Circular slide rule, 1660-1680.

Circular slide rule, 1660-1680.
A circular slide rule, made by John Brown, with two brass radial arms and an stronomical quadrant engraved on the back. A spiral slide rule affords a long and therefore accurate logarithmic line in a small amount of space. The potential of spiral rules was not really utilized until the Victorian period, when several spiral and helical designs appeared on the market.
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