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The variety of calculations required ran into a problem: to calculate (multiply or divide) or convert between different quantities one needs to move from a number on one scale to the number at the equivalent point on another scale.

This required the two scales to touch each other, so the equivalent points could be pinpointed accurately.

Mannheim devised a slide rule with two two-cycle log scales and two single cycle scales, and added a brass cursor to allow moving between them.

He may not have invented the cursor, but he had the practical streak in him that allows some inventors to influence the real world.

The earliest slide rules took many forms, but by the late 17th century had standardized on a few useful form factors, among them the Coggeshall carpenters rule.

This instrument had a variety of useful scales and tables on one side and a standard 2-foot measuring rule on the other.

The slide carries two identical scales, wasting one interface. The Soho rule shown here is a beautiful exemplar, having been made by the renowned Gravet-Lenoir workshop in Paris near the middle of the 19th century.

This "Gunter's Rule" was the original device introduced by Edmund Gunter in 1620, which remained in use for some two centuries.

This allowed one to multiply and divide with reduced accuracy on the top two scales, and to do squares and square roots on the bottom two when they were aligned properly.

One couldnt, however, multiply with the greater accuracy that the single-cycle scale would allow were there a matching one sliding above it, on the slide, which is curiously lacking.

To do that, youd need a cursor, a sliding member that traces a perfect perpendicular line across the rule. Who invented the Cursor (or, as it was sometimes called, Index, indicator, or Runner) is debatable.

There is mention of some form of cursor devised by Isaac Newton in 1675, and another is described by Frenchman Philippe Mouzin in 1837.

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