What Was Swift's Reasoning?

The Martian day is 24.6 hours (similar to Earth). If the inner moon revolved in 10 hours it would be zipping around 2.5 times every day. There was no precedent for that. We know Hall thought it "very improbable". So why would Swift have guessed 10 hours? What was his reasoning? Astronomers can't think it was just another lucky guess!