Complex sentence: During the 18th century ships carrying cargo, passengers and mail between Europe and America would sail only when they were full, but starting in the early 19th century, as trade with America became more common, schedule regularity became a priority.
Split into simpler sentences: During the 18th century ships carrying cargo, passengers and mail between Europe and America would sail only when they were full. Starting in the early 19th century, as trade with America became more common, schedule regularity became a priority.