Slide Rule for Chemical Equivalents, by John Newman, London, c. 1830

This rule is based on the scale for chemical equivalents devised by the chemist William H. Wollaston in 1814. They facilitsted calculations in analysis, by allowing the 'practical chemist' to calculate the composition of any weight of any chemical substance ('salts') on the scale, the necessary quantity of another chemical to decompose it, and the quantity of the new compound that would be formed, according to contemporary interpretration of chemical composition. Both John Newman and William Cary were well-known makers of chemical and other apparatus.