I would guess that the first careful measurements that are ever referenced were performed by Jacques Charles. In 1787 he did an experiment where he filled 5 balloons with with equal volumes of oxygen, hydrogen, nitrogen, carbon dioxide, and air and he heated them all to 80oC. He noticed that all five balloons increased in volume by the exact same amount, but never published his results. Charles' law (which describes the relationship between the volume of a gas and its temperature) was actually developed by Guy-Lussac, but named after Charles because of this initial unpublished experiment. It could also be Boyle.
Jacques Charles (1746-1823), along with the brothers Anne-Jean Robert (1758-1820), and Marie-Noël Robert (1760-1820), launched the first unmanned hydrogen balloon in August, 1783. On December 1, Charles and the younger Robert made the first manned hydrogen balloon flight over Paris, covering some 36 kilometers (22 miles).
Robert Charles died in 1900.
Robert Charles Chambers was born in 1952.
Robert Henry Charles died in 1931.
Robert Henry Charles was born in 1855.
Charles Robert Sanger died in 1912.
Charles Robert Sanger was born in 1860.
Charles Robert Broberg was born in 1870.
Charles Robert Broberg died in 1937.
Robert Charles Sands was born in 1799.
Robert Charles Sands died in 1832.