Complex sentence: Accordingly, Naval Captain Matthew Fontaine Maury made a deal with whalers--if they would provide him with their logbooks from which he could extract wind and current information, he would in return prepare for them maps showing where whales were most concentrated.
Split into simpler sentences: Accordingly, Naval Captain Matthew Fontaine Maury made a deal with whalers. If they provided him with their logbooks, from which he could extract wind and current information, he would in return prepare maps for them showing where whales were most concentrated.