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This is a circular slide rule made Fowler's Ltd in th e1920's. The entire scale disk is rotated by turning the top knob. the second knob, rotated a second hairline (in black) while the other (red) hairline is fixed relative to the metal case. This calculator is 9' in diameter and ruggedly made for traveling with a heavy duty felt lined thick leather case. The instrument contain 4 separate scales arranged concentrically on a single dial. Besides the typical multiply and reciprocal scales, it included scales for converting twelfths to/from tenths, meters to yards, Kg to lbs, inch to cm, etc. |
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