Who was the French professor that the Robert brothers built the world's first hydrogen balloon for?
Robert brothers: Les Frères Robert were two French brothers. Robert brothers:  Anne-Jean Robert (1758–1820) and Nicolas-Louis Robert (1760–1820) were the engineers who built the world's first hydrogen balloon for professor Jacques Charles; which flew from central Paris on August 27, 1783. Jacques Charles: Jacques Alexandre César Charles (November 12, 1746 – April 7, 1823) was a French inventor, scientist, mathematician, and balloonist.
Jacques Alexandre César Charles