In our brief conversation Dufour mentioned that the differential posed a bit of a headache; he said it was "very difficult" to visualize how it would work. In particular, he knew that if he stopped one balance, the other should continue to run but he wasn't sure that would actually be the case until he had completed one of the movements. (Even now, years later, when he talks about it you can sense the pleasure and relief he must have felt at seeing it work properly). Another interesting piece of information, which I hadn't previously been aware of, is that if you do stop one balance completely, the other will continue to run but the seconds hand will take two minutes, not one minute, to go once around the dial. This makes sense in retrospect, when you remember that the whole rationale behind the Duality is to take the average of the rate of two balances, but it certainly would never have occurred to me that this was the case if Dufour hadn't told me.