Two slide rules, 1663 and 1881.
© Science Museum / Science & Society Picture Library
The earlier slide rule of Henry Sutton (bottom) consists of fixed scales and moveable index arms. A spiral slide rule affords a long and therefore accurate logarithmic line in a small amount of space. Sutton was one of the best known engravers of scales and quadrants of his day. The slide rule of Thacher (top) is an extremely important American contribution to slide rules, capable of accurate reading to four decimal places.