Circular slide rule, 1660-1680.
© Science Museum / Science & Society Picture Library
A circular slide rule, made by John Brown, with two bras 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.