You know, when you've got an idea or theory and you're desperate to commit it to paper before it evaporates. I was in Cambridge the other day talking with some Darwin scholars about the manuscripts of "On the Origin of Species". I asked them whether there was any evidence of Darwin's fluidity of thought. Sometimes you write in a staccato way, struggling to formulate or express ideas, and sometimes it just flows - the ideas are clear in your mind and stream onto the page as if your pen is just a conduit.
Their eyes lit up "Yes, yes!" they said. Apparantly on occasion Darwin was writing so fast that, when he turned a page, almost the entire recto was still wet with ink, and they can see the impression of that wet ink on the facing verso. Sometimes this happened for page after page, Charles Darwin writing faster than the ink could dry.
Almost 150 years after the publication of the "Origin", we're still finding out much about how it was written.
That's why I love working with libraries.