A slide rule adds the logarithms of two numbers to multiply them. Slide the C scale so that the 1 or 10 is over on the D scale. Find on the C scale; will be underneath on the D scale. You have to figure out the decimals yourself.
Slide rules were invented in the early 1600s and continued to be used until the 1970s. They typically had many scales, allowing calculations of reciprocals, squares, cubes, roots, trig functions, and so on. There were even circular and cylindrical slide rules.
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