John Dicas

John Dicas was a scientific instrument maker, active in Liverpool, England, between 1774 and 1797. He was succeeded by his daughters Mary and Ann, who continued to operate the business until some time after 1837.

In 1780 John Dicas was awarded UK patent 1,259 for “constructing hydrometers with sliding-rules to ascertain spirit strength”. According to Peter Hopp, the rule was of very simple form: “actually a line of numbers, 0-360, on a rod opposite which is placed the strength of spirits. This shows the strength of spirits at any degree of heat.”

In 1790 Dicas's hydrometer was listed as the US standard for measuring the proof of alcohol in an Act of Congress passed on 19 July of that year and effective 1 January 1791. Dicas's hydrometer continued to be the standard for the next 60 years.