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The device known as a logarithm was invented in the late 1500s by a Scottish baron named John Napier as a tool to simplify arithmetic. It was useful because it replaced multiplication with addition. This was accomplished by a property of logarithms:

ln xy = ln x + ln y

You could multiply two positive numbers x and y by looking up their logarithms in a table, adding the logarithms, finding the sum in the body of the table, and reading the table backwards to find the product xy.
Of course, you needed an actual table to do this, and Napier spent the last 20 years of his life working on a table he never finished. The table was later completed after Napier's death by his friend Henry Briggs.