Sentence:  McGonagall is earnestly trying to gather the resources of a metrical tradition, not subvert it, but the mismatch of <WSD>duple</WSD> and triple measure in the first line alone means that, while it's made of recognisable metrical feet, the line doesn't feel like it belongs to any specific metrical pattern (iambic, dactylic, anapestic etc) or mode (pastoral, elegy or ballad). 
Meaning of the word marked with <WSD> tags:  Having two beats in each foot.