James WATT (1736-1819)

James Watt considered that the usual slide rules of the time (about 1780), were not adapted to the production of his steam machines.
"Mr. Watt and Mr. Southern arranged a series of logarithmic lines upon a sliding rule, in a very judicious form, and they employed the most skilful artists to graduate the original patterns, from which the the sliding rules themselves were to be copied. “ Sliding rules of this kind are still called Soho rules, and they are so correctly divided by some of the best makers of mathematical instruments in London, that they are capable of performing ordinary calculations with sufficient accuracy for parctice. " (from J. Farey, A treatise on the steam engine 1827).
Watt regarded this new type of rule as an industrial secret. “Particular formulae are required, which were confined to a very few of the principal engineers in Soho, and have not been at all disseminated in the profession “. They made it possible to make all kinds of calculations with sufficient precision.