Roget's Log Log Scales on Post Versalog 1460
USA 1951+, duplex, 25 cm scales, celluloid on bamboo.
In 1815 Peter Roget, an English physician (and author of Roget's Thesaurus), invented a log log scale, which he used to calculate roots and powers to any number or fraction thereof.
It was regarded at the time as a mathematical curiosity.
Fifty years later, advances in engineering and science made these scales so necessary that they were rediscovered.
In the next fifty years they increased from three, to six, to eight scales on the slide rule.