Complex sentence: He returned to Britain in 1802 and subsequently joined the ship of the line HMS "Ganges", remaining in command until 1812 when he was made a rear-admiral.
Split into simpler sentences: He returned to Britain in 1802 and subsequently joined the ship of the line HMS "Ganges". Halkett remained in command of "Ganges" until 1812 when he was made a rear-admiral.