Slide rule

William Oughtred is widely credited with inventing the slide rule in the 1620s, and his first design was circular. It turns logarithms into distance: because the scales are logarithmic, sliding one bar along another adds lengths, and adding lengths multiplies numbers. Aligning different scales gives trigonometry as well as multiplication. The answer is only as precise as the reader's eye, but that was good enough for generations of engineers, and slide rules stayed in wide use through the 20th century. It belongs here as the longest-lived analogue computer in everyday hands, and the clearest way to understand what "analogue" actually means.