Complex sentence: In 1818 he served under Lieutenant (afterwards Sir) John Franklin in David Buchan's Arctic expedition, of which at a later period he published a narrative; and in the following year he accompanied Lieutenant W. E. Parry in the "Hecla".
Split into simpler sentences: In 1818 he served under Lieutenant (afterwards Sir) John Franklin in David Buchan's Arctic expedition, of which at a later period he published a narrative. In the following year he accompanied Lieutenant W. E. Parry in HMS "Hecla".