<p style='text-align: justify;'>This narrow two-foot-long print is the only evidence we have of the very first slide-rule. A close look shows that the numbers are reversed—this is because it was made directly from the slide-rule itself (long since lost), rather than from a normal printing plate. It dates from 1638, more than a decade after the principle of this kind of calculating device had been developed by the mathematician William Oughtred. The reason for the delay is not known, but in an accompanying letter to the instrument maker Elias Allen, Oughtred says that he ‘would gladly see’ the slide-rule, ‘wch yet I never have done’. From this we can be certain that the print is a record of the first example of an instrument that was in use right up to the invention of modern electronic calculators in the 1960s.</p>
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