I believe it is the compass. It aided sailors locate the direction to which they needed to go. Even before the age of exploration, people had ships capable of long voyages. As the years pass, the designed improved to an extent that they can travel farther.
A secondary source is a source of information that was created later by someone who did not experience first-hand the events or conditions you are researching.
• A biography is an account of someone's life, written by someone else. This means it is a secondary source.
Hope this helps! :D
~PutarPotato
"When I said you were a friend of Tom's, he started to abandon the whole idea. He doesn't know very much about Tom, though he says he's read a Chicago paper for years just on the chance of catching a glimpse of Daisy's name" (p.152)
