Why did benjamin franklin know about the ocean current pattern in the north atlantic?

As a deputy postmaster, he became interested in the North Altlantic Ocean circulation patterns. While in England, Franklin heard a question about why it took mail longer to reach than RI. He then asked his cousin who told him that merchant ships routinely avouded a strong eastbound mid-ocean current while the mail packets captain sailed dead into it, fighting an adverse current. 

The exchange of foods between the new and old worlds