Complex sentence: He entered the medical service of the Royal Navy as a surgeon's mate, and in 1757 became surgeon, and was attached to the ship HMS "Edgar" till 1763, when, on the conclusion of peace, he left the service.
Split into simpler sentences: He entered the medical service of the Royal Navy as a surgeon's mate, and in 1757 became a surgeon. Attached, till 1763, to the ship HMS "Edgar", when, on the conclusion of peace, he left the service.