Instruction for Using a Slide Rule

Can two sliding scales turn 2.12 × 3.16 into 6.70 without a pencil? W. Stanley’s answer begins with a moving hairline—and a trick for keeping the decimal point in its place.

In the age when engineers, surveyors, chemists, and students relied on slide rules for calculations, Stanley sets out to teach a beginner how to use one. He names its scales, then guides the reader through multiplication and division, where the rule supplies the significant digits but leaves the decimal point to the user’s judgment. From reciprocals and powers to roots and the dimensions of a circle, each operation reveals another use for the sliding instrument. How much arithmetic can you do by moving a single line?

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