A mechanical device used for mathematical calculations such as multiplication and division. In the simplest form, one piece slides alongside another piece, each piece being marked with a logarithmic scale. To multiply x and y, the reading ‘x’ on one scale is placed opposite the ‘l’ on the other scale, and the required product ‘xy’ then appears opposite the ‘y’. Slide rules have now been superseded by calculators and computers.

http://www.sliderule.ca/ Descriptions of various slide rules and how they work.