 As part of his apprenticeship, Cook applied himself to the study of algebra, geometry, trigonometry, navigation and astronomy—all skills he would need one day to command his own ship.His three-year apprenticeship completed, Cook began working on trading ships in the Baltic Sea.After passing his examinations in 1752, he soon progressed through the merchant navy ranks, starting with his promotion in that year to mate aboard the collier brig "Friendship".