Charles, Jacque Alexandre César (1746–1823)
Jacque Charles was a French physicist, inventor, and mathematician who, with Nicholas Robert, made the first ascent in a hydrogen balloon (1783). He discovered the law relating the expansion of a gas to its temperature rise. Gay-Lussac published this work some 15 years after Charles' discovery and it is alternately known as Charles' law or Gay-Lussac's law. Charles is credited with inventing a thermometric hydrometer and improving Fahrenheit's aerometer and Gravesande's hydrometer.