Analysis of Modern Love XVIII: Here Jack and Tom

Here Jack and Tom are paired with Moll and Meg.
Curved open to the river-reach is seen
A country merry-making on the green.
Fair space for signal shakings of the leg.
That little screwy fiddler from his booth,
Whence flows one nut-brown stream, commands the joints
Of all who caper here at various points.
I have known rustic revels in my youth:
The May-fly pleasures of a mind at ease.
An early goddess was a county lass:
A charmed Amphion-oak she tripped the grass.
What life was that I lived? The life of these?
Heaven keep them happy! Nature they seem near.
They must, I think, be wiser than I am;
They have the secret of the bull and lamb.
'Tis true that when we trace its source, 'tis beer.