Complex sentence: In 1660 Hartlib was at work writing to John Evelyn, an important broker of the royal charter for the eventual Society, but promoting not a purist Baconian model, but an "Antilia".
Split into simpler sentences: In 1660 Hartlib was at work writing to John Evelyn, an important broker of the royal charter for the eventual Society. He was, however, not promoting a purist Baconian model, but an "Antilia".