Now doesn't that sound weird? Sounds like they just stop and add a beat every now and then. Haydn creates this illusion by fooling you as to where the downbeat is:
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If you notice, the first note is tied over from the third beat to the first so that you think that it is a half note on the downbeat. But no. You aren't quite sure where the downbeat us until the eighth measure. And then it starts all over again.