"A computer who must make many difficult calculations usually has a slide rule close at hand."
Pickett manual
The slide rule is a mechanical device used to carry out complicated mathematical functions. It is based on two logarithmic scales that move parallel to each other and are aligned according to the desired calculation. To multiply two numbers, for example, the logs are added and raised to the power ten; to divide, the logs are subtracted.
In 1620 Edmund Gunter (1581-1626), an English clergyman and Gresham College Professor of Astronomy, produced a logarithmic scale and used dividers to take off specific distances to do the calculations. William Oughtred (1574-1660), mathematician and rector of Albury, did away with the dividers by using two sliding Gunter rules side by side in circa 1622 and described his circular slide rule in Circles of Proportion and the Horizontal Instrument (1632). Sliding different distances multiplied and divided by different quantities. Seth Partridge (1617- 1689) invented the modern slide rule in which the inner scale (the slide) is held by, and moves within, the outer scales, known as the stock or body of the rule.
In 1775 John Robertson added a cursor (an etched line in a transparent sliding attached plate) so that settings could be noted and transferred to any of a series of parallel scales. By 1815, P. M. Roget had added log-log scales, enabling powers, exponentials, and roots to be assessed easily.
