Pascaline

Blaise Pascal invented this adding machine in the 1640s to help his father, a tax collector, with endless sums. You entered a digit by putting a stylus into a toothed wheel at that number and turning until it hit a stop, and the machine carried into the next wheel by itself. That carry was the hard part. Pascal's mechanism used falling weights rather than linked gears, and it was complicated and unreliable, so a handsome exterior hid a fragile heart. Even so, he built about 50, making it the first adding machine produced in any quantity, though only about 15 sold. It shows the problem every later designer had to solve: carrying a one across many digits at once.