An adding machine is a class of mechanical calculator, usually specialized for bookkeeping calculations. In the United States, the earliest adding machines were usually built to read in dollars and cents. Adding machines were ubiquitous office equipment until they were phased out in favor of calculators in the 1970s and by personal computers beginning in about 1985. The older adding machines were rarely seen in American office settings by the year 2000. Blaise Pascal and Wilhelm Schickard were the two original inventors of the mechanical calculator in 1642. For Pascal, this was an adding machine that could perform additions and subtractions directly and multiplication and divisions by repetitions, while Schickard's machine, invented several decades earlier, was less functionally efficient but was supported by a mechanised form of multiplication tables. These two were followed by a series of inventors and inventions leading to those of Thomas de Colmar, who launched the mechanical calculator industry in 1851 when he released his simplified arithmometer (it took him thirty years to refine his machine, patented in 1820, into a simpler and more reliable form). However, they did not gain widespread use until Dorr E. Felt started manufacturing his comptometer (1887) and Burroughs started the commercialization of differently conceived adding machines (1892). Operation To add a new list of numbers and arrive at a total, the user was first required to "ZERO" the machine. Then, to add sets of numbers, the user was required to press numbered keys on a keyboard, which would remain depressed (rather than immediately rebound like the keys of a computer keyboard or typewriter or the buttons of a typical modern machine). The user would then pull the crank, which caused the numbers to be shown on the rotary wheels, and the keys to be released (i.e. to pop back up) in preparation for the next input. To add, for example, the amounts of 30.72 and 4.49 (which, in adding machine terms, on a decimal adding machine is 3,072 plus 449 "decimal units"), the following process took place: Press the key in the column fourth from the right (multiples of one thousand), the key in the column second from right (multiples of ten) and the key in the rightmost column (multiples of 1). Pull the crank. The rotary wheels now showed 3072. Press the key in the third column from the right, the key in the second column from right, and