After Briggs popularised Napier's idea of logarithms, Gunter had the brilliant idea of mapping logarithms onto a line in which the linear distance is represented by the logarithm. A simple piece of wood engraved with expertly positioned lines took the sting out of laborious calculations. Initially multiplication and division were completed with the aid of dividers. Gunter's navigational scale was used by the Royal Navy up to the 1840s. Eventually,(1630c)two Günter's scales were juxtaposed to make an even more powerful calculator-the slide rule. The initial use of the slide rule was used by Customs and Excise to calculate the duty due on spirits and by tradesmen, carpenters, glaziers and stone masons to calculate the price of raw materials. After Mannheim (1850c) regularised the slide rule's scales many designers, draftsmen, engineers and scientists used the slide rule on a regular basis. Every significant invention of the 20th century up to about 1975 was completed with the aid of a slide rule - the Spitfire and Hurricane, radar, the atom bomb, the Concorde, etc. Even the Apollo missions were equipped with 6" slide rules to get them to earth safely in case their computer failed.