Slide rules are the original mechanical calculators. They could quickly multiply and divide large numbers.
Slide rules are based on logarithms. These are tables of the number another number is raised to produce a third number. Scales of roots do the opposite.
John Napier realized sets of log scales placed next to one another easily and accurately multiply and divide.
William Oughtred, a minister, took this to the next step placing scales on a piece of wood with a slider to align the numbers. By sliding the device to the right two numbers the user could quickly and accurate multiply and divide large numbers.
For hundreds of years, mathematicians and engineers relied on slide rules.
Newton used them to develop his rules of physics. James Watt, after joining with Boulton, used them to refine and build his condensing steam engine that kicked off the first Industrial Revolution. Centuries later, during the computer era, NASA engineers still used them while planning the Apollo missions.
Virtually every entry before 1970 on innowiki relied, to some extent, on slide rules.
Many “computer museums” feature the slide rule as the second computing device ever invented, after the ancient abacus which was more focused on addition.
After hundreds of years, computers superseded slide rules. However, the impact of slide rules cannot be overstated. These primitive yet vital calculating machines built the modern world.