small hole in the moving scale. Complex tables are provided on the back of the rule giving conversion factors for weight per unit volume of various substances.

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